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 W_v′ 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 W_v′ ∼43 kcal/mole and the cumulative yield with W_v′≳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.