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

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 A state surface, transfer to the ground-state surface uia the conical intersection in the N-H exit channel and production of CH,NH fragments carrying substantial internal excitation, behaviour very reminiscent of that exhibited by ammonia following photoexcitation to its corresponding A 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: Do(H-NHCH,) = 34550 fr 200 cm-'. 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 A state. RRKM calculations assuming complete energy randomisation in the CH,NH,# species prior to dissociation suggest, respectively, major and minor contributions to the total H atom yield from channels leading to the products H + CH,NH, and H + CH,NH, 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 CH,NH(A) fragments, supplements the slow H atom yield observed at the shortest excitation wavelengths.