Activation of the CH stretching vibrations in CH4–OH entrance channel complexes: Spectroscopy and dynamics

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The vibrational spectroscopy and decay dynamics of CH4–OH reactant complexes have been studied in the CH4 symmetric and antisymmetric stretching regions (ν1 and ν3). The vibrational spectra have been obtained using both infrared and stimulated Raman excitation with ultraviolet probe laser-induced fluorescence detection. Stimulated Raman excitation of CH4–OH in the symmetric stretching region reveals two blended Q branch features at 2912.5 and 2911.8 cm−1. An extremely weak infrared spectrum is also seen in the CH4 symmetric stretching region, which is induced by the presence of the nearby OH partner. Infrared excitation in the asymmetric stretching region results in an intense, yet enormously broad spectrum centered at 3020 cm−1 that extends over 40 cm−1. The appearance of the spectra in the ν1 and ν3 regions has been explained in terms of a model in which the CH4 unit undergoes internal rotation within the CH4–OH complex. The ν1 features are attributed to transitions involving two different nuclear spin states of CH4. In the ν3 region, the CH4–OH complex can undergo a multitude of allowed transitions, each associated with a rovibrational transition of free methane, which give rise to the enormous span of the spectrum. The vibrational spectra also exhibit extensive homogeneous broadening (⩾1 cm−1) arising from the rapid decay of vibrationally activated CH4–OH complexes due to vibrational predissociation and possibly reaction. The OH fragments are produced with minimal rotational excitation, indicating that the dominant inelastic decay channel involves near-resonant vibrational energy transfer within the CH4 unit from the initially prepared CH stretch to an overtone bend (2ν4) state.