Nonlinear Spectroscopy

Nanodroplet/particle platform

We have developed a nanoparticle/droplet platform to study aqueous interfaces, taking into account the possibility to measure simultaneously multiple length scales, charge and surface structure. In addition to having the possibility of performing a simultaneous charge and structure measurement, the use of a nanoparticle or droplet dispersion prepared by mixing small volumes of chemicals significantly reduces the challenges posed by impurities that are inherently present in every chemical.

Femtosecond time and Angstrom interfacial length scale information can be obtained with vibrational sum frequency scattering. Information on a larger (~nm) length scale can be obtained with non-resonant second harmonic scattering.

Sum Frequency Scattering

 

Left: Energy scheme for sum frequency generation. Middle: Schematic sketch of a sum frequency scattering geometry. An IR femtosecond and visible femtosecond or picosecond pulse are overlapped in a sample containing a dispersion of particles or droplets in a liquid or solid phase. From the interface a SF is scattered according to a particular anglular distribution (indicated by the formula). Right: the optical setup.

 

We developed vibrational sum frequency scattering spectroscopy through several smaller steps:

  • We demonstrated the experiment and improved the optical layout by using particles in infrared transparent non-aqueous liquids [Proc. Nat. Acad. Sci 2006, 103, 13310-13314], and more brightly emitting crystallites in solids [Phys. Rev. Lett. 2009, 102 095502, J. Phys. Chem. B (2013),117, 8906].
  • By designing a high power sum frequency spectroscopy system with an extended infrared frequency range from 4000 cm-1 down to 500 cm-1 (2.6 – 20 microns) we can probe ionic head groups of amphiphiles at interfaces [App. Phys. B. 2008, 91, 315 PDF, and 3D structures of surface macromolecules, J. Phys. Chem. C (2008), 112, 7531].

 

Plasma from a focus of the high power amplifier needed to generate broadly tunable femtosecond infrared pulses.

  • Theoretical models were developed to interpret the scattering data (see page: Theory).
  • We characterized all optical parameters for performing nonlinear light scattering experiments in aqueous solutions [Chem. Phys. Lett. (2011), 512, 76,Phys. Chem. Chem. Phys. (2012), 14, 6826]. Dynamic nonlinear light scattering was also developed [J. Chem. Phys. (2009), 130, 214710-1], which has the potential to probe the water contained within the hydrodynamic radius of a droplet (nm length scale). The detection limit of our instrument is 1 molecule per 27 nm2 of droplet surface of ~ 100 nm particles [Angew. Chem. Int. Ed. (2012), 51, 12938].

Second Harmonic Scattering

 

Sketch of the second harmonic process and scattering instrument

To obtain information about the molecular directionality of water molecules in the interfacial region (on a nanometer length scale), we have designed a second harmonic scattering instrument and implemented a new optical layout, light source and detection method [Opt. Express, 21, 815, 2013], which allows us to probe interfacial changes on the millisecond time scale; a significant improvement over existing methods.

Theory

Second-order nonlinear optical methods are inherently sensitive to molecular (a)symmetry. A centrosymmetric arrangement of molecules cannot emit second harmonic or sum frequency photons. Thus, vibrational sum frequency scattering and elastic non-resonant second harmonic scattering can be used to probe interfacial chemical composition and structures, charge distributions, and short lived or short ranged (nm – micron) sized inhomogeneities. The light-matter interaction processes that determine how effects of molecular composition, orientation, chirality, dynamics and charge appear in, and can be extracted from nonlinear light scattering data have been unraveled during the past years in our lab [Phys. Chem. Chem. Phys. (2012), 14, 6826 ,Annu Rev. Phys. Chem (2012), 63, 353]. We have developed an exact nonlinear light scattering (Mie) theory [Phys, Rev. B. 2009, 79, 155420-1-9, PDF] as well as a number of approximations:
 
We investigated the effect of chirality [Phys. Rev. B 2007, 75, 245438-1-8, PDF] and clustering [J. Chem. Phys. 2009, 130, 214710, PDF], the separation of charge and structural effects [Phys. Rev. B, 2010, 82, 235431, PDF], and the effect of anisotropic particle shape [J. Opt. Soc. Am. B 2011, 28, 1374, PDF] on nonlinear light scattering. We further developed a method to extract molecular orientation with respect to the surface normal [J. Chem. Phys 2010, 132, 234702, PDF], and developed a model to probe bulk organization [Phys. Rev. Lett. (2009), 102, 095502-1,J. Phys. Chem. B (2013),117, 8906].
In addition, to understand the light-matter interactions we have also developed several methods to extract molecular interferences (and thus e.g. local orientation, a sub-nm effect) from spectral intensity data, by employing maximum entropy methods [J. Chem. Phys 135, 224701 (2011), PDF] and complex Fourier filtering [J. Phys. Chem. C  (2013), 117, 26582].

 

Calculated scattering pattern for a 200 nm particle containing both chiral (blue) and non-chiral (red) chemical groups.

Simulation of nonlinear light scattering patterns from water droplets in air using the Rayleigh Gans Debye approximation (top) and exact Mie theory (bottom).