No abstract

Theoretical approximations to the interface specific sum frequency generation (SFG) spectrum of O–H stretching at the water/vapor interface are constructed using time correlation function (TCF) and instantaneous normal mode (INM) methods. Both approaches lead to a (SSP polarization geometry) signal in excellent agreement with experimental measurements; the SFG spectrum of the entire water spectrum, both intermolecular and intramolecular, is reported. The observation that the INM spectrum is in agreement with the TCF result implies that motional narrowing effects play no role in the interfacial line shapes, in contrast to the O–H stretching dynamics in the bulk that leads to a narrowed line shape. This implies that (SSP) SFG spectroscopy is a probe of structure with dynamics not represented in the signal. The INM approach permits the elucidation of the molecular basis for the observed signal, and the motions responsible for the SFG line shape are well approximated as local O–H stretching modes. The complexity of the broad structured SFG signal is due to O–H stretching motions facing toward the bulk or vacuum environments that are characteristic of the interface. The success of both approaches suggests that theory can play a crucial role in interpreting SFG spectroscopy at more complex interfaces. It is also found that many-body polarization effects account for most of the observed signal intensity.

An improved time correlation function description of sum frequency generation (SFG) spectroscopy was applied to theoretically describe the water/vapor interface. The resulting spectra compare favorably in shape and relative magnitude to extant experimental results in the O-H stretching region of water. Further, the SFG spectra show a well-defined intermolecular mode at 875 cm(-1) that has significant intensity. The resonance is due to a wagging mode localized on a single water molecule. It represents a well-defined population of water molecules at the interface that, along with the free O-H modes, represent the dominant interfacial species.

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An improved time correlation function (TCF) description of sum frequency generation (SFG) spectroscopy was developed and applied to theoretically describing the spectroscopy of the ambient water/vapor interface. A more general TCF expression than was published previously is presented-it is valid over the entire vibrational spectrum for both the real and imaginary parts of the signal. Computationally, earlier time correlation function approaches were limited to short correlation times that made signal processing challenging. Here, this limitation is overcome, and well-averaged spectra are presented for the three independent polarization conditions that are possible for electronically nonresonant SFG. The theoretical spectra compare quite favorably in shape and relative magnitude to extant experimental results in the O-H stretching region of water for all polarization geometries. The methodological improvements also allow the calculation of intermolecular SFG spectra. While the intermolecular spectrum of bulk water shows relatively little structure, the interfacial spectra (for polarizations that are sensitive to dipole derivatives normal to the interface--SSP and PPP) show a well-defined intermolecular mode at 875 cm(-1) that is comparable in intensity to the rest of the intermolecular structure, and has an intensity that is approximately one-sixth of the magnitude of the intense free O-H stretching peak. Using instantaneous normal mode methods, the resonance is shown to be due to a wagging mode localized on a single water molecule, almost parallel to the interface, with two hydrogens displaced normal to the interface, and the oxygen anchored in the interface. We have also uncovered the origin of another intermolecular mode at 95 cm(-1) for the SSP and PPP spectra, and at 220 cm(-1) for the SPS spectra. These resonances are due to hindered translations perpendicular to the interface for the SSP and PPP spectra, and translations parallel to the interface for the SPS spectra. Further, by examining the real and imaginary parts of the SFG signal, several resonances are shown to be due to a single spectroscopic species while the "donor" O-H region is shown to consist of three distinct species-consistent with an earlier experimental analysis.