VIBRATIONAL LIFETIMES AND VIBRATIONAL LINE POSITIONS IN POLYATOMIC SUPERCRITICAL FLUIDS NEAR THE CRITICAL-POINT

Picosecond infrared pump-probe experiments are used to measure the vib rational lifetime of the asymmetric (T-1u) CO stretching mode of W(CO) (6) in supercritical CO2, C2H6, and CHF3 as a function of solvent dens ity and temperature. As the density is increased at constant temperatu re from low,,gaslike densities, the lifetimes become shorter. However, in all three solvents, it is found that within a few degrees of the c ritical temperature (T-r=T/T-c approximate to 1.01), the lifetimes are essentially constant over a wide range of densities around the critic al value (rho(c)). When the density is increased well past rho(c), the lifetimes shorten further. At higher temperature (T-r=1.06) this regi on of constant vibrational lifetime is absent. Infrared absorption spe ctra of W(CO)(6) and Rh(CO)(2)acac in supercritical CO2, C2H6, and CHF 3 acquired for the same isotherms show that the vibrational spectral p eak shifts follow similar trends with density. The peak positions shif t to lower energy as the density is increased. Near the critical point , the peak positions are density independent, and then redshift furthe r at densities well above rho(c). It is shown that critical fluctuatio ns play a dominant role in the observed effects. Theoretical calculati ons ascribe the density independence of the observables to the cancell ation of various rapidly changing quantities near the critical point. The theory's calculation of density independence implicitly involves a verages over all local densities and does not involve any form of solu te-solvent clustering. (C) 1997 American Institute of Physics.