Translational energy dependence of the branching fraction and cross sections for the decay of collision complexes: K+CsF, RbF

Abstract

The reactions CsF+K→KF+Cs−1.8 kcal mol−1 (I) and RbF+K→KF+Rb+1.5 kcal mol−1 (II) have been studied by the crossed molecular beam technique over a range of relative translational energy ?tr from 3 to 8 kcal mol−1. Both reactions, (I) and (II), are found to proceed via the formation of long-lived MFK complexes (M≡Cs, Rb). However, a strongly different reactivity behavior is observed, attributable mainly to the difference in sign of the exoergicities of the reactions. Angular distributions of reactive (R) and nonreactive (N) scattering show characteristic bimodal structure due to the (nearly) symmetric forward–backward peaking in the center-of-mass (c.m.) system. The reactive branching fraction FR=σR/(σR+σN) for each reaction has been determined at five values of ?tr by analysis of the angular distributions of the reactive and nonreactive scattering (and appropriate integration in the c.m. system). For the endoergic reaction (I) FR is small, increasing with ?tr, while for the exoergic reaction (II) FR?1/2, decreasing slightly with Ētr. These results are qualitatively in accord with simple phase-space branching theory. The cross section for complex formation σC=σR+σN is found to decline with energy, as expected on the basis of the orbiting-capture (centrifugal barrier) model. Based on an analysis of the low angle, ’’pseudoelastic’’ scattering, dominated by the long-range attractive potential, it has been possible to scale the measured scattered intensities and thus estimate the absolute magnitude of the cross sections. Values of σC were found to be of the order of 100 Å2. Approximate c.m. differential cross sections have been obtained by an iterative fitting procedure that satisfactorily reproduced the laboratory scattering data. A simple, uncoupled, angle-recoil velocity distribution function of the conventional 1/sinϑ form (with a slightly energy-dependent osculation) and a modified Maxwellian recoil distribution has been found to be adequate. The average relative translational energy of the scattered products, ?′tr, is found to increase essentially linearly with increasing ?tr. The theoretical implications of the present experimental results are discussed.