L. Wang / M. Chergui (Dir.)  

Electrons generated by Landau damping of the plasmon excitation on gold nanoparticles that can be injected into an adjacent semiconductor e.g. anatase TiO2, enhancing the light harvesting capabilities of solar energy conversion devices. The understanding of the electron injection processes is crucial for the optimization of the devices. The first step is to have an understanding of the bare gold nanoparticle itself, we thus report on ultrafast transient absorption studies of surface plasmon-excited ~25 nm diameter gold nanoparticles in solution, monitoring the surface plasmon resonance response in the visible (1.7-3.0 eV) as well as the inter- and intra-band transitions in the near-to-deep-UV (3.4-4.5 eV). However, in order to distinguish the thermal effects in absorption spectrum on gold NPs, we present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core-shell nanoparticles (core diameter: ~25 nm) in the range from 1.5 eV to 4.5 eV, which encompasses the localized surface plasmon resonance and the interband transitions. The changes of absorption spectra are well reproduced by theoretical calculations based on the experimentally measured temperature-dependence of the real (e_1) and imaginary (e_2) part of the gold nanoparticle dielectric constant. We also compare the results to ultrafast photo-induced transient absorption spectra upon excitation of the plasmon band, and conclude that while the plasmon band can be used to monitor the heating of the nanoparticles, the interband region is little affected by thermal effects and is dominated by electronic ones. Our deep-UV probed transient results on gold nanoparticles show that after the initial ultrafast electron-electron and electron-phonon scattering processes, low-energy hot electrons above the Fermi level remain in equilibrium with the hot lattice for tens to hundreds of ps. Then we investigate the photo-induced charge carrier transfer and relaxation in Au/TiO2, our results show that the bleach at the exction resonance in Au/TiO2 has a significant slow decay speed upon plasmon excitation, and its transient absorption intensity at long term decay gives a quadratic dependence on excitation fluence, implying the appearance of two-photon absorption in anatase TiO2 that enhanced by the surface plasmon resonance of gold nanoparticles. Besides, in the Au/SiO2 and Au/TiO2 NPs, we both observe acoustic vibrations in the UV region for the first time but with significantly different oscillation frequencies which were attributed to the different elastic properties of the shell.

B. Bauer / M. Chergui (Dir.)  

The absorption, conversion and transport of electronic energy in molecular aggregates is at the heart of many important natural and artificial photochemical systems, including organic solar cell materials, photosynthetic light-harvesting complexes and DNA oligomers. The photochemical function of these systems is built on the interactions between their aggregated chromophores, which determine the dynamic evolution of their photoexcited states. In this thesis, DNA oligomers are investigated as multi-chromophoric model systems, where the coupling of individual nucleobases is key to their photoprotection mechanism against high-energy ultraviolet (UV) radiation. In this respect, the pairing and stacking interactions of the nucleobases enable the formation of exciton states with charge transfer character, which govern the photochemical dynamics in DNA. In this thesis, the electronic and structural dynamics of adenine single strands are studied with broadband, polarization-controlled transient absorption spectroscopy in the deep-UV. This spectral region gives direct access to the excited state dynamics encoded in the UV-transitions of the nucleobases and by comparing different base-sequences, strand-lengths, solvent environments, and photoexcitation conditions, the role of the base-stacking interaction in energy and charge transfer processes in DNA systems is investigated. By comparing adenosine homopolymers of different strand lengths, it is found that the initially formed charge-transfer (CT) exciton spans two stacked bases, whereas the maximum charge separation sensitively depends on the structural arrangement and solvation shell of the stack. Importantly, these aspects also determine the lifetime of the excitons: in dimers with large inter-base distances, charge recombination dynamics are a factor of two slower than in closely packed oligomers with 20 bases. Through transient absorption anisotropy experiments in deuterated buffer solutions, an intra-strand proton transfer is identified as the main quenching process governing the CT exciton lifetime and its structural relaxation to a minimum-energy configuration on the 10 ps scale is observed. 2-aminopurine (2AP) is used as a sensitive local structural probe as its strong fluorescence is quenched via stacking. By integrating 2AP in adenine strands charge and energy transfer processes are studied. Varying the base sequence reveals that two coupled 2AP bases play an important role in the excited state dynamics. By comparing the dynamics after direct excitation of 2AP and indirect excitation via adenine bases, the fast energy transfer from adenine to 2AP is observed. A charge transfer state, responsible for the quenching of 2APâ s strong emission, is detected.

S. Polishchuk / M. Chergui (Dir.)  

Natural foundations defining the manner in which a solid material interacts with optical radiation are primarily encoded in the material’s momentum-dependent band structure and the microscopic details of the electron-lattice interactions. An access to this fundamental information can facilitate an understanding and help to predict technologically-relevant optoelectronic properties, driving a further optimization of current solar material and device designs, and fueling the search for novel compounds, eliminating unresolved issues. Lead halide perovskites (LHPs) and transition metal oxides (TMOs) are promising solar materials, commonly constituted by earth-abundant components. Particularly, hybrid and fully-inorganic LHP and ZnO semiconductors show a great potential in photonic applications, going beyond light harvesting. Their electronic structure was directly accessed by time- and angle-resolved photoelectron spectroscopy (TR-ARPES). The capabilities of this technique in conjunction with extreme ultraviolet (XUV) Harmonium light source were exploited by performing valence band (VB) mapping of the entire Brillouin zone of three lead tribromide perovskites featuring a different central cation, and of ZnO; and by investigating the ultrafast photocarrier dynamics in CsPbBr3. High quality VB mapping of CsPbBr3 revealed the polaron formation-induced effective mass renormalization, governing carrier transport properties in weak excitation regime. Whereas the temporal and momentum resolutions, surface sensitivity of XUV TR-ARPES, and transport simulations, helped to show the importance of many-body effects and electron-phonon coupling in the ionic lattice in explaining the nature of fluence-dependent quasi-ballistic to diffusive surface-to-bulk transport crossover in strong excitation regime. Future experiments with tunable excitation and surface deposition capabilities can elucidate the influence of excess energy on charge transport in LHPs and TMOs, and contribute to the knowledge of surface properties in presence of adsorbents.

O. Cannelli / M. Chergui; G. F. Mancini (Dir.)  

In this thesis, we exploited optical and X-ray pump-probe methods in a synergistic approach to study the interplay of the electronic, spin and structural degrees of freedom in two class of complex systems relevant for solar energy conversion applications: lead-halide perovskites and transition metal oxides. CsPbBr3 perovskites are promising optoelectronic materials characterized by a flexible ionic inorganic network that strongly interacts with the charge carriers. Upon photoexcitation, the lattice response leads to the polaronic distortions affecting the optical properties of the system. In our study, we combined time-resolved X-ray absorption spectroscopy and ab-initio simulations to quantify the structural deformations induced in the lattice through electron-phonon coupling. Complementing our pump-probe results with temperature-dependent X-ray absorption measurements, we disentangled the photodynamics of the system from thermally-induced effects. Additionally, we clarified the thermal response of CsPbBr3 comparing temperature-dependent X-ray investigations with first principles computations. A consistent atomic level picture is provided, in which the role of thermal fluctuation and phonon anharmonicity are rationalized with the experimental evidence. Spinel Co3O4 represents a model system for the investigation of the correlated electronic-spin-nuclear degrees of freedom in transition metal oxides. By combining femtosecond broadband reflectivity and ultrafast time-resolved X-ray emission spectroscopy, we monitored the material’s photoresponse upon selective excitations of the ligand-to-metal and metal-to-metal charge transfer optical transitions. In the former case, sub-picosecond spin and electronic dynamics occurs together with a displacive excitation of coherent phonons, in the latter a slower electronic relaxation is measured in presence of impulsively stimulated Raman scattering phonons. We propose a possible explanation in terms of excitation-selective ultrafast intersystem crossing. We also present the preparation of parallel time-resolved X-ray emission and X-ray diffraction experiments, which will harness the structural- and spin-sensitivity of these techniques to disentangle the interactions determining the photodynamics of the system. In the last part of the thesis, we show a preliminary study aiming at extending the temperature-jump pump-probe method to the X-ray domain. In this experiment, a near-infrared pump is used to vibrationally excite the water molecules of the solvent, which undergo ultrafast relaxation causing a sudden increase of the temperature in the bulk of the solution. This process triggers a thermal chemical reaction of the dissolved solutes, which is monitored in a time-resolved fashion. We studied a multistep ligand substitution reaction of a hexacoordinated Cobalt ion complex in chlorinated water solution, showing the great sensitivity of the X-ray absorption technique to subtle structural changes. This work opens new perspectives for the investigation of thermally-driven reactions with element-selective and site-specific X-ray methods.

L. Longetti / M. Chergui (Dir.)  

Investigating molecular excitations with femtosecond time resolution is of pivotal importance to understand the out-of-equilibrium processes taking place in molecular systems upon light absorption. The photochemistry of solvated species is heavily determined by the early steps of the relaxation dynamics. In this thesis, ultrafast electronic spectroscopies are exploited to study relaxation dynamics of photoexcited molecules in solution by probing the transient electronic structure. Vacuum-ultraviolet (VUV) ultrashort monochromatized light pulses (HARMONIUM, 15-100 eV) are exploited for steady-state photoelectron spectroscopy (PES) and time-resolved PES in a pump-probe scheme. This is a powerful tool for investigating electronic structure and ultrafast processes of solvated molecules with high time (50fs) and energy (0.12eV) resolution. PES retrieves binding energies of valence and core electronic states with elemental specificity and without the constraints of selection rules, and is here applied to liquids and solutions by means of a liquid micro-jet. With the aim of extending this technique to a larger class of solutes, we demonstrate the capability to perform studies on volatile liquids such as organic solvents. PES spectra of gas phase and liquid phase aromatic compounds are presented and their valence orbitals are compared and identified. The solvation is discussed and the vertical ionization energies are reported. This retrieves important parameters for electrochemistry and opens new perspectives towards PES studies of molecules in organic solvents. Upon photoexcitation, molecules can undergo several relaxation processes such as nonradiative relaxation processes which include dissociation, internal conversion and intersystem crossing. Energy can also be released to the solvent environment by intermolecular processes such as vibrational energy transfer. All of these phenomena are reflected in a transient electronic configuration. PES can hence probe electronic and structural dynamics along the entire reaction coordinates. The ferric trisoxalate complex represents an ideal coordination compound showing an intriguing cascade of processes upon photoexcitation. We have directly observed the metal photoreduction due to an instantaneous intramolecular charge transfer. We then suggest a branching between photodissociation and a back-electron transfer channels, which completes the picture of the compoundâ s photochemistry. This experiment shows PES sensitivity to oxidation state changes, providing an interesting perspective for photoredox reaction studies in solution and for intra- and inter-molecular charge-transfer phenomena. Vibrational wave packets (WP) are interesting observables to map non-adiabatic dynamics, relaxation pathways and coupling to solvent of the excited molecule. We have investigated molecular iodine in ethanol, which we have studied first by optical transient absorption spectroscopy using a white light continuum probe. The electronic absorption spectrum of I2 in ethanol is strongly modified compared to the case of other organic solvents, due to a strong coupling of covalent and ionic states. We have mapped the vibrational WP dynamics, in particular its trajectory and damping along the I-I bond axis. This study is intended to prepare for an ultrafast PES experiment aimed at linking the response of inner shells to the WP launched in the valence states. Preliminary steady-state PES spectra are reported and discussed.

