Vibrational energy transfer studies in gas, liquid and supercritical rare gases

The applicability of gas-phase energy transfer concepts to relaxation in the liquid phase is examined. The technique of laser-induced fluorescence was employed to vibrationally excite HF, NO and HCl in gas and liquid-phase mixtures with rare gases. The vibrational energy relaxation rates of each system were followed as a function of rare-gas density and the results are examined for correspondence with the predictions of the Isolated Binary Collision model.; Both the HF({dollar}upsilon{dollar} = 1) and NO({dollar}upsilon{dollar} = 1) studies resulted in liquid phase relaxation rates smaller than the rates given by linear extrapolations of the gas-phase results. The same rates also were a factor of 2 smaller than the rates given by the corresponding predictions of the Attractive Hard Sphere model. Attempts to improve on the agreements between the experimentally observed and the IBC-predicted rates were introduced; soft-cored interaction potentials, rather than the AHS hard-sphere potentials were employed but led to no definite conclusions as the ratio of the radial distribution functions required by the IBC model could not be obtained with accuracy.; The study of the vibrational relaxation of HCl({dollar}upsilon{dollar} = 1) by Xe was performed over a very wide range of densities in the gas phase, as well as in both supercritical fluid and the liquid phase. The supercritical signals were identical to the liquid-phase signals obtained at the same densities, thus confirming that vibrational relaxation rates only depend on density and not on phase. The gas-phase results were not linearly dependent on density. Instead, the relaxation rates increased faster than linearly at high densities. The reason for the exhibited behavior involves the role of van der Waals complexes in vibrational relaxation processes. A model was formulated which shows that at sufficiently high densities, the contributions to the relaxation rate due to the collisional relaxation of the parent molecule and the vibrational predissociation of the van der Waals complex are saturated, and only the collisional relaxation of the van der Waals complexes is linearly dependent on density.