Reorientation-Induced Spectral Diffusion in Vibrational Sum-Frequency-Generation Spectroscopy

There is a growing appreciation that dynamic processes play an important role in determining the line shape in surface-selective, nonlinear spectroscopies such as vibrational sum-frequency-generation (VSFG). Here we analyze the influence that reorientation can have on VSFG spectra when the vibrational transition frequency is a function of orientation. Under these circumstances, reorientation-induced spectral diffusion (RISD) causes the underlying spectral line shape to become time dependent. Unlike previously reported mechanisms through which reorientation can contribute to the VSFG signal, RISD influences the line shape regardless of the degree of polarization of the Raman transition that is probed. We assess the impact of RISD on VSFG spectra using a model system of liquid acetonitrile at a silica interface. Comparison of delay-time-dependent VSFG spectra with simulations that employ static line shapes suggests that RISD contributes substantially to the spectra, particularly at delay times that are comparable to or greater than the probe pulse duration. The observed behavior is in qualitative agreement with a two-state RISD model that uses orientational distributions determined from previous molecular dynamics simulations.