D. Kinschel / M. Chergui (Dir.)  

With the advent of X-ray free-electron lasers (XFELs) time-resolved X-ray spectroscopic techniques have advanced to the femtosecond regime. These are element selective techniques which offer unique insight into the electronic and chemical environment and dynamics of a sample. Specifically, X-ray emission spectroscopy probes the occupied density of states and is sensitive to the spin and local structure of the element of interest, whereas X-ray absorption spectroscopy is a tool for probing the unoccupied density of electronic states which makes it sensitive to the oxidation state, ligation and local structure around a specific atom. The tunable photon energy of the X-ray from XFELs allows to selectively probe the element of interest in the sample and additionally, the high intensity (1011 to 1012 photons per pulse) makes it possible to study dilute biological samples in physiological conditions. The sample studied in this work is myoglobin with nitric oxide as ligand, which has long been used as a model system to gain deeper understanding of the class of heme proteins. These proteins all have an iron porphyrine (heme) as an active center and play a crucial role in oxygen storage and transport in all mammals for example, amongst many other functions. In heme proteins, the change of the low-spin (LS) hexacoordinated heme (ground state) to the high spin (HS) pentacoordinated domed form (excited state) is promoted by a reversible light induced ligand detachment, representing the â transition stateâ that ultimately drives the respiratory function. Here we investigate Myoglobin-NO (MbNO) by employing femtosecond Fe Kα and Kβ non-resonant X-ray emission spectroscopy (XES) at an XFEL upon photolysis of the Fe-NO bond. We find that the photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2) deoxy-myoglobin (deoxyMb) heme occurs in ~800 fs, and it proceeds via an intermediate (S = 1) spin state. The XES results also show that upon NO recombination to deoxyMb, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ~30 ps. Thus, the en-tire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process. Femtosecond X-ray absorption near edge spectroscopy (XANES) experiments also performed at an XFEL show that NO dissociates in <75 fs and the intermediate (S = 1) spin state which has antibonding character is populated in ~110 fs. The XANES spectrum at short time delays (t=1 ps) shows a similarity to the steady state difference spectrum (deoxyMb minus MbNO) suggesting that at 1 ps the present species is very similar to deoxyMb in terms of electronic and local geometric structure. XAS time-traces at the pre- and rising-edge (7112, 7122.5 and 7127 eV) reveal the shortest pathway of geminate recombination which takes ~30 ps.

T. Palmieri / M. Chergui (Dir.)  

The strength of the electron-hole interaction in bulk semiconductors is not only determined by the dielectric environment, but also depends on the presence of other quasiparticles – free charge carriers or phonons – that populate the system. In the former case, a high density of charge carriers is expected to screen the Coulomb attractive force, eventually driving the transition from an insulating exciton gas to a conductive electron-hole plasma. In the case of lattice vibrations, the coupling between the exciton and the phonon field can give rise to significant modulations of the exciton amplitude and binding energy. Understanding how excitons react to these perturbations is of pivotal importance, as the strength of the Coulomb interaction affects the way light is absorbed and emitted, and determines how energy is converted and transported in several optoelectronic technologies. The aim of this thesis is to investigate the nonequilibrium exciton physics in highly-excited semiconductors. For this scope, we will combine different steady-state and ultrafast optical techniques, spectro-temporal analysis, and advanced theory calculations, with the aim of investigating the fundamental processes that lead to the renormalization of the excitonic states in hybrid lead-halide perovskites and titanium dioxide. In our experiments, we use an ultrashort laser pulse to excite the system above the fundamental energy gap; a broadband pulse then monitors the optical properties in the exciton spectral region at different time delays upon photoexcitation. The temporal evolution of the exciton parameters are retrieved via quantitative lineshape analysis, which is achieved by modelling the steady-state optical quantities and combining them with the time-resolved spectra. Thanks to this approach, we are able to elucidate the dynamics of exciton renormalization in hybrid perovskites and titanium dioxide in presence of elevated carrier densities and coherent strain pulses, and disentangle the many-body effects that lye at the origin of the observed optical nonlinearities.

L-H. Mewes / M. Chergui (Dir.)  

Unraveling the interplay between electronic- and vibrational degrees of freedom on the earliest time scales of physical, chemical and biological processes is crucial to gaining insight into the mechanisms that govern the world around us, since it is during these primary steps that the fate of the excitation energy – be it solar, chemical or physical – is decided. A tiny structural change during the first 10s of femtoseconds can for example predetermine the dynamics of a system over microseconds, and newly developed techniques in time-domain spectroscopy have proved to successfully capture exactly these pivotal mechanisms. In this thesis I use ultrafast spectroscopy to investigate the non-equilibrium dynamics of a prototypical di-platinum complex. Transient absorption spectroscopy is complemented with transient absorption anisotropy and fluorescence up-conversion to identify the spectral features of the intermediate excited states. The results show that the intersystem crossing from the singlet- to the triplet manifold of states happens on a 0.6 to 0.9 picosecond time scale, slower than previously assumed. As a consequence, the observable coherence is associated with a singlet excited state, rather than a triplet, and dephases prior to the intersystem crossing. The study further illustrates the importance of having a good understanding of the transient spectral signatures and yields insight into the structural dynamics of di-platinum complexes. In addition, I have built and commissioned a two-dimensional photon echo experiment that is capable of acquiring broadband excitation- and detection frequency resolved transient spectra in the visible with a temporal resolution of approx. 10 femtoseconds. All relevant steps of the development are outlined in this thesis and 2D spectra of four different dye solutions highlight the capabilities of the experiment. The temporal evolution of the 2D spectra of nile blue is analyzed as an example and shows the information content that is contained within the data.

T. C. H. Rossi / M. Chergui (Dir.)  

In modern ultrafast optoelectronic technologies based on wide band gap insulators, the non-equilibrium dynamics of photogenerated charges plays a major role. Unravelling the mechanisms of interaction between these charge carriers and their environment is crucial to support the design of more efficient devices. Especially, the absorption of photons with more energy than the optical band gap generates electrons-hole pairs (EHP) whose transport and dissociation as excitons or free charge carriers is of pivotal importance for the charge separa- tion at photovoltaic interfaces. The advent of ultrafast pump-probe optical spectroscopy in the deep-UV provides access to the dynamics of EHP through a variety of transient modifications of the optical spectrum. In this Thesis, the cooling of electrons in ZnO and methylammonium lead bromide perovskite (MAPbBr3), two broadly used direct band gap semiconductors, is tracked as they end up and accumulate at the bottom of the conduction band. ZnO has a fast cooling time of the order of 100 fs which efficiently converts photon excess energy into lattice heat while MAPbBr3 has a rather slow electron cooling time of the order of 400 fs, favorable for charge separation at interfaces as it drives the injection of hot carriers. Additionally, the high energy carried by the UV probe photons provides access to the photodynamics at different high symmetry points in the Brillouin zone. In a second step, these two semiconductors have been associated to other materials to form prototypical photovoltaic assemblies such as dye-sensitized (ZnO/N719) and solid-state lead perovskite sensitized (TiO2/MAPbBr3) interfaces. Upon electron injection into the transition metal oxide (ZnO or TiO2), dramatic changes are observed in the absorption close to the optical band gap from which the timescale of electron injection is obtained. It highlights how slight screening length and chemical potential changes can generate large modifications in the optical properties through many-body effects. At both interfaces, a delayed appearance of the electron at the bottom of the conduction band is observed which is due the formation of a bound state. This Thesis extends beyond the band insulators to the photodynamics of NiO, a strongly correlated charge-transfer insulator. Excitation above the optical band gap generates a large photoinduced absorption below the optical band gap which is characteristic of the interplay between electronic and low energy bosonic excitations. The dressing of electronic excitations with bosonic modes generates low energy excitation modes on ultrafast timescales which generate a competition between itinerant and localized resonances with a high degree of tunability. The progress made in resonant X-ray based time-resolved spectroscopies allows the investigation of the degree of charge localization around specific atoms in a system. Photogenerated electron localization in NiO is studied by time-resolved X-ray absorption spectroscopy, causing bond elongations and the formation of an electron-polaron in less than 100 ps. In a last part, the linear dichroism of the steady-state X-ray absorption spectrum of anatase TiO2 at the K-edge is studied which provides an unambiguous assignment of the pre-edge transitions as their orbital composition is strongly anisotropic and sensitive to the crystal orientation.

