Velocity relaxation of S(1D) by rare gases measured by Doppler spectroscopy

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Abstract
Velocity relaxation of S(1 D) by He, Ar, and Xe has been monitored by measuring the Doppler profile of the S(1 D) for variable collision partner pressures at a fixed time delay following creation of S(1 D) by pulsed laserphotolysis of OCS at 222 nm. The nascent S(1 D) has a mean speed about three times that at room temperature and an effective anisotropy parameter of β=0.5. A calculation assuming elastic hard‐sphere collisions is performed to model the process. The data are in qualitative agreement with the model in that the angular distribution relaxes more rapidly with collision number as the mass ratio between the collision partner and sulfur approaches infinity, whereas the speed distribution relaxes more rapidly as the mass ratio approaches one. Helium behaves as predicted by the hard‐sphere model with a collision cross section of σcol=26±2 Å2. However, the cross sections for argon and xenon are found to depend on the collision energy. The dependence allows an estimation of the following Lennard‐Jones parameters: for argon σ=3.6±0.5 Å and ε=2.5±0.5 kJ/mol; for xenon σ=3.9±0.5 Å and ε=3.9±0.8 kJ/mol.