Temperature evolution of the low-frequency optical Kerr effect spectra of liquid benzene in quasicrystalline approach

Abstract

Theoretical calculations of low-frequency spectra, due to intermolecular collective modes in liquid benzene, are presented. The molecules in a liquid are assumed to be organized in instantaneous, quasicrystalline, short-range structures of the lifetime of order of 10−13 s, which perform rotational and translational vibrations. Those vibrations are responsible for the intermolecular part of the femtosecond optical Kerr effect response. The spectra, obtained for five different temperatures, show a good agreement with the experimental results of other authors. We prove that the low-frequency feature of the spectrum, which is evidently augmented at low temperatures, is associated with collective translational intermolecular vibrations.

Ultrafast optical Kerr effect spectroscopy of water confined in nanopores of the gelatin gel

“…34,35 In our model the fast part of the signal is due to the coherent vibrational response of the local H-bonded structures, whereas the slow, "diffusionlike" part of the signal results from, both, the rotational diffusion of single molecules and the overdamped translational vibrations of H-bonded molecular clusters. 39,40 We have found the lifetime of the coherence to be very short, of the order of 120 fs, which is in agreement with the recent reports of Cowan et al 41 and Fecko et al 42 The long-time relaxation term in the diffusionlike response, determined in our model, is of the order of the longer diffusion lifetime found by other authors. 36,29,43,44 The interpretation of this term as due to single molecule rotational relaxation is in agreement with the arguments given by Winkler et al 44 Although the process of a single molecule rotational diffusion in water involves the breakage and formation of hydrogen bonds, it can be well described by the classical StokesEinstein-Debye theory of the molecular reorientation in viscous liquids.…”

“…47,48 It has been shown by Winkler et al, 47 that despite the complicated nature of the relaxation process, it can be well described by the Stokes-Einstein-Debye theory of single molecule rotating in a viscous liquid. Thus, the total nuclear part of the nonlinear polarizability of the system consists of two parts, 40 …”

“…The averaging is performed according to the Maxwell-Boltzmann distribution with the unperturbed Hamiltonian. The third order medium polarizability consists of two separable parts: the first one which is due to the electronic hyperpolarizability of molecules, ijkl ͑3͒ , and the second nuclear one, P ij nuclear ͑t͒, which is connected with the internal and external vibrations of the whole molecules 40,30 and includes the dependence on the nuclear coordinates,…”

“…40,30 We assume that the clusters are randomly oriented with respect to the electric field of the incident light wave, and, for simplicity, they perform the same normal vibrations.…”

“…Thus, the increased variety of those structures in the gel, in comparison to that in bulk water, leads to the enlarged inhomogeneous broadening of the spectral line ⌬ ␣ corresponding to a given normal vibration. The polarizability of a cluster is expressed as the sum of effective polarizabilities of molecules, modified by induced dipole-induced dipole interaction with their surrounding, 40,30 …”