F. G. Santomauro / M. Chergui (Dir.)  

This thesis investigates the photoinduced charge carrier dynamics of TiO2 nanoparticles and CsPbX3 (X = Cl, Br) nanocrystals, by means of ultrafast x-ray absorption spectroscopy (XAS) and transient absorption spectroscopy (TAS). TiO2 is among the most promising transition metal oxides for applications such as photocatalysis and photovoltaics. In the latter, TiO2 has been used over the years in dye-sensitized solar cells (DSSCs) as electron transport material. CsPbX3 belongs to the class of lead halide perovskites and is currently one of the most investigated materials for solar energy applications, due to the high conversion efficiencies and ease of preparation. These materials are commonly used in solar cells with an all-solid-state DSSC architecture as light absorbers. Solar energy conversion is governed by the generation of charge carriers, their subsequent evolution as excitons or free charge carriers, and eventually their localization. Ultrafast spectroscopy can gain insights into the evolution of charge carriers by following their dynamics in real time. For this reason, ultrafast XAS was the main technique used in this work, as it combines elemental and structural sensitivity to study the fate of charge carriers and their evolution under operating conditions. The early stages of electron localization in TiO2 anatase nanoparticles upon photoexcitation at 3.5 eV, are investigated by fs-XAS at the Ti K-edge using the synchrotron slicing technique. The results show that localization of electrons at Ti atoms occurs in < 300â fs, forming Ti3+ centres, in or near the unit cell where the electron is excited. Moreover, electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. Similar conclusions are drawn for another polymorph of TiO2, rutile, from ps-XAS at the Ti K-edge. Here electrons are trapped next to oxygen vacancies at 100 ps after photoexcitation. Electrons in rutile though, show a weaker tendency to localization than in anatase and this could explain the differences in photocatalytic performances between these two polymorphs. In the second part of this thesis, the ultrafast charge carrier dynamics of CsPb(ClBr)3 nanocrystals is investigated using fs-TAS in the visible region (~1.8-3.1 eV) upon photoexcitation at 3.1 eV. This material represents an ideal system to study the ultrafast physics of lead halide perovskite in general, because ultrafast TA studies suggest that the charge carrier dynamics are similar to the organic-inorganic materials but do not suffer from the same stability issues. Here, the ultrafast transition from free charge carriers to excitons is observed in a fluence dependent study, which sheds light on the interpretation of a long lived spectral feature rising from a transient electroabsorption effect. Finally, the charge carrier dynamics of CsPbX3 (X = Cl, Br) is investigated using ps-XAS at the Br K-edge, the Pb L3-edge and Cs L2-edge upon photoexcitation at 3.5 eV. The Br K-edge transients at 100 ps delay show evidence for a full electron charge being withdrawn from the Br atoms, i.e. the hole is localized due to formation of a small polaron. The transients at the Pb L3-edge point to the opposite, i.e. the electrons are fully delocalized as conduction band electrons and there is no hint of trapping. Lastly, the Cs L2-edge shows no transient signal, in agreement with predictions based on the partial density of states in the material.

E. Baldini / M. Chergui; F. Carbone (Dir.)  

Revealing the emergence and the dynamics of collective excitations in complex matter is a subject of pivotal importance, as it provides insight into the strength and spatial distribution of interactions and correlations. At the same time, collectivity lies at the origin of several cooperative phenomena in many-body systems, which can lead to profound transformations, instabilities and, eventually, phase transitions. Mapping the interactions of the collective bosonic excitations with the fermionic particles and among themselves leads to the comprehension of the many-body problem. In this Thesis, we investigate the dynamics of collective excitations in strongly interacting and correlated systems by means of ultrafast broadband optical spectroscopy. Within this approach, a material is set out-of-equilibrium by an ultrashort laser pulse and the photoinduced changes in its optical properties are subsequently monitored with a delayed probe pulse, covering a broad spectral region in the visible or in the ultraviolet. Collective excitations can be unraveled either in the frequency domain as spectral features across the probed range or in the time domain as coherent modes triggered by the pump pulse. Studying the renormalization and temporal evolution of these collective excitations gives access to the hierarchy of low-energy phenomena occurring in the solid during its path towards the thermodynamic equilibrium. This framework is explored in a number of prototypical materials with an increasing degree of internal complexity beyond conventional band theory. Among the most remarkable results obtained in this work, we observe crosstalk phenomena between distinct electronic subsystems in MgB2, discover bound excitons coupled to the phonon bath in anatase TiO2, provide a selective and quantitative estimate of the electron-phonon coupling in La2CuO4, reveal precursor superconducting effects in NdBa2Cu3O(7-ÎŽ) and unravel a phonon-mediated mechanism behind the magnetic order melting in the multiferroic TbMnO3.

J. J. P. Ojeda Andara / M. Chergui (Dir.)  

This thesis focuses on the commissioning and characterization of HARMONIUM, an ultrafast tunable EUV source based on high-harmonic generation for performing photoelectron spectroscopy of liquids. In ultrafast photoelectron spectroscopy a short pump pulse induces an excited state in a molecular system while a time-delayed short pulse interrogates the electronic distribution of the transient species. This method is an important tool in photochemistry as it combines features like element specificity and surface sensitivity with the absence of forbidden transitions and the ability to retrieve absolute binding energies. Ultrafast photoelectron spectroscopy of liquid media became possible by combining efficient turbomolecular pumping with a liquid micro jet in vacuum from which collisionless evaporation takes place. In HARMONIUM, it is possible to choose between high energy (up to 0.16 eV) or high temporal (up to 40 fs) resolution modes by selecting one of the 4 different gratings in a pulse-preserving single-grating EUV monochromator. High photon flux ranging from $2.3\cdot10^{11}$ photons/s at 36 eV to $1.5\cdot10^{8}$ photons/s at 102 eV has been demonstrated. Tunability of the laser repetition rate allows distributing the laser power among the maximum number of pulses per second for a given EUV photon energy, thereby minimizing EUV-induced space charges without compromising the acquisition time of photoelectron spectra. An ellipsoidal mirror in a 4:1 configuration yields an EUV focal spot size similar to the $\sim$ 20 $\mu$m diameter of the liquid jet. Apart from its implementation on photoelectron spectroscopy of liquid samples, the versatility of this beamline makes it suitable to study different states of matter. To this end, HARMONIUM will be coupled to a molecular beam and an angle-resolved photoemission spectroscopy (ARPES) end stations. A photoelectron spectrum of a liquid sample is composed of electrons emitted from the liquid surface and those emitted from the surrounding evaporated molecules. In this thesis, a systematic approach for separating the gas contribution from the liquid+gas photoelectron spectrum is presented. This procedure opens the possibility to acquire photoelectron spectra of dilute aqueous media under conditions of moderate electrokinetic charging. In this regime, preliminary photoelectron measurements of protonated water in the absence of counterions suggest that changes in the water orbitals take place as a function of the hydrogen bond coordination. The laser assisted photoelectric effect in liquids is fully described for the first time. Convolution of its retrieved response function with the unpumped photoelectron spectrum reproduces the pumped photoelectron spectrum. This contribution is important as usually the laser assisted photoelectric effect masks photoinduced dynamics transients in time-resolved photoelectron spectra around the region of zero time delay. For the first time ultrafast photoelectron spectroscopy is used to temporally resolve photo induced dynamics in an aqueous metal complex. Impulsive photoreduction of the iron center followed by ultrafast oxidation in $\sim$ 550 fs was shown to take place upon excitation of the ligand-to-metal charge-transfer transition of aqueous ferricyanide solution.

G. Capano / M. Chergui; I. Tavernelli (Dir.)  

Cu(I)-phenanthrolines are an important class of metal-organic molecules that exhibits much promise for solar energy harvesting and solar-driven catalysis applications. Although many experimental studies have been performed calling for high-level simulations to elucidate their photophysics, a complete picture is still missing. This is the goal of the present thesis. On the ultrafast (femtosecond) timescale we studied the non-adiabatic relaxation of a prototypical Cu(I)-phenanthroline, [Cu(dmp)2]+, by performing excited state simulations using two approaches: quantum dynamics and trajectory surface hopping. These simulations help to identify several mechanisms, internal conversion, pseudo Jahn-Teller distortion, intersystem crossing, occurring in the subpicosecond time scale. Surprisingly, we have found that intersystem crossing does not take place between the lowest singlet and triplet excited states, as previously proposed, but between the lowest singlet and higher triplet states. Moreover, we observed the initial stages (< 100 fs) of the solvent reorganization due to the electronic density changes in the excited state. This leads to an energy stabilization of the excited states that is associated with an increase of the non-radiative decay rate. The quantum dynamics simulations allowed us to provide indications for performing additional spectroscopy measurements by using the recently developed X-ray Free Electron Lasers (X-FELs). This technology can monitor both electronic and structural changes with an unprecedented time resolution of tens of femtoseconds and, therefore, is capable of revealing the aforementioned processes. In addition, we questioned the feasibility of such experiments and calculated the signal strengths for XAS and XES transient spectra. Finally, we analyzed the luminescence quenching, which has been observed for all Cu(I)-phenanthroline complexes when they are dissolved in strongly donating solvents. By performing Molecular Dynamics calculations we showed that, in contrast with the previously accepted model based on the formation of an exciplex (a species formed by two molecules, one in the excited state and one in the ground state), no stable exciplex is formed and that quenching is due to electrostatic solute-solvent interactions. In addition, we investigated how the geometry configuration can affect the luminescence lifetime in these molecules. We found a correlation between rigidity of the copper complex - inhibition of the pseudo Jahn-Teller distortion - and lifetime of the emission. The more the metal complex retains the ground state structure (large substituents), the longer its lifetime. This effect is attributed to a higher energy gap (excited state minus ground state energy) due to the reduction of relaxation. Our research reveals important insights into the relaxation mechanism and the complex interplay between geometry and electronic structure in Cu(I)-phenanthroline. These results can be exploited for guiding the synthesis of complexes with the desired physical properties.

