Statistical mechanics of sum frequency generation spectroscopy for the liquid–vapor interface of dilute aqueous salt solutions

0009-2614/$ see front matter 2009 Elsevier B.V. A doi:10.1016/j.cplett.2009.01.028 * Corresponding author. Address: Department of C fornia, Berkeley, CA 94720, United States. E-mail address: [email protected] (P.L. Geissle We demonstrate a theoretical description of vibrational sum frequency generation (SFG) at the boundary of aqueous electrolyte solutions. This approach identifies and exploits a simple relationship between SFG lineshapes and the statistics of molecular orientation and electric field. Our computer simulations indicate that orientational averages governing SFG susceptibility do not manifest ion-specific shifts in local electric field, but instead, ion-induced polarization of subsurface layers. Counterbalancing effects are obtained for monovalent anions and cations at the same depth. Ions held at different depths induce an imbalanced polarization, suggesting that ion-specific effects can arise from weak, long-ranged influence on solvent organization. 2009 Elsevier B.V. All rights reserved.