Molecular beam reactive scattering of Br 2 from Pd(111) using an electrochemical effusive source

Abstract

A computer-controlled modulated molecular beam source is used to investigate the kinetics of the surface reactions which occur when bromine is reactively scattered by Pd(111). The reaction products are atomic bromine and molecular bromine: the latter species arises from an adatom recombination process and gives rise to a product vector modulated at twice the frequency of the incident beam (2ω.) By making suitable measurements of the temperature dependence of the product vector phase shifts at ω and 2ω, the four kinetic parameters which characterise the first-order and second-order rate processes are obtained. These are: A 1 = 2.5×10 9 s −1, E 1 = 177 kJ mol −1, A 2 = 3.6×10 −10 m 2 s −1, E 2 = 131 kJ mol −1. The significance of these values is discussed in terms of the properties of the transition state to desorption.