M. Silatani / M. Chergui (Dir.)  

A wide variety of physicochemical processes at a molecular level, in particular charge or energy transfer, electronic and vibrational relaxation, are at the origin of biological functionality of proteins and organometallic compounds. The work reported in this thesis is devoted to the study of these molecular dynamics in selected organometallic systems. In particular, it focuses on the role of these dynamics in the ultrafast photophysics and photochemistry of Myoglobin and iridium complexes having a transition metal active center of d6 configuration. The nature of ligand recombination to the active centre has a strong impact on the reactivity and activation of heme proteins. Using picosecond (ps) Fe K-edge X-ray absorption spectroscopy (XAS) we probed the NO-heme recombination with direct sensitivity to the Fe-NO binding in physiological solution. The transient XAS at 70 ps and 300 ps are identical, but they deviate from the difference between the static spectra of deoxyMb and nitrosyl myoglobin demonstrating the formation of an intermediate species, supposedly the 6-coordinated domed form that is populated on a time scale of ~200 ps and relaxes in ~30 ps. A broadband femtosecond (fs) luminescence study of Ir(ppy)3 (Ir1), Ir(ppy)2(pic) (Ir2), [Ir(ppy)2(bpy)]+ (Ir3) and Ir(ppz)3 (Ir4) provides the first extensive picture of their ligand dependence. After excitation to a ligand-centered state (266 nm), we directly clock the relaxation cascade of Ir1 leading to the lowest metal-to-ligand charge transfer (3MLCT) state. The cascade proceeds within â € 10 fs, being faster than some of the high-frequency modes of the system. In Ir4 the 3MLCT state decays (530 fs) by non-radiative channels. For Ir1-3, the early emission (<1ps) upon 400 nm excitation is dominated by an intermediate 3MLCT state on the ppy moiety. At t>100s ps the emission evolves to the steady-state showing evidence of dual emission for Ir1,2 due to a double-well minimum of the lowest 3MLCT state. For Ir3, the final emission stems from a ppy to bpy inter-ligand CT state. The studies of the electronic and geometric structure of Ir1-4 by Ir L3 XAS in solution reveals overall identical above-edge multiple scattering resonances and EXAFS indicating similar bond lengths and angles. Simulation of the XAS using TDDFT and FEFF9 shows good overall agreement with the experiment. The near-edge region of the spectra shows distinct differences among the studied complexes. Ir4 has stronger WL intensity than Ir1, indicating weaker Ï -back donation. The XAS spectra of Ir2,3 indicate larger 5d-electron density on the iridium. The XAS spectra of Ir1 and Ir4 show weak pre-edge resonance due to metal (2p3/2)-to-ligand charge transfer excitations. The absence of the pre-edge in Ir2,3 is due to the reduced covalency of the bonds. However, the existence of such transitions remains open for debate since Ir+3 has only occupied t2g orbitals in the low spin configuration. Using ps XAS, we have successfully captured the transient spectrum at the Ir L3-edge of both Ir1 and Ir4 probed 150 ps after laser excitation in solution. Upon excitation of a MLCT band (355 nm), we observed oxidation state change of Ir from +3 to +4. In DCM, the transient spectra are almost identical for both Ir1,4. The only significant changes in the transient spectra are those observed around the WL. The calculations reproduce the blue shift of the absorption spectrum due to the reduced electron density on the Ir, consistent with the MLCT excitation.

A. Al Haddad / M. Chergui (Dir.)  

The first section of the thesis discusses the development and implementation of a two dimensional Fourier transform spectroscopy setup, that permits the recording of electronic 2D spectra in the visible regime. A hollow core fiber was used to generate broadband pulses, compressed down to sub-15 fs, allowing high spectral and temporal resolution measurements. The design relies on basic optical components (no diffractive optics or liquid crystal display) with pair-wise manipulation of the beams, which lead to reduction of the signal modulation to the difference between the transition frequency and the laser frequency. The required timing precision as well as the mechanical stability is reduced due to the used geometry. For testing the integrity of the setup, stability measurements over several hours are presented as well as measurements on rhodamine 101 dye and Pentacene thin films. The photophysics of porphyrins has been widely investigated because of their importance in biology and their relevance in a wide range of applications such as light harvesting applications, molecular electronics, and gas sensors. The energy of the photon absorbed by these systems is converted into electronic and vibrational energy, eventually triggering photochemical processes such as charge transfer, energy transfer, and bond breaking. Deep understanding of the specific early relaxation mechanism and nuclear wavepacket motion occurring in this family of molecules is therefore fundamental for further comprehension of the role of porphyrins in nature and future application using porphyrin-based molecular devices. In the third chapter we discuss the impulsive heating of free-base porphyrins and the electronic and vibrational relaxation dynamics upon photo-excitation. Transient absorption and fluoresce up-conversion spectroscopy techniques with high temporal resolution were utilized to investigate the relaxation processes, and the heating/cooling dynamics. Wavepacket dynamics of free-base porphyrins are discussed in the fourth chapter, where the phase and spectral intensity of the vibrational modes were extracted using Fourier transform spectroscopy. Based on the mode phase and spectral dependent intensity each mode is assigned to being in-plane or out-of-plane mode and to whether it exists in the excited or ground state. In the last chapter, excited state dynamics of triply fused diporphyrins in toluene is investigated using transient absorption spectroscopy. These highly Ï -conjugated organic systems are receiving considerable attention because they incorporate ease of synthesis with tunable chemical structure that can be tailored for various applications, such as molecular wires, mimic light harvesting complexes, and saturable absorbers dyes. These porphyrin tapes can chelate different metal centers, changing their optical and electronic properties. In this chapter a comparison between free-base, Zn, and Pt based triply fused diporphyrins is done in order to understand the role of the metal in the relaxation dynamics of these systems. By further comparing these systems with earlier studies, the peripheral substituents effects are also discussed. In the end, we show that the high ISC yield, long triplet lifetime, and high cross-section of Pt-3DP makes it a perfect candidate to be used as NIR sensitizer for photochemical up-conversion via triplet-triplet annihilation.

R. Monni / M. Chergui (Dir.)  

The aim of this thesis is to investigate the ultrafast inter¿ and intra¿molecular processes, occurring in both metal¿containing molecular complexes and in proteins, by means of ultrafast Transient Absorption (TA) spectroscopy. The first part of the thesis focuses on understanding the origin of the solvent dependent Intersystem Crossing (ISC), from the lowest excited singlet state (1A2u) to the lowest excited triplet state (3A2u), in [Pt2 (P2O5H2)4]4¿ (Pt(pop)) and its perfluoroborated derivative [Pt2 (P2O5(BF2)2)4]4¿ (Pt(pop)¿BF2). Our UV pump¿Visible probe TA experiments (exciting both in the 1A2u state and in higher¿lying states) highlight the presence of an intermediate state, in agreement with the previous hypothesis of Milder and Brunschwig. The energy of the latter state is strongly modulated by both the solvent and the ligands present in the pop cage, causing the large difference in ISC timescales (upon excitation in the 1A2u state) between Pt(pop) in acetonitrile (MeCN) (< 1 ps), Pt(pop) in water (~13 ps) and Pt(pop)¿BF2 in MeCN (1.6 ns). Excitation at higher¿lying excited states populates both the 1A2u and the 3A2u state in different ratios, depending on the investigated system. Finally, we report on the presence of wave¿packets in the ground and excited states, as well as the coherence transfer from the initially excited state to the 3A2u state. The second part focuses on the photo¿excited tryptophan (*Trp) quenching by metal complexes in protein systems, such as ferrous Myoglobins (Mbs) and Prion Proteins (PrPs). Mb is a small globular protein composed by 153 amino acids and containing two Trp residues (positions 7 and 14). Trp7 is ~20 Å far away from the heme and its fluorescence is only quenched via Förster energy transfer (FRET), while Trp14 is closer (~15 Å) and it is quenched via both FRET and electron transfer to the heme. Our results demonstrate that *Trp14¿to¿heme electron transfer process occurs, surprisingly, also in ferrous Mbs (e.g. deoxy¿Mb and MbCO) and highlight the generation of a long¿lived product, namely a FeII¿porphyrin¿¿. We also report the interesting case of MbNO, in which the *Trp14 transfers the electron on the NO instead of the heme. Prion proteins are also globular proteins composed of 209 amino acids (in humans) and involved in metal binding. Even though a large amount of experiments and theoretical work have been performed on these proteins their main physiological role is not completely determined, but it is clear that the formation of a scrapie isoform (PrPSC) of the cellular PrPs (PrPC) is the infectious agent and functions as a template for the PrPC ¿ PrPSC conversion. Our UV pump¿Visible probe TA experiments aim to unravel the ultrafast processes occurring in the octarepeat region of PrPs and fill the gap between ultrashort and biological timescales. We find that both the minimum Cu binding sequence HGGGW (OPS) and the total octarepeat region (PHGGGWGQ)4 (OP4) display a non¿exponential quenching of the Trp residues, which was assigned to *Trp¿to¿nearby amino acids (H, G or Q) in different peptide¿s conformations. Upon Cu2+ complexation by OPS, a feature related to the formation of a Cu+ complex was detected. The same was not detected upon Cu2+ complexation by OP4, showing longer quenching timescales than the free peptide. Our results highlight the relationship between structural conformation of the peptide and quenching timescales of the *Trp.

M. H. Rittmann-Frank / M. Chergui (Dir.)  

M. E. Reinhard / M. Chergui (Dir.)  

A. Bouwens / T. Lasser; M. Chergui (Dir.)  

Fluorescence microscopy techniques are well established research tools and have proven their use in a large variety of biomedical applications. Microscopic molecular contrast is achieved by imaging fluorescent dyes that bind specifically to a molecule of interest and gene expression can be imaged by genetically modifying organisms to express fluorescent proteins. This versatile technique has, however, three limitations: fluorescent labels can cause toxicity or have an unwanted influence on the process under study; the photobleaching effect limits observation time; and the highest acquisition speed is restricted by the fluorophore’s brightness and by the raster scan required for 3-D imaging. For applications where direct molecular contrast is not essential, label-free imaging offers a solution to these issues and can simplify imaging experiments. In this work, we present methods to perform 3-D label-free imaging of the anatomy (structural imaging) and physiology (functional imaging) of living tissue and cells with optical coherence microscopy (OCM). OCM is founded on low-coherence interferometry and acquires 3-D images of the local light back-scattering strength from within highly scattering biological tissue. By measuring in the spectral or Fourier domain, image acquisition is multiplexed over depth and achieves high sensitivity. OCM can therefore offer an important gain in acquisition speed, under the condition that lateral resolution is maintained over the imaging depth. This is realized by extended focus OCM (xfOCM), which uses Bessel beam illumination to create a high lateral resolution over an extended depth of field. The combination of high 3-D resolution and fast acquisition is especially useful for in vivo experiments, where measurement time is limited, and when fast processes are studied. We demonstrate long-term in vivo imaging with xfOCM of amyloid plaque in a mouse model of Alzheimer’s disease (AD), without administration of extrinsic contrast agents. By implementing a 3-D image segmentation algorithm, we further show how xfOCM can be employed to perform a label-free ex vivo transversal study of the development of amyloid plaque in the mouse brain. The high acquisition speed obtained from multiplexing depth acquisition is further exploited to image cerebral blood flow. We first analyze the Doppler frequency spectrum measured by OCM when scatterers such as erythrocytes flow through the system’s focus and show how it relates to the axial and lateral flow velocity components. This then enables quantitative blood flow imaging with xfOCM, with in an unprecedented combination of 3-D resolution and acquisition speed. We apply this technique to label-free in vivo angiography and quantitative blood flow imaging in the murine brain. xfOCM achieves deep tissue penetration due to the use of a near-infrared spectrum. The study of cell cultures, however, does not require such deep penetration, but instead could benefit from higher resolution. For these applications, we introduce visible spectrum OCM (visOCM), enabling label-free and long-term imaging of living cell cultures with 3-D sub-micron resolution. visOCM allows cell morphology to be imaged in thin single layer cultures, but also in thicker three-dimensional cultures and organotypic slices, for which no label-free tomographic microscopy exists to date.

