Evidence for non-thermalized vibrationally excited molecule production during azomethane pyrolysis in methane desorption from Cu(001)

A Cu(001) surface was exposed to products of an azomethane pyrolysis doser at varying temperatures. In addition to methyl radical adsorption, for certain doser conditions one or more doser emergent species can undergo an activated adsorption on the copper face. Directly after exposures, temperature programmed desorption between 170 K and 500 K was used to indicate the relative concentrations of adsorbed atomic hydrogen and methyl species, and thermally induced surface reactions. Two methane desorption features were invariably observed, indicating the presence of adsorbed methyl groups (CH₃) and transient adsorbed atomic hydrogen. The deduced relative surface concentrations levels of both H and CH₃ depend on the total exposures and the operating temperatures of the azomethane pyrolysis doser. The initial H concentrations apparent at surface temperatures between 275 K and 375 K are shown to arise from defect-related methyl decomposition and, at high operating doser temperatures, from the initial adsorption of one or more activated Cu incident species. It is proposed that the distributions of vibrational energies of emergent molecular hydrogen or methane species from higher temperature dosers are non-thermal. Hence, with doser temperatures of 800 °C or above, the effects of subsequent dissociative molecular adsorption on the copper surface can dominate over Cu defect chemistries.