Velocity-averaging effects on polarisation measurements in hot atom reactions

The effects of thermal velocity averaging in polarised 'hot atom' chemical dynamics experiments have been analysed. Linearly polarised photolysis of a molecular precursor is assumed to produce anisotropically recoiling fragments which collide with target molecules. Analytical expressions are deduced which describe the degradation of the degree of alignment of the collision velocity by the isotropic motions of the precursor and target molecules. The efficiencies of different routes to the solution are discussed. Expressions are given for the overall alignment moments averaged over the distribution of collision speeds, both for the number density of collision pairs and the rate of collisions. The results are illustrated by typical realisticexamples. A method is provided for the rapid assessment of the likely significance of these effects in any case of interest.