Differential reaction cross section of the C2H5X(X=Br, I) + K → KX+ C2H5systems

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Abstract
Using the crossed molecular beam method complete laboratory differential reaction cross sections for the exoergic reaction C2H5Br → BrK + C2H5 have been measured as a function of relative translational energy from 0·11 to 0·41 eV. An analysis has been carried out of both the present KBr laboratory angular distributions and that of KI from the K + C2H5I molecular beam reaction obtained by Aoiz et al., over the range of reactive translational energy, Ē t, from 0·17 eV to 0·55 eV. By using the uncoupled approximation for the centre of mass (c.m.) angular and recoil energy distributions to recover the laboratory angular distributions it was found the c.m. differential (solid angle) reaction cross sections to be backward-peaked, characteristic of a direct, rebound mechanism, with a large fraction of the available energy going into product translation. The average translational energy of the products, Ē′ t, increases approximately linearly with increasing collision energy E′ t = 0·57 Ē t + 0·59 and Ē′ t = 0·65 E t + 0·72 (in eV) for the K + C2H5Br and C2H5I reactions respectively. The present data for the K + C2H5 X (X = I, Br) systems are compared with previous results for the analogous CH3 X reaction from where the role played by the halogen and alkyl group is discussed and qualitative effects are noted as, for example, the fact that the heavier the alkyl group the broader the backward cone of the MX angular distribution. Comparison with several theoretical impulsive models, e.g. the photodissociation model of Herschbach and the information-theoretic form of Levine and coworkers is made. A modified hard sphere collision is also found to account satisfactorily for the main features of the present differential reaction cross sections.