THE LOW-FREQUENCY DENSITY-OF-STATES AND VIBRATIONAL POPULATION-DYNAMICS OF POLYATOMIC-MOLECULES IN LIQUIDS

Instantaneous normal mode calculations of the low frequency solvent mo des of carbon tetrachloride (CCl4) and chloroform (CHCl3), and experim ents on the vibrational population dynamics of the T-1u CO stretching mode (similar to 1980 cm(-1)) of tungsten hexacarbonyl in CCl4 and CHC l3 are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump-probe experimen ts measuring the vibrational lifetime of the T-1u mode from the meltin g points to the boiling points of the two solvents show a dramatic sol vent dependence. In CCl4, the vibrational Lifetime decreases as the te mperature is increased; however, in CHCl3, the vibrational lifetime ac tually becomes longer as-the temperature is increased, The change in t hermal occupation numbers of the modes in the solute/solvent systems c annot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the an harmonic coupling matrix elements are considered. The calculated diffe rences in the temperature dependences of the densities of states appea r too small to account for the observed difference in trends of the te mperature dependent lifetimes. This suggests that the temperature depe ndence of the liquid density causes significant changes in the magnitu de of the anharmonic coupling matrix elements responsible for vibratio nal relaxation. (C) 1995 American Institute of Physics.