A. Odeh / M. Chergui (Dir.)  

Physically, the colour of an object is a complex result of its intrinsic surface properties, its transmission properties and its emission properties, all of which contribute to the mix of wavelengths in the light leaving the surface of the object. Structural colour is the production of colour by microscopically structured surfaces that disperse visible light. In nature, one of the best examples for structural colour are butterflies. Thanks to the periodicity found in the scales of their wings, iridescent colours arise not only because of pigmentation but also due to the interaction of light with such a photonic arrangement. Currently, there is a strong research orientation in the field of simulating such behaviour. Many attempts have been successful in replicating these nanoscale features and consequently, only the simplest and cheapest methods will prevail and eventually, be transferred to applications. The aim of this thesis is to demonstrate the control of the interaction of femtosecond laser pulses with matter for metal surfaces nanostructuring and colouring. An analysis of the specific physical mechanism of laser-matter interaction in the femtosecond regime is presented. The laser induced periodic surface structures (LIPSS) are identified along the generated surface textures. The analysis of the topography of the textured surfaces using different techniques (SEM, AFM) show the effectiveness of this method for generating multi-scale texturing. The different textures morphologies can be associated with different regimes of the laser-matter interaction (LIPSS, microstructures…). The evolution of periodic, self-assembly structuring is also investigated with the change in experimental parameters such as: laser polarisation and fluence, scanning speed of laser on the surface, number of laser passages and distance between adjacent scanning lines. By this investigation, we can control the structuring of aluminium surfaces and obtain iridescent or uniform colours (yellow, grey and black). Theses colours are reproducible under the same experimental parameters. Thus, new optical characteristics of structural colours are found and remarkable enhancement in the absorption of aluminium surface is obtained by nanostructuring (≈90% between 400-2000 nm). Moreover, a significant relationship between the total surface reflectance and laser-texture characteristics (morphology/topography and periodicity) is demonstrated. The chemical content analysis (EDX) of treated surfaces revealed an increase of aluminium oxide on the surface. An eventual role of defects in the oxide layer revealing the broadband absorption of the structured yellow surface of aluminium is suggested in a time-resolved transient reflectivity experiment. The similar chemical content of defects in all coloured surfaces leads us to attribute the major role to the structural colouring rather than to pigmentation. Thus, by changing the morphology of the surface we created yellow, grey and black colours. The femtosecond laser technology presents a powerful tool to study the temporal evolution of the relaxation dynamics of systems. We describe the construction and characteristics of a setup for recording of reflectivity/absorption spectra with a time resolution in the femtosecond domain. In addition, new experimental results on the exploration of high-temperature superconductivity are presented. These investigations are part of a larger project dealing with the femtosecond dynamics of Cooper pair condensates in superconductors.

C. Consani / M. Chergui (Dir.)  

Proteins are dynamic macromolecules, which accomplish biological function driven by long-range interactions and global motions. While their biological function is often clear, little is known about the collective interactions which make these systems real molecular devices. The last decades showed an increased interest in accessing the very early events of protein function and the weak couplings among different sub-units. This implies the capability to observe events which occurs on sub-ps time scales. Ultrafast spectroscopy is a powerful tool to investigate in real time electronic processes and vibrational dynamics in photo-excited systems. The last years showed an increased interest in the UV spectral region, to access spectroscopic features of molecules with electronic excitations located exclusively below 400 nm. Among them, are the high- spin states of metal complexes and the aromatic aminoacids in proteins. Especially, exploring the UV range below 300 nm opened the possibility to employ naturally occurring aminoacids as local probes of protein dynamics and intra-proteins correlations. This allowed to access the dynamical evolution of sites which were previously spectroscopically silent, or whose spectra were too congested, in wild type proteins. In this thesis work, we focus on the study of Fe-based metal complexes, and in partic- ular on wild type haem-proteins in native environment, and we address the questions of the relaxation of the prosthetic group and its interaction with the Trp residues. Despite intensive study, the details of the early haem photocycle in haem proteins is still under debate. With this thesis work, we show the successful combination of our UV- extended broadband transient absorption setup with time gated fluorescence in unraveling the early mechanisms of haem relaxation. In particular, for the first time we found evidence that the electronic relaxation in ferric met-myoglobin is specific for the system and that the widespread analogy among the electronic relaxation in all myoglobin forms should be reconsidered. Despite their capability to access the time scales of Trp de-excitation, these techniques are not the most suitable to investigate the mutual interaction among several residues, nor can they access the dynamics of environmental fluctuations. Two-dimensional spec- troscopies are instead the right tool to solve these issues. To this purpose, we designed an ultra-broadband transient absorption two-dimensional setup in the UV, with a band- width exceeding 60 nm in excitation and 80 nm in detection. The performance of the setup, which is described in details, was successfully demonstrated on UV dyes and bio- logical samples and it represents an excellent complement to Fourier-transform 2D setups, whose biological application in the UV is often compromised by the limited observation bandwidth.

F. Alves Lima / M. Chergui (Dir.)  

Over the last decade, ultrafast time-resolved X-ray absorption spectroscopy (XAS) has evolved to be now a mature and well-established experimental technique, giving extremely detailed information about the local geometrical and electronic structure in the early stages of a chemical reaction or biological process. Electronic structure changes are the fundamental driving forces triggering structural modifications in many chemical and biological reactions. Ultrafast time-resolved XAS is an ideal experimental technique to probe these changes in “real time” during the course of a chemical reaction, a biological function or a physical process. Ideally, the observation of ultrafast processes is made in conditions as close as possible as the natural ones, i.e., biological processes in physiological media (instead of crystals or frozen films). In this sense XAS offers unique capabilities, since it can be applied to any kind of system. In this thesis we successfully implemented a new scheme for measuring time-resolved XAS spectra with picosecond temporal resolution at MHz repetition rate. The increase in the data acquisition repetition rate provided an increase in the signal-to-noise (S/N) of a factor of > 20 compared to previous experiments in the kHz regime. To assess the improvements in this new data acquisition methodology, we used the light-induced spin transition in aqueous solutions of [Fe(bpy)3]2+ to benchmark our experiments. This system has been well-characterized by both ultrafast laser and x-ray time-resolved spectroscopies. The expected gain in data quality was confirmed, which allowed the recording of subtle changes in the pre-edge region of the XAS spectrum, reflecting the different electronic structure of [Fe(bpy)3]2+ upon the spin transition. We also present the investigation of the electronic and geometric structures of a series of metalloproteins (Myoglobin) in physiological solutions by means of X-ray absorption spectroscopy. To our knowledge, this is the first study of the structure of the different forms of Myoglobin in physiological media using XAS. The analysis of the XANES region of the spectrum using full-multiple scattering (FMS) formalism revealed that the ironnitrogen bond length in the porphyrins ring converged to a common value of about 2 Å, in contrast to the wide variation found in the crystallographic structures. Porphyrins are known to be very rigid structures due to the big number of carbon-carbon double bounds, supporting our conclusion. The relative geometry of the ligands with respect to the heme is reported for the whole series of Myoglobins investigated. In addition, time-resolved XAS has been measured for two types of Myoglobin, Carboxy-Myoglobin (MbCO) and (Nitrosyl-Myoglobin) MbNO. It has been shown that, as expected from previous studies, the transient structure of photo-excited MbCO resembles that of the deligated ferrous Myoglobin (deoxyMb). On the other hand, the transient structure of photo-excited MbNO at 50 ps deviates slightly from that of deoxyMb. The analysis of the transient XANES indicates that the NO molecule moves 2.88 Å away from the heme, staying closer to the iron atom than in the case of photo-excited MbCO. The NO geminate recombination time with the heme was also measured in “real time”, and it has been found to occur in 216 ± 24 ps. This is the first direct measurement, i.e. sensitive to structural changes, of this geminate recombination time.

