The probability of hydrogen atom release, following photoexcitation of methylamine, CH(3)NH(2), is found to increase extensively as higher vibrational states on the first excited electronic state are accessed. This behavior is consistent with theoretical calculations, based on the probability of H atom tunneling through an energy barrier on the excited potential energy surface, implying that N-H bond breaking is dominated by quantum tunneling.

The mechanism of H and D atom loss, following ultraviolet photolysis of methylamine-d(3), CD(3)NH(2), has been studied via electronic action and Doppler spectroscopies. The N-H bond is preferentially cleaved and the yield of both H and D photofragments increases gradually, but differently, as higher vibrational states on the first excited electronic state, A, are accessed, leading to some drop in H/D branching ratios. The average translational energies of the H photofragments are somewhat higher than those of D, implying lower energy content left in the internal degrees of freedom of the CD(3)NH than in the CD(2)NH(2) partner fragment. These results provide evidence for discrimination between the two channels and mechanistic insight into the N-H and C-D bond cleavage.

The N-H and C-D bond fission in partially deuterated methylamine, CD(3)NH(2), has been investigated using vibrationally mediated photodissociation. Jet-cooled action spectra and Doppler profiles of the H and D photofragments were monitored following approximately 243.1 nm photodissociation of the parent pre-excited to two, three or four N-H stretch quanta. The action spectra were analyzed in terms of simplified local mode/normal mode (LM/NM) and NM models, allowing band assignment and determination of the strong resonances involved in the coupling. The Doppler profiles show that the released H and D photofragments have low translational energy content and that the H is the dominant product, although its yield decreases as higher pre-excited N-H vibrational states are dissociated. The dynamics of the site-dependent bond fission in CD(3)NH(2) is discussed.

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