NORMAL MODE THEORY OF 2-STEP RELAXATION IN LIQUIDS - POLARIZABILITY DYNAMICS IN CS2

An instantaneous normal mode (INM) theory is given for relaxation in l iquids by a fast beta process followed by a slow alpha process. The be ta process is harmonic dynamics in the wells of the N-body potential, while the alpha process is structural relaxation coincident with barri er crossing to a neighbor well. The theory introduces a new parameter, the ''harmonic fraction'' denoted F-H, which is the fraction of the m ean-square fluctuations of a dynamical variable capable of being relax ed by the harmonic beta process. Theory and computer simulation are co mpared for the polarizability correlation function, PC(t), and the pol arizability time derivative correlation function, DPC(t), in a model o f CS2 including internal degrees of freedom. Agreement is good, with t he INM theory clearly showing the ''signature'' time dependence of a c orrelation function undergoing alpha beta relaxation in a low temperat ure liquid; there are no adjustable parameters in the theory. The pola rizability is calculated in the ''point atomic polarizability approxim ation'' (PAPA) which is sensitive to molecular vibrations, so a prelim inary classical INM treatment of Raman scattering is obtained. The PAP A overestimates the derivative of the polarizability with respect to t he internal coordinates, and in reality the vibrations behave quantum mechanically, so the Raman intensities are inaccurate, but otherwise a plausible description is obtained for several features of the spectru m. It is explained how an improved PAPA will be combined with a quantu m INM theory in future Raman calculations. (C) 1996 American Institute of Physics.