Near-UV photolysis of methylamine studied by H-atom photofragment translational spectroscopy

Abstract

H(D) atom photofragment translational spectroscopy has been used in a detailed study of the near-UV photolysis of methylamine and its deuteriated analogues at a number of wavelengths in the range 203.0–236.2 nm. Analysis of the total kinetic energy release spectra so obtained serves to reinforce recent suggestions that at least two dissociation pathways lead to H(D) atom fragments. One, ‘direct’, route involves N—H bond extension on the à state surface, transfer to the ground-state surface via the conical intersection in the N—H exit channel and production of CH₃NH fragments carrying substantial internal excitation, behaviour very reminiscent of that exhibited by ammonia following photoexcitation to its corresponding à excited state. Determination of the kinetic energies of the fastest H atoms formed at various of the longer excitation wavelengths yields an improved value for the N—H bond strength in methylamine: D₀(H—NHCH₃)= 34 550 ± 200 cm^–1. A second fragmentation channel yields H(D) atoms with a kinetic energy distribution peaking much closer to zero. Many of these are presumed to arise from the unimolecular decay of highly internally excited ground-state methylamine molecules, themselves formed via internal conversion from the initially prepared à state. RRKM calculations assuming complete energy randomisation in the CH₃NH^#₂ species prior to dissociation suggest, respectively, major and minor contributions to the total H atom yield from channels leading to the products H + CH₂NH₂ and H + CH₃NH, but the observed variations in total H(D) atom signal strength upon isotopic substitution indicate a dominant role for the N—H(D) fission process. Analyses of the measured total kinetic energy release spectra suggest that another component of the ‘direct’ dissociation pathway, yielding electronically excited CH₃NH(Ã) fragments, supplements the slow H atom yield observed at the shortest excitation wavelengths.