O. C. Bräm / M. Chergui; A. Cannizzo (Dir.)  

A wide variety of physical and chemical processes at the molecular level, as charge or energy transfer, solvation, electronic as well as vibrational relaxation, is at the origin of the biological functionality of proteins. The work reported in this thesis is devoted to the study of these molecular dynamics in different chromophore systems. More specifically, this study is focused on the role of these dynamics in the ultrafast photophysics and photochemistry of haemoproteins. This important class of proteins has been widely studied through various spectroscopic techniques, in order to understand their functionality. These proteins contain several chromophore moieties, among which one aminoacid residue, Tryptophan, and the haem prosthetic group, which are of particular interest in this work. While Tryptophan is used as a probe of the local environment via intermolecular relaxation dynamics, we investigated the role of haem intramolecular dynamics on the protein functionality. While different kind of ultrafast spectroscopic techniques are indicated to tackle these issues, like pump probe or photon-echo peak shift experiments, we choose a different approach to probe directly the energetic relaxation, which consists in following the ultrafast changes in the photoluminescence spectrum. The dynamics are then extracted from the spectral evolution of the emission. With this purpose, we implemented a fluorescence up-conversion set-up, which allows us resolving temporally and spectrally the photoluminescence of the sample under investigation, excited with an ultrashort light pulse, and with a temporal resolution on the femtosecond timescale. The thesis is structured in 9 chapters. Chapter 1 resumes the theoretical background required for the interpretation of the experimental results. Chapter 2 describes the photophysical properties of the different systems investigated. Chapter 3 presents the experimental technique of fluorescence up-conversion, used to achieve broadband femtosecond detection of photoluminescence. In Chapter 4, we present a study on the role of ultrafast vibrational and structural dynamics on the time-resolved fluorescence spectra from two UV dyes. The measurements allow us exploring the capabilities of the set-up in revealing subtle relaxation dynamics with an unequaled accuracy. In Chapter 5, we move on to the study of the relaxation dynamics of Tryptophan (Trp) in water. This aminoacid served as an in vivo probe of solvation dynamics, owing to its high dipolar moment in its excited state. Our study focuses on water reorganization around the chromophore and on the distinction of this important process from the electronic internal conversion occurring on a similar timescale. In the framework of the study of protein dynamics, Chapter 6 reports the investigation of a family of molecules extensively found in nature, namely porphyrins. In these molecular systems, a quite complex pattern of intramolecular relaxation mechanisms occurs, which needed to be clarified. A systematic study on the free base and on a wide series of metallo-porphyrins definitely clarified the picture of their electronic relaxation pathways. We finally elaborate a comprehensive scheme of this relaxation, including substituent’s influence. The mechanisms observed are intrinsically related to the photophysical properties used by nature, as will be illustrated in the next Chapter. Trp and metallo-porphyrin constitute the key natural chromophore molecules for the non-invasive spectroscopic investigation of complex biological systems as haemoproteins. In Chapter 7, we follow relaxation dynamics of photo-excited Tryptophan and Haem (Fe-porphyrin) in ferro- and ferricytochrome c and in Myoglobin in its met form. In the former protein, an ultrafast energy transfer from Tryptophan to Heme is evidenced, the efficiency of which is found to depend on the oxidation state. The energetic relaxation pathway of the haem, also dependent on the oxidation state, is characterized by a porphyrin to metal charge transfer that triggers the ligand dissociation. In metMyoglobin, the fluorescence contribution of the two Tryptophans (W14 and W7), also quenched by resonant energy transfer to the haem, are temporally and spectrally separated, and the solvation dynamics of W7, located in the water-protein interface, is followed. The haem of metMb, which is in the ferric form, shows an electronic relaxation similar to that of ferricytochrome c. In parallel to the biology oriented investigations presented above, we extended our study to the metal-polypiridine complexes. Due to their specific photophysics, mainly characterized by a photo-induced Metal-to-Ligand Charge Transfer (MLCT), they are extensively used in photochemical applications involving charge transfer dynamics. In particular, they are used as sensitizer in the Dye-Sensitized-Solar-Cells (DSSCs). In Chapter 8, we present studies of these dyes, both in solution and adsorbed on a substrate, in order to understand the role of intra- and intermolecular dynamics on the electron injection process occurring upon absorption of light. Finally, Chapter 9 summarizes the conclusions of the investigations of the various systems studied.

A. Ajdarzadeh Oskouei / M. Chergui; A. Tortschanoff (Dir.)  

This thesis work concerns the development and applications of ultrafast optical techniques and in particular photon echo in the UV range (< 300 nm). Development of these techniques in the UV range is important for studying molecular dynamics in solution, since standard UV dyes are small molecules and are therefore, appropriate for theoretical and experimental characterization. The UV techniques are also important for the investigation of protein dynamics, since most proteins contain UV – absorbing amino acids. The processes we studied include electronic dephasing, non-polar solvation dynamics and tryptophan dynamics in water. The UV three-pulse photon echo set-up which we developed is presented in the first step of this thesis. Pulse compression is performed in two stages by the prism-based compression on visible and UV pulses. The sample is excited at 287 nm with 20 nJ UV pulses with a temporal width of about 50 fs. Frequency Resolved Optical Gating (FROG) technique is implemented to characterize the UV pulses. In the study of non-polar solvation dynamics, solvent dependent ps decays are detected in transient grating (TG) and photon echo peak shift (PEPS) experiments, which were attributed to solvation dynamics and rotational diffusion. Fluorescence up-conversion (UC) measurements did not show similar dynamics, which highlights the high sensitivity of TG and PEPS techniques with respect to the non-polar solvation. Sub-100 fs homogeneous dephasing times are measured for p-terphenyl (pTP), diphenylacetylene (DPA) and tryptophan in solutions. Homogeneous dephasing times can not be detected with conventional ultrafast optical methods, such as fluorescence UC and pump-probe techniques. Our results emphasize the role of intramolecular processes as the primary dynamics causing electronic dephasing in the case of non-polar solvation. Excited state dynamics of tryptophan in water is investigated by a combination of TG, UV pump – broadband UV probe and fluorescence UC techniques. It is suggested that a Stokes shift with characteristic times of τ1 ≅ 0.16 ps and τ2 ≅ 1 ps results in stronger coupling of the excited state population to a higher state, resulting in an increase of the TG and pump-probe signals. These measurements highlight the higher sensitivity of TG technique to detect molecular dynamics in solution compared to the pump-probe technique. At the next part of the thesis work, the dependence of the PEPS traces (depletion at short times and long time offset variation) on experimental parameters and, in particular, the pulse chirp was investigated. A model is presented which describes these effects in terms of phasematching and triangular beam geometry. It is concluded that a precise control on the experimental parameters and, in particular, on the chirp of the pulses is necessary, in order to obtain reliable dynamics on the UV PEPS traces. Finally, the experimental aspects of extending two-dimensional photon echo (2D-PE) experiments into the UV range are discussed. The new 2D-PE set-up, which we developed using a compact design is presented. This set-up is capable of supporting the necessary phase stability for UV 2D-PE experiments.

R. M. Van der Veen / M. Chergui (Dir.)  

In this thesis we followed the synergetic approach of combining ultrafast optical and X-ray spectroscopies to unravel the electronic and geometric structural dynamics of the solvated binuclear transition metal complex [Pt2(P2O5H2)4] 4- (PtPOP). This molecule belongs to a broader class of d8 – d8 compounds that are known for their interesting photophysical properties and rich photochemical and photocatalytic reactivity. Broadband femtosecond fluorescence up-conversion and transient absorption spectroscopy have revealed the ultrafast vibrational-electronic relaxation pathways following excitation into the 1A2u (σ*dz2 → σpz) excited state for different solvents and excitation wavelengths. Both sets of data exhibit clear signatures of vibrational cooling (∼2 ps) and wave packet oscillations of the Pt-Pt stretch vibration in the 1A2u state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing into the 3A2u state within 10-30 ps. The vibrational relaxation and intersystem crossing times exhibit a clear solvent dependence. We also extract from the transient absorption measurements the spectral distribution of the wave packet at given time delays, which reflects the distribution of Pt-Pt bond distances as a function of time, i.e. the structural dynamics of the system. We clearly establish the vibrational relaxation and coherence decay processes and we demonstrate that PtPOP represents a clear example of an harmonic oscillator that does not comply with the optical Bloch description due to very efficient coherence transfer between vibronic levels. We conclude that a direct Pt-solvent energy dissipation channel accounts for the vibrational cooling in the singlet state. Intersystem crossing from the 1A2u to the 3A2u state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the 1A2u excited state. The particular structure, energetics and symmetry of the molecule play a decisive role in determining the relatively slow rate of intersystem crossing, despite the large spin-orbit coupling strength of the Pt atoms. Ultrafast X-ray absorption spectroscopy (XAS) is a powerful tool to observe electronic and geometric structures of short-lived reaction intermediates. We have measured the photoinduced changes in the Pt LIII X-ray absorption spectrum of PtPOP with picosecondix nanosecond resolution. A rigorous analysis of the time-resolved EXAFS results allowed us to establish the structure of the lowest triplet 3A2u excited state. We found that the Pt atoms contract by as much as 0.31(5) Å due to the formation of a new intermetallic bond. In addition, a significant, though minute, elongation of 0.010(6) Å of the coordination bonds could be derived from the transient X-ray absorption spectrum for the first time. Using state-of-the-art theoretical XAS codes, we were also able to assign photoinduced changes in the XANES spectrum, to changes in Pt d-electron density, ligand field splitting and orbital hybridization in the excited state. Spectral changes in the XANES multiplescattering features were used to derive a semi-quantitative value for the Pt-Pt contraction of ∼0.3 Å, which is in excellent agreement with the time-resolved EXAFS results. Application of ultrafast XAS and the data analysis methods to other chemical and biological systems in liquids offers an exciting perspective; in particular, in view of the recent development of intense free electron laser sources delivering ∼100 fs X-ray pulses, opening new venues in X-ray science that scientists could hitherto only dream of.

A. El Nahhas / M. Chergui (Dir.)  

The photocycle of rhenium carbonyl complexes, type facial-[Re(L)(CO)3(diimine)]n [L=halide / n=0, L=4-Ethyl-pyridine (Etpy)- Imidazole (ImH) /n=+1, diimine= bpy , 10- phenanthroline (phen), 4,4′-dimethyl-2,2′-bpy (dmb)] was studied using a set of ultrafast spectroscopic techniques: Fluorescence up-conversion, transient absorption and X-ray absorption spectroscopy. Relaxation of the initially excited singlet charge transfer (CT) b1A’ state was investigated using fluorescence up-conversion (FlUC). The excited b1A’ state undergoes intersystem crossing (ISC) simultaneously to two triplet states (a3A” and b3A”) with a time constant τ1 in the 85-160 fs range that depends on the ligand L. An internal conversion process between the involved triplet states was found to occur with a solvent dependant lifetime τ2 of ∼500 fs in CH3CN and ∼1.4 ps in DMF. Femtosecond transient absorption measurements of the bpy-containing complexes revealed two processes. The first one, occurring in the same time range as the internal conversion measured using FlUC, also showed a solvent dependence. We attribute it to vibrational and electronic relaxation as well as solvation leading to population of the a3A” state. The second strongly solvent-dependant process, is manifested mainly by a continuous rise of the bpy- absorption band at 370 nm. We assign it to reorganization within a supramolecular cluster consisting of the [Re(L)(CO)3(bpy)]n chromophore and several strongly interacting local solvent molecules, probably intercalated between the ligands. In this series of complexes, theory predicts that the ligand (L) is important in tuning the character of the lowest excited state. For an electron-rich L ligand, the time-dependent density functional theory (TD-DFT) calculations characterize the lowest CT state as a metal-ligand-to-ligand-charge-transfer (MLLCT) state which leads to the reduction of the diimine ligand. To confirm this prediction and to extract structural information about the excited state, we used X-ray absorption spectroscopy. We first characterized the ground state by static measurements at the Re L3 and the Br K edges of the Re(Br)(CO)3bpy complex. Picosecond X-ray absorption measurements of the excited complex revealed a charge transfer from both the Re and the Br sites, confirming thus the mixed excited state character. These results constitute the first evidence about charge transfer from a monoatomic ligand containing Re complex. The excited state structure was also extracted by an analysis of the XANES and EXAFS spectra at the Re L3 edge. It involved contraction of the Re-N and Re-Br bond distances, and elongation of the Re-C distance, supporting thus the MLLCT character. The structural analysis confirms the prediction from the TD-DFT calculations.

