Reactive Scattering in Molecular Beams: Velocity Analysis of KBr Formed in the K+Br2 Reaction

Abstract

The velocity distributions of scattered K and KBr from collisions of crossed thermal beams of K+Br2 have been measured over the range 200–850 m/sec at four laboratory angles Θ=10°, 15°, 20°, 30°. The results confirm that most of the energy released in the reaction (∼45 kcal/mole) appears in internal excitation rather than in translational motion of the products, as inferred previously from qualitative kinematic analysis of angular-distribution measurements. An approximate transformation to the center-of-mass coordinate system is carried out to evaluate for each Θ the distribution of the final relative translational kinetic energy E′ of the products. These distributions have a roughly Maxwellian shape (broader by ∼20%). The position of the peak varies from E′=1 to 4 kcal/mole for Θ=30°→10° (as compared with E=1.2 kcal/mole for the most probable initial relative kinetic energy of the reactants); the integrated intensity of cumulative yield with E′≳5 kcal/mole is ∼25% at 10° and ∼50% at 30° and with E′≲15 kcal/mole it is ∼75% at 10° and ∼90% at 30°. The distribution of the vibrational excitation Wv′ of the KBr for Θ=30° is also estimated by combining these results with data from electric-deflection experiments which provide an estimate of the rotational excitation. The vibrational excitation peaks near Wv′ ∼43 kcal/mole and the cumulative yield with Wv′≳30 kcal/mole is ∼90%. The ``electron-jump'' mechanism suggested by previous results, K+Br2→K++Br2−→K+Br− +Br, again offers plausible qualitative interpretations (perhaps not unique) for the main features of the observed energy distributions.