A CROSSED MOLECULAR-BEAM INVESTIGATION OF THE REACTION CL-]HCL+C3H7 USING VUV SYNCHROTRON-RADIATION AS A PRODUCT PROBE(PROPANE)

We have used the crossed molecular beam technique to study the hydroge n atom abstraction from propane by atomic chlorine over a wide range o f collision energies. The experiments were carried out using a recentl y constructed crossed molecular beam apparatus that utilizes tunable V UV synchrotron radiation for product photoionization. We have measured laboratory TOF spectra and angular distributions for E-coll = 8.0, 11 .5, and 31.6 kcal/mol. Center-of-mass flux maps were generated from th e measured laboratory distributions. The results demonstrate two disti nct reaction mechanisms that depend on the impact parameter of the rea ctive collision. Large impact parameter collisions proceed via a strip ping mechanism resulting in forward scattered products with very littl e momentum change in going from reactant to product. The stripping rea ctions are most likely dominated by abstraction of secondary hydrogen atoms. Smaller impact parameter collisions lead to direct reactions wi th an impulsive recoil and are consistent with a preference for a coll inear transition state geometry, C-H-Cl. The larger energy along the l ine of centers in smaller impact parameter collisions most likely make s the effect of a larger barrier to abstraction of primary hydrogen at oms negligible leaving the ratio of primary to secondary hydrogen abst raction to be dictated by simple statistics. (C) 1998 Published by Els evier Science B.V. All rights reserved.