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

The vibrational spectroscopy and decay dynamics of CH4-OH reactant complexe s have been studied in the CH4 symmetric and antisymmetric stretching regio ns (nu (1) and nu (3)) The vibrational spectra have been obtained using bot h infrared and stimulated Raman excitation with ultraviolet probe laser-ind uced fluorescence detection. Stimulated Raman excitation of CH4-OH in the s ymmetric 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 ne arby OH partner. Infrared excitation in the asymmetric stretching region re sults 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 nu (1) an d nu (3) regions has been explained in terms of a model in which the CH4 un it undergoes internal rotation within the CH4-OH complex. The nu (1) featur es are attributed to transitions involving two different nuclear spin state s of CH4. In the nu (3) 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 vib rational spectra also exhibit extensive homogeneous broadening (greater tha n or equal to 1 cm(-1)) arising from the rapid decay of vibrationally activ ated CH4-OH complexes due to vibrational predissociation and possibly react ion. The OH fragments are produced with minimal rotational excitation, indi cating that the dominant inelastic decay channel involves near-resonant vib rational energy transfer within the CH4 unit from the initially prepared CH stretch to an overtone bend (2 nu (4)) state. (C) 2001 American Institute of Physics.