OPTICAL KERR EFFECT SPECTROSCOPY OF SIMPLE LIQUIDS

Abstract

Optical Kerr effect (OKE) spectroscopy has become a prominent nonlinear optical technique for studying liquids, which allows for the direct, time-resolved probing of collective orientational diffusion as well as Raman-active intermolecular and intramolecular modes.

The temperature-dependent orientational dynamics of 1, n-dicyano n-alkane liquids ranging from dicyanomethane to 1,8-dicyanooctane has been investigated by ultrafast OKE spectroscopy. The dependence of the reorientational times on temperature and viscosity is consistent with the molecules adopting a largely extended structure in the liquid state, with a preference for gauche conformations at the methylenes bonded to the cyanide groups. The data are also suggestive of temperature-dependent, collective structural rearrangements in these liquids.

Ultrafast OKE spectroscopy has also been used to study the intermolecular dynamics of aromatic liquids. A model that links the differences in the OKE spectra to corresponding differences in the local ordering of the liquids has been proposed previously based on the temperature-dependent OKE study of five aromatic liquids. The spectra of some other aromatic liquids such as pyridine, pyridine-d5, 2,4,6-trifluoropyridine and 1,3,5-tris(trifluoromethyl) benzene has been obtained to test this model, and the relative importance of molecular shape and electrostatic forces in determining the form of the OKE reduced spectral density for such liquids has been realized. It has recently been shown that liquid tetrahydrofuran (THF) has an unusual structure that features voids of significant dimension. Such voids should affect other observable properties of this liquid. Temperature-dependent, optical Kerr effect spectra for THF and a number of related liquids (furan, cyclopentane, tetrahydropyran, cyclohexane, diethyl ether, hexamethylphosphoramide and n-pentane) has been obtained to test whether the shape of the spectra can be used to reveal the presence of sizeable voids in liquids. Liquid under tension is another interesting system to study: A method based on Berthelot tube technique has been developed to hold benzene and acetonitrile under tension successfully.

A new scheme for measuring different tensor elements of the OKE response is developed. A dual-ring, polarization dependent Sagnac interferometer is used to create two co-propagating probe pulses that arrive at the sample at different times but that reach the detector simultaneously and collinearly. The tensor element of the response that is measured is determined by the polarization of the pump pulse. By controlling the relative timing of the probe pulses it is also possible to perform optical subtraction of two different tensor elements of the response at two different times, a strategy that can be used to enhance or suppress particular contributions to the OKE response.