Vibrational Energy Distribution in the Dichloroethylene Photoelimination Chemical Lasers

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Abstract
The HCl laser emission caused by photolysis of the three dichloroethylenes has been examined in detail. All three emit in 3 → 2, 2 → 1, and 1 → 0 vibrational transitions, and 1,1‐dichloroethylene also emits in the 4 → 3 transition. The highest‐gain transition at 15°C is P 3→2 (5) for 1,1‐dichloroethylene, P 2→1 (5) for cis‐1,2‐dichloroethylene, and P 2→1 (6) for trans‐1,2‐dichloroethylene (100 torr total pressure with 200 to 1 ratio of argon to dichloroethylene). Equal‐gain temperature measurements give population ratios N 3 / N 2 = 0.87 ± 0.02 for the 1,1 isomer, N 2 / N 1 = 0.81 ± 0.02 for the cis‐1,2 isomer, and N 2 / N 1 = 0.70 ± 0.02 for the trans‐1,2 isomer. These ratios are all below unity, as was found in the chemically activated trifluoroethane HF elimination laser. Further, the data show that the HCl vibrational mode is decreasingly significant in the sequence 1,1 > cis‐1,2 > trans‐1,2. The results might be attributable, in part, to competition between α, α and α, β elimination. Even so, the differing HCl excitation may indicate, by microscopic reversibility, a stereospecific influence of HCl vibration in the reverse, addition reaction between HCl and chloroacetylene.

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