Tandem Chemical Laser Measurements of Vibrational Energy Distribution in the Dichloroethylene Photoelimination Reactions

Equal-gain temperature measurements with two laser tubes in the same optical cavity (a ``tandem'' laser) are described. In this way, rate constant ratios can be measured for vibrational transitions that do not have highest gain. The HCl laser emissions caused by photolysis of the 1, 1- and cis-1, 2-dichloroethylenes were examined in this manner. For 1, 1-dichloroethylene, k3/k2=0.85± 0.02 and k2/k1=1.08± 0.03 and for cis-1, 2-dichloroethylene, k3/k2=0.71± 0.03 and k2/k1=0.76± 0.02. The trends confirm the earlier inference that HCl vibrational excitation is more significant in the 1, 1-dichloroethylene photo-elimination, with the attendant implication that HCl vibrational excitation tends to favor, 1, 1 addition (Markovnikov addition) in the reverse reaction between HCl and monochloroacetylene.