A femtosecond snapshot of crystalline order in molecular liquids

Abstract

A theoretical model of ultrafast dynamics in the optical Kerr effect in molecular liquids is developed. It assumes that for short times there exist dynamic quasicrystalline structures including a central molecule and its nearest neighbors. The interaction of such structures with a femtosecond laser pulse leads to excitation of vibrational modes (local phonons) which are responsible for subpicosecond nonlinear polarizability of the liquid. The results of numerical calculations performed for benzene show a very good agreement with the experimental data. The lifetime of the dynamic quasicrystalline structures in benzene, at room temperature, is estimated to be about 200 fs.

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

“…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.…”

“…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,…”

“…The mean normal coordinate ͗Q ␣ ͑t͒͘ k and mean square normal coordinate ͉͗Q ␣ ͑t͉͒ 2 ͘ for each H-bonded tetrahedral structure can be found as the solutions of the following dynamic equations, obtained from Hamilton equations: 40,30 …”

“…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 …”

“…Optical Kerr effect ͑OKE͒ spectroscopy is a powerful tool for investigating the intermolecular dynamics of the short-living molecular structures. [4][5][6]9,[26][27][28][29][30][31] Herewith we are using this technique to study the dynamics of water molecules imprisoned within the pores of the gelatin gel. Gelatin is the protein obtained from collagen by the denaturing process.…”