Vibrational relaxation in liquids

Abstract

A new theory is presented for vibrational dephasing in a molecular liquid. It is shown that the vibrationally excited molecule relaxes through interaction with the collective orientation density fluctuations in the liquid. By making suitable approximations the vibrational dephasing time and the infra-red absorption lineshape are expressed in terms of the intermolecular potential and the Van Hove correlation function of the liquid F(k, t). In this way measurements of F(k, t) made, for example, by neutron scattering or molecular dynamics can be used to calculate vibrational dephasing times. It is shown that the low-frequency part of the infra-red spectrum within about 10 cm^-1 of the band centre corresponds to diffusional motions in the liquid, on a distance scale appropriate to the first peak of the fluid structure factor S(k). Both self and exchange relaxation processes are described and the spectral distortions arising from slow molecular motions are evaluated. It is shown that unless the spectrum corresponds to extreme motional narrowing it is not possible generally to factorize the reorientational and vibrational contributions to the relaxation.