Photofragmentation dynamics of CH3I at 248 and 266 nm: Vibrational distributions in the CH3(ν2) ‘‘umbrella’’ mode

Abstract

Vibrational population distributions in the ν2 out-of-plane bending mode of CH3 when produced by photofragmentation of CH3I at 266 and 248 nm are analyzed by infrared emission spectroscopy. In both cases, there is excitation of the ’’umbrella’’ mode vibration up to 10 quanta, and the peak of the excitation occurs at ν2 = 2 for both wavelengths. The vibrational distribution produced at 266 nm agrees well with molecular beam time-of-flight results. The distribution at 248 nm is only moderately enhanced, in contradiction to theoretical predictions which suggest that the peak of the distribution should be shifted to ν2 = 5. The infrared emission spectra suggest that there is no additional rotational excitation in the radical over the ambient rotational temperature of the parent CH3I. In addition, there is no detectable excitation in other vibrational modes, produced either directly in the dissociation or upon collisions.