V. T. Pham / M. Chergui; C. Bressler (Dir.)  

Solvation dynamics, the process of solvent reorganization upon electronic excitation of a solute, is central to our understanding chemical reactions in liquid phase. Ultrafast optical studies of solvation dynamics have so far been carried out on polyatomic molecules, which have internal degrees of freedom. This property does not allow the unambiguous extraction dynamics of the solvent shell. Because of their atomic character (i.e. lacking internal degrees of freedom) and of their solvent sensitive absorption bands (the so-called CTTS or charge-transfer-to-solvent bands), atomic halides represent ideal systems for the study of electronic solvation dynamics. Although these systems have received detailed attention in femtosecond optical studies, very little has been learned about the response of the caging solvent, which results from the fact that optical tools do not extract structural movement in a direct fashion. In this thesis, we combined ultrafast laser and structure-sensitive X-ray spectroscopies to probe in real-time the formation and decay of a nascent iodine atom created by photodetachment of a valence electron from the parent iodide anion. Optical pump-probe experiments are used to assess the photoproduct concentrations on time scales ranging from femtoseconds to nanoseconds. We also carried out detailed optical studies employing 1- and multiphoton detachment of the valence 5p electron from iodide, and confirmed an ultrafast thermal heating of the entire sample on a ps time scale. Static L1 and L3-edge X-ray absorption spectra of aqueous iodide have been recorded and analysed in terms of simulations based on classical, quantum mechanical molecular mechanics (QMMM) and density functional theory (DFT) molecular dynamics. QMMM yields the solvent shell structure that best fits the EXAFS spectrum. Picosecond X-ray absorption near edge structure (XANES) spectra were recorded from 50 ps up to several tens of nanoseconds. They were analyzed with respect to the different photoproducts observed on these time scales, delivering for the first time new spectra for the intermediate reaction products, I0 and I-2. By analyzing the transient extended x-ray absorption fine structure (EXAFS) data of the iodine atoms, we derived a dramatic expansion of ∼0.6 Å of the solvent shell with respect to that of iodide. Femtosecond XANES studies reveal an increase in the binding energies of the 5p and 2s, with respect to those at 50 ps. An increase in the 2s →5 p transition probability is consistent with the increase of the ionization energy of 2s electron of iodine atom at 300 fs, as compared to 50 ps. The 2s →5 p transition probability is found to decrease by ∼1.1 times from 300 fs to 50 ps.

J. Liu / M. Chergui (Dir.)  

This thesis presents the development of a scattering scanning near-field microscope (s-SNOM) and investigations on the photoluminescence study of solid C60. The first part concerns setting up of such a scattering SNOM where a tuning fork with an attached AFM tip is used as the force sensor and as a light-scattering probe. The light is coupled into the microscope and detected in a confocal arrangement. The characterization of the system and the force and optical scanning measurement of nanoparticles on glass substrates are presented and discussed. In order to know the tip vibration amplitude of quartz tuning-fork based sensors, we have developed a simple method, requiring only the measurement of a few mechanical properties of the fork (dimensions and Q factor), which can be easily obtained without changing the experimental setup in a matter of minutes. This method uses the (known) electrical energy absorbed by the system and the Q-factor to derive the elasto-mechanical energy stored in the tuning fork, and from this, the amplitude of motion through the elastic constants of the system. In order to improve the sensitivity for controlling the tip-sample distance, we have also made experimental and theoretical investigations on the performance of a different resonator, based on a tuning-fork + optical fiber mechanical scheme for use in shear-force mode. We have found that both the quality-factor and the spring constant play the main role in determining the behavior of such a force sensor. Based on this understanding, we are able to control both of these important parameters and, hence to optimize the force sensor and accurately model its response to an external force. The second part presents a time resolved local spectroscopic study of the photoluminescence of single C60 microcrystals performed with the optical microscope and with time-correlated photon counting techniques. C60 crystals prepared by the vapor sublimation method and the solution evaporation method are discussed. The photoluminescence of C60 crystals prepared by the vapor sublimation method is tentatively assigned to phosphorescence from the lowest two triplet states. The emission quantum efficiency and decay dynamics shows dependence on temperatures for C60 samples made by both methods, while the sample with a less ordered crystal structure made by the solution evaporation has a higher energy barrier for opening a non-radiative decay channel, pointing to quenching effects related to exciton migrations. This work helps clarify the origins of the C60 emitting states.

C. Bonati / M. Chergui (Dir.)  

This thesis presents an experimental study of the time-resolved optical response of three different nanoscale systems: CdSe and PbSe quantum dots, and silver triangular nanoplates. The first part of the thesis is devoted to the understanding of the effects of quantum confinement on carrier-carrier interaction in a “model” system: CdSe quantum dots. This issue is addressed by investigating the evolution of the early-time fluorescence spectra of quantum dots of different sizes and lattice structure. The experiment is performed using a femtosecond photoluminescence up-conversion technique, with polychromatic detection. The transient photoluminescence spectra reveal the emission from short-lived multiexciton states. By combining a detailed spectral and kinetic analysis, it is possible to: (i) evaluate the binding energies of these states and therefore acquire insight on the strength of multi-particle interactions, (ii) understand how these interactions affect the lifetime of multiexciton states and, (iii) infer their mechanisms of formation upon optical excitation. We find that confinement-enhanced Coulomb interaction between carriers leads to large binding energies (> 20 meV) and activates efficient Auger-type recombination. This last mechanism points to somewhat different carrier interactions with respect to bulk semiconductors. Surprisingly, we observe that “tailoring” the lattice structure of the quantum dot does not significantly affect the spectral and dynamic properties of multiexciton states. The second part of the thesis addresses the effects of quantum confinement in semiconductor nanocrystals from a slightly different point of view, by investigating PbSe quantum dots. This material is supposed to exhibit a mirror-like, sparse, energetic structure due to extreme quantum confinement which should profoundly alter the carrier relaxation dynamics. We analyze the inter- and intra-band relaxation by combining several techniques. In order to characterize the evolution of the particles luminescence from the nanosecond to the femtosecond range, we perform time-correlated single photon counting and femtosecond near-infrared photoluminescence up-conversion measurements. The results are compared with near infrared, broadband transient absorption measurements. Overall, we observe extremely fast intraband relaxation times, on sub-ps time scales, slightly increasing with decreasing dot size. From our analysis we can estimate a weak electron-phonon coupling between excited states, and we observe that surface mediated relaxation does not play a relevant role in this system. The third part of this work concerns the investigation of the time-resolved optical response of silver triangular nanoplates. The optical response provides fundamental information about the relaxation mechanisms of plasmons, electrons and phonons in metal nanocrystals, and access to the mechanical properties of metal nanoparticles. The anisotropy of the system under study is found to influence the physical properties: we observe for the first time two different excitation mechanisms of mechanical vibrations. In order to disentangle homogenous and inhomogeneous contributions, we present a model which takes into account a realistic distribution of particle size and shape, and which is able to capture the relevant dynamics in these complex systems.

A. Al Salman / M. Chergui (Dir.)  

This thesis presents an experimental study of the energy and time-resolved optical response of chemically prepared CdSe nanoparticles with different sizes, shapes (dots, rods, and tetrapods), and lattice structures (wurtzite and zinc blende). The first part of the thesis concerns a model system: spherical CdSe quantum dots with a wurtzite lattice structure. We have investigated their fluorescence spectra and decay kinetics as a function of size, laser excitation power, detection energy, and temperature. The experiments were performed using a nanosecond time-correlated single photon counting technique and have revealed kinetics from different short and long-lived states. In contrast to comparable literature studies, we covered a wide temporal window (up to 1 µs). The study shows at least four order of magnitude of signal to background ratio. Generally, at low excitation power, we always find four independent decay components covering a range of lifetimes between one nanosecond up to hundreds of nanoseconds. The first component is < 2 ns and probably corresponds to fast relaxation and trapping processes. A second component of 3-8 ns can be attributed to the lifetime of charged excitons, while the third one is in the range of 20-30 ns is due to the radiative electron-hole recombination. Finally, a longer decay time of 80-100 ns with low amplitude (< 10 %) appears for all sizes and experimental condition. This component is related to trap states. At high power (corresponding to more than one exciton per particle), an additional fast component, in the same range as the first one (< 2 ns), appears and is due to the multiexciton effect. The second part of the thesis concerns CdSe semiconductor nanocrystals from a slightly different point of view focusing on the crystal structure, by investigating quantum dots with a cubic lattice structure, synthesized in our group. We performed low- and high-resolution luminescence and excitation spectroscopy, and time-resolved spectroscopy as a function of size and temperature from room temperature down to 4K. These measurements reveal the optical properties and relaxation processes, and finally infer the amount of the structure-dependent field effect on the band edge exciton structure. Overall, we observe no difference in all these properties compared to wurtzite dots. We conclude that the crystal field is much less important than shape asymmetries and exchange interaction. The appearance of a permanent dipole moment, in a cubic lattice structure of spherical quantum dots, can explain the similar response of the two different lattice structure dots in our experiments. The third part of this work concerns the investigation of the shape effect on the spectroscopic and kinetic properties of CdSe nanocrystals. New fabrication methods have enabled the synthesis of high-quality CdSe nanorods and tetrapods. We investigated the temperature dependence of the absorption and fluorescence spectra of the different shapes of CdSe nanocrystals in the range of 4 to 300 K. We find that, while the shift of the fluorescence maximum indicates a little dependence on the shape, the broadening of the emission spectrum behaves very differently for dots and rods, indicating major differences in the broadening mechanisms for different shapes. Tetrapods behave more similar to dots, which suggest that the lowest exciton state is centered at the core. This is confirmed by the decay kinetics, which is again identical between dot and tetrapod nanocrystals, while an opposite temperature dependence decay was recorded for the kinetics of nanorods. We attribute this behaviour in tetrapods to the dot like centre. Nanorods show very different kinetics, because the lowest exciton state becomes allowed.

