Effect of molecular motion on surface vibrational spectra

Summary form only given. Surface vibrational spectroscopy via infrared-visible sum-frequency generation (SFG) has been developed into a powerful tool to study structures of surfaces and interfaces. With various input/output polarization combinations, the SFG vibrational spectra can yield detailed information about orientational distributions of selected atomic groups at a surface or interface. However, in all quantitative SFG studies reported so far, the dynamic nature of molecules have been ignored. In some cases, this practice has resulted in difficulty to understand the observed spectra. In this paper we show that the effect of rotational or librational motion of molecules on their vibrational spectra can be significant.