Statistical mechanics of sum frequency generation spectroscopy for the liquid-vapor interface of dilute aqueous salt solutions
Chem. Phys. Lett. , 470 (2009), p. 21-27
We have employed a simplified theoretical description of sum frequency generation (SFG) to examine the vibrational spectroscopy of aqueous liquid-vapor interfaces in the presence of dissolved ions. This approach makes transparent the relationship between SFG lineshapes and the statistics of fluctuations in molecular orientation and electric field. Using molecular dynamics simulations, we have found that halide anions near an interface on average maintain nearly centrosymmetric first coordination shells. As a result the orientational averages that determine SFG susceptibility do not manifest ion-specific shifts in local electric field, which constitute the predominant influence of ions in bulk vibrational spectroscopy. These orientational averages instead reflect an ion-induced polarization of subsurface layers that is independent of anion identity. Precisely counterbalancing effects are obtained for a cation at the same depth. A pair of oppositely charged ions at different depths, however, 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 ranged influence on solvent organization, reflecting spontaneous charge layering at the interface.
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