Density dependent vibrational relaxation in supercritical fluids

The vibrational lifetimes of the asymmetric CO stretching mode (1990 cm−1) of W(CO)6 in the gas phase and as a function of density in supercritical ethane are examined using infrared pump–probe experiments. The gas phase measurement provides the zero density rate constant, which permits the effect of the ethane solvent to be separated from internal relaxation dynamics. The non-exponential gas phase vibrational dynamics are briefly discussed. The density dependence of the lifetime is compared to an extension of a hydrodynamic theory of vibrational relaxation in supercritical fluids to the regime of large wavevector. Agreement between experiment and theory is found to be good.