TEMPERATURE-DEPENDENT VIBRATIONAL LIFETIMES IN SUPERCRITICAL FLUIDS NEAR THE CRITICAL-POINT

Vibrational relaxation measurements on the CO asymmetric stretching mo de (similar to 1980 cm(-1)) of tungsten hexacarbonyl (W(CO)(6)) as a f unction of temperature at constant density in several supercritical so lvents in the vicinity of the critical point are presented. In supercr itical ethane, at the critical density, there is a region above the cr itical temperature (Tc) in which the lifetime increases with increasin g temperature. When the temperature is raised sufficiently (similar to T-c + 70 degrees C), the lifetime decreases with further increase in temperature. A recent hydrodynamic/thermodynamic theory of vibrational relaxation in supercritical fluids reproduces this behavior semiquant itatively. The temperature dependent data for fixed densities somewhat above and below the critical density is in better agreement with the theory. In fluoroform solvent at the critical density, the vibrational lifetime also initially increases with increasing temperature. Howeve r, in supercritical CO2 at the critical density, the temperature depen dent vibrational lifetime decreases approximately linearly with temper ature beginning almost immediately above T-c. The theory does not repr oduce this behavior. A comparison between the absolute lifetimes in th e three solvents and the temperature trends is made. (C) 1998 American Institute of Physics. [S0021-9606(98)51938-1].