The use of van der Waals molecules to detect a mode selective nonradiative process in chromyl chloride

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Abstract
A free jet expansion was used to form van der Waals complexes of helium and chromyl chloride of the type CrO2Cl2–He(x), x = 1,2,... The vibronic bands of both the complex and the corresponding parent molecule have been excited using a tunable cw dye laser, and the fluorescence excitation spectra were recorded. The lifetimes of the complexes were measured and were found to be in the range 11–121 ps. The complexes undergo vibrational predissociation after excitation. For the van der Waals molecules we have evidence of competition between the vibrational predissociation and a nonradiative process in the CrO2Cl2 portion of the complex. The fluorescence intensity of a vibronic band of the complex relative to the corresponding band of the uncomplexed parent molecule shows that there is a mode selective nonradiative process in the 1B1 excited state of CrO2Cl2. The cross section for this nonradiative process increases with increasing excitation of the ν4 mode, the CrCl2 bend. A kinetic scheme is presented to interpret these observations, and the utility of this effect to measure vibrational predissociation rates in polyatomic molecule–rare gas van der Waals complexes is discussed.