Laser-initiated chemical chain reactions

A detailed kinetic and experimental analysis is presented for chemical chain reaction processes initiated by well-controlled, low power laser pulses. Realtime evolution of the chain reaction is followed by direct detection of infrared chemiluminescence from vibrationally excited HCl product molecules produced by one of the propagation reactions in the chain. By appropriate choice of conditions, the chain reactions may be analyzed separately for pseudofirst-order, radical–reagent processes as well as for second-order, radical–radical events. The pulsed laser initiation technique is applied to three sample chain systems which exhibit distinctly different chain lengths, rates, and termination behaviors. These systems are Cl2/H2S, Cl2/H2, and Cl2/CH3SH. In the case of Cl2/H2S, detailed rate constant data are obtained for the fundamental chain propagation steps, and appropriate chain termination steps are assigned from the observations. The results demonstrate a new, general technique for the quantitative study of chemical chain reactions and related combustion processes.