The Saykally Group

Experimental and Theoretical Physical Chemistry

Nonlinear Optical Spectroscopy Studies of Liquid Interfaces

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P. Jungwirth, D. J. Tobias, J. Phys. Chem. B 105, 10468 (2001)

Interfacial chemistry can be found in myriad environments of scientific significance including biological membranes, ocean and atmospheric chemistry, and electrochemistry. The interface represents a unique coordination environment with properties distinct from those of the bulk. In the image charge picture, derived from early surface tension measurements, ions have traditionally been considered to be depleted from the liquid/vapor interface. However, recent works have contradicted this simple picture, finding some ions exhibit enhanced surface concentration, with their respective surface affinities often following the well-known Hofmeister series. Of particular importance is that of the air-water interface as it is involved in many atmospheric reactions. However, it is difficult to probe the interface because the interfacial signal tends to be overwhelmed by the bulk signal, and thus not readily discernible. Fortunately, with the advent of ultrafast laser spectroscopy, we are now able to employ second order nonlinear optical techniques, Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), to probe ions and molecules at aqueous interfaces. Under the electric dipole approximation, due to necessary symmetry constraints, these techniques are interface specific and thus give no signal from bulk.

1. Jungwirth, P. & Tobias, D. J. Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B 105, 10468–10472 (2001).

2. Yamaguchi, S. & Tahara, T. Precise electronic χ(2) spectra of molecules adsorbed at an interface measured by multiplex sum frequency generation. J. Phys. Chem. B 108, 19079–19082 (2004).

3. Rizzuto, A. M., Irgen-Gioro, S., Eftekhari-Bafrooei, A. & Saykally, R. J. Broadband Deep UV Spectra of Interfacial Aqueous Iodide. J. Phys. Chem. Lett. 7, 3882–3885 (2016).

4. Petersen, P. B. & Saykally, R. J. Probing the interfacial structure of aqueous electrolytes with femtosecond second harmonic generation spectroscopy. J. Phys. Chem. B 110, 14060–14073 (2006).

5. McCaffrey, D. L. et al. Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water. PNAS 114, 13369–13373 (2017).

6. Mizuno, H., Rizzuto, A. M., Saykally, R. J. "Charge-Transfer-to-Solvent Spectrum of Thiocyanate at the Air/Water Interface Measured by Broadband Deep Ultraviolet Electronic Sum Frequency Generation Spectroscopy" J. Phys. Chem. Lett., 9, 4753-4757 (2018).