G. Zgrablic / M. Chergui; S. Haacke (Dir.)  

Photo-induced excited-state reactions stand in the center of function of the photosensitive biological systems. These reactions can be accompanied by structural changes of the chromophore (for example isomerization) which are influenced by the nearest environment of the chromophore, and vice versa. In bacteriorhodopsin (bR), the effects of the protein environment are crucial to assure high rate and outstanding bond selectivity of isomerization. In order to identify if these effects are of steric or electrostatic origin, we carried out an extensive femtosecond time resolved fluorescence study on the retinal chromophore of bR in a large class of solvents. The latter differ in viscosity, dielectric constant, polarizability and hydrogen bonding abilities. To carry out this study a novel experimental setup – the polychromatic fluorescence up-conversion, has been designed and constructed that allows broad band detection of the fluorescence spectra, with the time resolution of 100 fs. It is found that in all studied solvents here essential part of the ultrafast excited-state dynamics is dominated by intramolecular processes. Indeed, the relaxation times and the period of the vibrational coherences, which are the markers of the protein-solvent difference, do not show significant dependence with respect to viscosity and/or to dielectric constant of studied solvents. Additionally, we find that in solvents that are able to evacuate the excess energy from the Franck-Condon zone, the chromophore is less likely to take a non-reactive path (return to the initial state). Consequently, the photoisomerization efficiency gets enhanced. Nevertheless, this solvent-induced enhancement is far smaller than the enhancement induced by the photocatalytic effect of the protein binding pocket. These observations lead us to conclude that in fact the dynamics of isomerization in protein are essentially determined by steric effects.

E. Portuondo Campa / M. Chergui (Dir.)  

This thesis presents a comparative study of the ultrafast solvation dynamics of liquids in bulk conditions, and at the interfaces of metal-IV oxides (specifically TiO2 and ZrO2), using ultrafast spectroscopy techniques. In the first part of the thesis we report on results of photon-echo peak-shift and pump super-continuum probe spectroscopy, that provided complementary ways for characterizing the solvation function of a dye, Eosin-Y, in aqueous solution. The solvation dynamics of the dye was studied both, dissolved in an aqueous buffer, and adsorbed to the surface of ZrO2 nanoparticles that were in turn, suspended in an aqueous medium. The results from both techniques indicate that only minor changes between the bulk and interfacial environments, are manifested in the solvation dynamics of Eosin-Y on the fastest timescales of the process (sub-picosecond). On the other hand, on the longest timescales ( >1 ps), we obtained consistent evidences for a slower solvation dynamics at the interface than in bulk water. From these results we concluded, that the presence of the ZrO2 surface affects the dynamics of librational motions and intermolecular vibrations of the hydrogen-bond network of water, only in a very narrow region of no more than 0.5 nm around the metal oxide. The long time behavior, on the other hand, was explained as due to hindered translational diffusion dynamics of the solvent molecules in the proximity of the interface. In order to overcome the limitations inherent to using dyes as probe targets of the solvation dynamics at interfaces, in the second part of this thesis we performed time-resolved Optical Kerr Effect experiments on liquids in the pores of nano-structured films of ZrO2. Three liquids were investigated: acetonitrile (an aprotic polar solvent), cyclohexane (apolar) and water (polar, strongly H-bond networked solvent). In all the cases, significantly slower dynamics were detected in the pores, as compared to the bulk behavior of each solvent. The most significant case was that of water, where the characteristic time constants of the dynamics in the bulk, were increased by a factor of 3 in the film. The results are discussed considering the possible physical models that determine the dynamics of solvation at the interface. In addition to this, using the same experimental setup, we carried out a detailed characterization of the non-resonant nonlinear optical response of nano-structured films of TiO2, by means of Transient Lensing, Cross Phase Modulation measurements, and Optical Kerr Effect spectroscopy. This investigation led us to define future experimental developments, that will allow the extension of our present investigations of solvent dynamics at the surface of ZrO2, to the interfaces of TiO2, of special relevance in several applications.

W. Gawelda / M. Chergui; C. Bressler (Dir.)  

Electronic structure changes are at the origin of the making, breaking and transformation of bonds. These changes can be visualized by measuring the geometric structure in “real-time” during the course of a chemical reaction, a biological function or a physical process. Time-resolved X-ray Absorption Spectroscopy (XAS) delivers information about both the electronic (via XANES) and geometric (via EXAFS) transient structural changes, when interfaced with an ultrafast laser in a pump-probe scheme. Moreover, XAS offers unique flexibility, since it is both element-selective and it can be applied to any kind of disordered or ordered systems. In this thesis, we successfully investigated the excited state electronic and geometric structures of two different transition metal complexes. In both cases, for the first time, their excited state molecular geometries were characterized “on the fly”, without any a priori assumptions about its excited state structure. More importantly, it has been shown that time-resolved XAS is the only method capable of delivering the transient molecular structures of their short-lived excited states. First, we investigated ruthenium(II)-tris(2,2′-bipyridine), [RuII(bpy)3]2+. This molecule has served as a prototype and a model system of intramolecular electron and energy transfer reactions, due to its unique excited state properties. Our studies focused on the energy and structural relaxation process of the short-lived excited states of this molecule. By using the combined ultrafast laser and x-ray spectroscopies, we have determined various relaxation pathways of its excited states down to 15 fs lifetimes. The geometrical distortion of its lowest-lying excited state (3MLCT state) has also been determined by picosecond XAS, delivering a Ru-N bond contraction of ∼ -0.04 Å. Second, our study focused on iron(II)-tris(2,2′-bipyridine) [FeII(bpy)3]2+. This class of compounds is being extensively studied in relation to the phenomenon of spin crossover, where a spin transition takes place, involving the low-spin (LS) ground state and the high-spin (HS) excited state. Here, we have characterized its excited states by means of both ultrafast optical and x-ray spectroscopy. The optical studies have revealed several new aspects concerning the relaxation pathways of its charge transfer and ligand-field states, including their corresponding lifetimes. The structural analysis has determined the geometric distortions taking place in the lowest-lying excited HS state of [FeII(bpy)3]2+.The extracted Fe-N bond elongation of 0.2 Å agrees well with previously predicted values and it is for the first time that the room-temperature solvated structure of the HS short-lived excited state of a ferrous transition metal is obtained.

L. Bonacina / M. Chergui (Dir.)  

This thesis reports on results of three different experiments of photo-induced structural dynamics in the condensed phase, investigated by time-resolved pump-probe spectroscopy with femtosecond time-resolution. In the first part, we address the ultrafast dynamics of a quantum solid : crystalline hydrogen. This is accomplished by optical excitation of a dopant molecule, Nitric Oxide (NO), to a large orbital Rydberg state, which leads to a bubble-like expansion of the species surrounding the impurity. The dynamics is directly inferred from the time-resolved data, and compared with the results of molecular dynamics simulations. We report the presence of three time-scales in the structural relaxation mechanism: the first 200 fs are associated with the ultrafast inertial expansion of the first shell of lattice neighbors of NO. During the successive 0.6 ps, as the interactions between the molecules of the first and of the successive shells increase, we observe a progressive slowing-down of the bubble expansion. The third timescale (~ 10 ps) is interpreted as a slow structural re-organization around the impurity center. No differences were observed between the dynamics of normal- and para-hydrogen crystals, justifying the simplified model we use to interpret the data, which ignores all internal degrees of freedom of the host molecules. The molecular dynamics simulations reproduce fairly well the static and dynamic features of the experiment. In line with the measurements, they indicate that the quantum nature of the host medium plays no role in the initial ultrafast expansion of the bubble. In the second part, we present the results of our study on the photo-physics of triangular-shaped silver nanoparticles upon intraband excitation of the conduction electrons. The picosecond dynamics is dominated by periodic shifts of the surface plasmon resonance, associated with the size oscillations of the particles, triggered by impulsive lattice heating by the laser pulse. The oscillation period compares very well with the lowest totally symmetric vibrational frequency of a triangular-plate, which we calculated improving an existing elastodynamic model. We propose an explanation for the unusual phase behavior of the oscillations, based upon the non-spherical shape, and size-inhomogeneity of the sample. Taking into account these effects, we are able to reproduce spectrally and temporally our data. In the last part, we present a comparative study of the ligand dynamics in heme proteins. We studied the photo-induced spectroscopic changes in the ferric CN complexes of Myoglobin and Hemoglobin I upon photo-excitation of the porphyrin ring to a low-lying electronic state (Soret), monitoring the UV-visible region of the Soret band, and the mid-infrared region of the fundamental C=N vibrational stretch. The transient response in the UV-visible spectral region does not depend on the heme pocket environment, and is very similar to that known for ferrous proteins. The infrared data on the MbC=N stretch vibration provides a direct measure for the return of population to the ligated electronic (and vibrational) ground state with a 3 ps time constant. In addition, the CN stretch frequency is sensitive to the excitation of low frequency heme modes, and yields independent information about vibrational cooling, which occurs on the same timescale. The similarity between ferrous and ferric hemes rules out the charge transfer processes commonly invoked to explain the ligand dissociation in the former.

M. Saes / M. Chergui (Dir.)  

The photocycle of aqueous ruthenium-(trisbipyridine) [Ru(bpy)3]2+ was studied under high laser excitation intensities and high sample concentrations with picosecond resolved x-ray absorption spectroscopy. In a pump-probe scheme a femtosecond laser pulse promotes a 4d electron from the ruthenium to the ligand orbitals, thus creating a metal-to-ligand-charge-transfer (MLCT) complex. A hard x-ray pulse from a synchrotron source probes the ruthenium L3 and L2 edges, monitoring the electronic and molecular structure of the ruthenium over the photocycle. The measured x-ray absorption spectrum of the MLCT state is in good agreement with the predictions of a theoretical calculation (TT-multiplet software). We extract from the spectrum that the excited-state complex can be described by D3 symmetry and has a 4d5 configuration. The decay kinetics of the MLCT state are found to be strongly dependent on the sample concentration, especially for solutions near the solubility limit of [Ru(bpy)3]Cl2 in water. Besides ground-state quenching and triplet-triplet annihilation a third fast decay component quenches the life-time of the MLCT state, tentatively attributed to a cluster effect. This study is the first application of sub-nanosecond time-resolved x-ray absorption spectroscopy on solvated systems and demonstrates its capability as a new tool for the observation of chemical dynamics in solvated systems.