Internal excitation of CH₃ radicals produced in the photolysis of acetone at 193 nm and the collisional enhancement of the infrared emission intensity in the ν₃ spectral region

Time-resolved IR emission spectroscopy has been used to monitor the fluorescence in the C-H stretch region of methyl radicals produced in the 193-nm photolysis of acetone. Spectra collected at 20-cm^-1 resolution in the ν₃ spectral region do not exhibit any structure. This indicates that the emission in this region is due to both the ν₃ fundamental of CH₃ and combination bands of the radical which overlap each other. Modes other than ν₃(00n0) must contribute to the observed emission in the 3000-3350-cm^-1 region. Translationally hot methyl radicals are also found to undergo very fast T → V energy-transfer processes via collisions with various noble gases, resulting in enhanced infrared emission. The intensity of the enhanced emission is a factor of 4 or 5 times the emission intensity in the absence of the noble gases, suggesting that most of the radicals are formed in other vibrational states. The results are explained by assuming that the CH₃ radical is initially produced in a broad range of vibrational states.