Density and temperature effects on vibrational phase relaxation and frequency shifts in liquid isobutylene

Abstract

The Raman line shapes of the ν4 (A1) symmetric C=CH2 stretching mode at 1657 cm−1 and the ν9 (A1) symmetric C–CH3 stretching mode at 805 cm−1 in isobutylene have been measured as a function of pressure from vapor pressure densities to 0.8 g cm−3 over the temperature range from −25 °C to 75 °C. The frequency shifts of the above isotropic bands have also been determined under the same experimental conditions. The important experimental finding is the opposite effect of density on the isotropic linewidths of the C–CH3 and C=CH2 stretching vibrations; namely, the former band broadens with increasing density whereas the latter band narrows with increasing density under isothermal conditions. Such narrowing is unexpected in view of the theoretical and experimental results on the mechanism of vibrational broadening. A very different behavior is also observed for the frequency shifts of these two stretching modes. There is a large red gas-to-liquid shift for the C=CH2 band while its frequency changes very little with density over the liquid range studied. The opposite is true for the C–CH3 band which has a small gas-to-liquid shift but exhibits a large blue shift with increasing density in the liquid region. A qualitative interpretation of the experimental frequency data is based on consideration of classical vibrational amplitudes of the two vibrations and the infrared activity of the C=CH2 mode. The experimental vibrational widths are discussed in terms of available theoretical models for dephasing and other mechanisms for broadening of isotropic Raman line shapes. It appears that the behavior of the C–CH3 band can be best interpreted considering the isolated binary collision model for dephasing and additional energy relaxation, whereas the unusual density effects on the linewidth of the C=CH2 band are discussed in a more general way using the Kubo stochastic line shape theory.