The low frequency density of states and vibrational population dynamics of polyatomic molecules in liquids

Abstract

Instantaneous normal mode calculations of the low frequency solvent modes of carbon tetrachloride (CCl4) and chloroform (CHCl3), and experiments on the vibrational population dynamics of the T1u CO stretching mode (∼1980 cm−1) of tungsten hexacarbonyl in CCl4 and CHCl3 are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump–probe experiments measuring the vibrational lifetime of the T1u mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl4, the vibrational lifetime decreases as the temperature is increased; however, in CHCl3, the vibrational lifetime actually becomes longer as the temperature is increased. The change in thermal occupation numbers of the modes in the solute/solvent systems cannot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the anharmonic coupling matrix elements are considered. The calculated differences in the temperature dependences of the densities of states appear too small to account for the observed difference in trends of the temperature dependent lifetimes. This suggests that the temperature dependence of the liquid density causes significant changes in the magnitude of the anharmonic coupling matrix elements responsible for vibrational relaxation.