Electronic spectroscopy of the deuterated isotopomers of the NO.methane molecular complex

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s<--pi* excitation (approximately A (2)Sigma(+)<-- approximately X (2)Pi) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH(4), CH(3)D, CH(2)D(2), CHD(3), and CD(4) as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.