Non-Markovian nature of the fluctuation of the polarizability in molecular liquids by time-domain and frequency-domain measurements

We measured the low‐frequency regions of several molecular liquids at various temperatures using time‐domain and frequency‐domain spectroscopy to investigate the origin of the relaxation mode. We employed high‐resolution light scattering spectroscopy in the frequency domain and femtosecond optical Kerr effect spectroscopy in the time domain. By comparing the light scattering spectra of a relaxation mode with a Lorentzian function fitted to the spectra, we found that the spectra only follow the Lorentzian curve within a limited wavenumber range around the peak, which is comparable with the half width at half maximum of each Lorentzian curve. We evaluated the spectral range and compared it with the half width of the Lorentzian curve for several liquid samples at various temperatures together with the data obtained by the time‐domain method. We found that they were surprisingly correlated with each other regardless of the sample and temperature used, which indicates that the relaxation mode systematically deviates from the Lorentzian function. This suggests that the dynamics governing the relaxation mode is clearly non‐Markovian. Copyright © 2013 John Wiley & Sons, Ltd. It is well known that spectra of molecular liquids in a low‐wavenumber region show Lorentzian‐like shape. However, we have found that the spectra are Lorentzian like in a limited wavenumber region, which is comparable with the half width ωd of the spectra. This is observed over three orders of magnitude of ωd ranging from ~0.005 to ~2 cm−1.