Toward a simple molecular understanding of sum frequency generation at air-water interfaces
J. Phys. Chem. B , 113 (2009), p. 4065-4074
Second-order vibrational spectroscopies successfully isolate signals from interfaces, but they report on intermolecular structure in a complicated and indirect way. Here we adapt a perspective on vibrational response developed for bulk spectroscopies to explore the microscopic fluctuations to which sum frequency generation (SFG), a popular surface-specific measurement, is most sensitive. We focus exclusively on inhomogeneous broadening of spectral susceptibilities for OH stretching of HOD as a dilute solute in D2O. Exploiting a simple connection between vibrational frequency shifts and an electric field variable, we identify several functions of molecular orientation whose averages govern SFG. The frequency-dependence of these quantities is well captured by a pair of averages, involving alignment of OH and OD bonds with the surface normal at corresponding values of the electric field. The approximate form we obtain for SFG susceptibility highlights a dramatic sensitivity to the way a simulated liquid slab is partitioned for calculating second-order response. For electrolyte solutions we find that (non-polarizable) halide anions near an interface maintain symmetric coordination shells, negating the ion-specific spectral shifts of adjacent protons observed in bulk solution. Changes in essential orientational averages instead reflect a polarization of subsurface layers that is independent of ion size. Precisely counterbalancing effects are obtained for a cation at the same depth. A pair of oppositely charged ions at different depths induce an imbalanced polarization, modifying orientational averages in ways consistent both with experiment and with more elaborate calculations. These results suggest that ion-specific effects on SFG, in contrast to bulk vibrational spectroscopy, can arise from weak, long-range influence on solvent organization, perhaps reflecting spontaneous charge layering at the interface.
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