Dissociation of CD3I at 248 nm studied by diode laser absorption spectroscopy

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Abstract
Transient diode laser absorption spectroscopy has been used to monitor the rotational and vibrational populations of CD3 radicals following photodissociation of CD3I by 248 nm light. Rotational lines in the ν2=1–0, 2–1, 3–2, and 4–3 bands have been analyzed to give time‐dependent CD3 populations following photodissociation of 100 mTorr samples of neat CD3I. Prompt absorption signals are small, but positive for all observed rotational lines of all observed bands, providing clear evidence against population inversion in the low levels of the ν2 vibration of methyl photoproducts. The vibrational population distribution in ν2 a few μs after photodissociation is 4:3:2 for v=0:1:2 with about 20% uncertainties. A strong variation in time dependence with rotational level is consistent with an initially hot rotational distribution (≥2000 K) that cools on the μs time scale of methyl translational moderation in the bath of undissociated methyl iodide. We believe the origin of the rotational excitation to be translation–rotation energy transfer, rather than the photodissociation itself. Time‐resolved Doppler line shapes provide an independent view of the translational energy loss.