INTERROGATION OF VIBRATIONAL STRUCTURE AND LINE BROADENING OF LIQUID WATER BY RAMAN-INDUCED KERR-EFFECT MEASUREMENTS WITHIN THE MULTIMODE BROWNIAN OSCILLATOR MODEL

The method used to deduce the spectral density distribution of intermo lecular and intramolecular (vibrational) degrees of freedom in the liq uid state from optical heterodyne detected optical (Raman-induced) Ker r effect (OHD-RIKE) measurements is reexamined within a multimode Brow nian oscillator model, The ramifications of nonlinear coupling of the nuclear degrees of freedom to the medium polarizability are explored f or discrimination between ''homogeneous'' and ''inhomogeneous'' contri butions to the vibrational spectral density, Under physically reasonab le assumptions, an estimation of the homogeneous contribution to the v ibrational line shape can be made from the OHD-RIKE observable (if non linear coupling is nonnegligible). The model is developed generally, a nd calculations are applied specifically to temperature-dependent OHD- RIKE measurements of liquid water. The results indicate that the line broadening in the low-frequency vibrational distribution due to the hy drogen-bonded network structure of liquid water is mostly inhomogeneou s, with an effective homogeneous relaxation time of 350 fs at 24 degre es C.