TIME EVOLUTION OF THE GAIN IN THE CHLORINE-FLUORIDE/HYDROGEN ROTATIONAL CHEMICAL LASER: EVIDENCE FOR V ---> R ENERGY TRANSFER

We have used the tandem chemical laser to study the origin of the very high rotational excitation which results upon flash photolysis of ClF/H(,2) mixtures. Advances in the time-delayed tandem method have enabled us for the first time to map the evolution of the laser gain for high rotational transitions as the photolytically initiated chain reaction produces HF('(NOT=)). A grating selective cavity was used to restrict the rotational emission to the transition of interest. In v = 0 the gain development was determined for the four transitions, ('0)30 (--->) 29, ('0)29 (--->) 28, ('0)28 (--->) 27 and ('0)22 (--->) 21. The three transitions mapped in v = 1 were ('1)32 (--->) 31, ('1)30 (--->) 29 and ('1)24 (--->) 23. Detailed pressure dependence studies were performed on ('0)29 (--->) 28 to determine the role of various collision partners in the development of laser gain.; The technique has proven to be extremely sensitive to the detection of small population differences in these states and particularly at early photolysis times when collisions have had a minimal effect on the reaction distribution. From the detection of gain as early as 0.3 (mu)sec, we have concluded that the H + ClF reaction directly populates these high J states. In addition, the rotational distribution which we derive from these gain measurements shows good agreement with the zero surprisal statistical distribution.; For transitions such as ('0)30 (--->) 29, ('0)29 (--->) 28 and ('0)28 (--->) 27, the initially measured negative gain distribution was quickly overcome by a process favoring population of the higher J state of the transition. Experimental results from the pressure studies indicated that ClF - HF('(NOT=)) and H(,2) - HF('(NOT=)) collisions were primarily responsible for this creation of positive gain. Arguments are presented in support of a V (--->) R,T mechanism which resonantly pumps molecules from the heavily populated higher vibrational states into the high J states of v = 0 and v = 1. HCl and Ar were found to have a detrimental effect on the positive gain of ('0)29 (--->) 28 as their pressures were increased. The effect of the various collision partners are discussed in detail.; In addition a computer model was developed to study the effects of various energy transfer processes on the primitive reaction distribution. A simple exponential gap model based on the experimental observations was used to describe the multiquantal V (--->) R,T processes. The results of the calculations were generally in good agreement with the experimental trends.