A comparison of the low-frequency vibrational spectra of liquids obtained through infrared and Raman spectroscopies

Dynamic solvation of charge-distribution rearrangements is often described using a (harmonic) solvent coordinate. It is not a priori clear whether such a solvent coordinate has a real physical meaning. We have studied five polar organic liquids (benzonitrile, benzyl alcohol, $N,N$-dimethylformamide, ethylene glycol, and glycerol triacetate) with high-resolution high signal-to-noise ultrafast optical heterodyne-detected Raman-induced optical Kerr effect spectroscopy (OHD-RIKES). The data, converted to the frequency domain, were analyzed entirely with a multimode Brownian-oscillator model. The infrared spectra of the same five liquids were obtained with a combination of terahertz spectroscopy and Fourier-transform infrared spectroscopy. The Brownian-oscillator fits to the OHD-RIKES spectra could be converted successfully to IR spectra by using a simple theoretical model and by keeping all Brownian-oscillator parameters the same except for the amplitudes. This suggests that there is a small set of harmonic oscillators describing ultrafast solvent nuclear dynamics that can be used to understand solvation, IR absorption, and Raman scattering spectra.