Photodissociation of acetone at 193 nm: Rotational- and vibrational-state distributions of methyl fragments by diode laser absorption/gain spectroscopy

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Abstract
Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of CD3 fragments formed in the photodissociation of acetone-d6 at 193 nm. Selected rotational lines have been measured in the ν2 ‘‘umbrella’’ fundamental and first two hot bands, and in the ν3 asymmetric stretching fundamental band. Substantial growth is observed in the vibrationless state on the time scale of vibrational relaxation. We estimate that only about 15% of the nascent CD3 population is formed in the vibrational states we detect: ν2=1 and 2, ν3=1, and the vibrationless state. Most of the nascent methyl population is evidently spread among many undetected vibrational states. These results complement previous measurements of acetone photofragments by infrared emission, multiphoton ionization, and laser-induced fluorescence. Our inferred global vibrational distribution is consistent with a two-step fragmentation.