Cl+HBr Pulsed Chemical Laser: A Theoretical and Experimental Study

The most powerful and efficient pulsed chemical lasers produced to date have resulted from the simple exothermic reactions between halogen atoms and hydrogen halides. These reactions have been initiated by flash photolysis. One such reaction is $$\mathrm{Cl}+\mathrm{HBr}\to \mathrm{HCl}+\mathrm{Br}\text{, −\hspace{0.17em}ΔH\hspace{0.17em}=\hspace{0.17em}15.5}\mathrm{kcal}/\mathrm{mole}.$$ A kinetic model has been developed for this laser system which calculates the intensity profile of the laser pulse as a function of time. The input data for the model include estimates of the reaction rate into the zeroth and first vibrational level of HCl and cross sections for collisional deactivation by the different chemical species in the laser medium. The rate of reaction into the second vibrational level of HCl is assumed to be zero. Any small finite rate into the second level would have a negligible effect on the calculated performance of the laser which operates only on the $\text{V(1\hspace{0.17em}→\hspace{0.17em}0)}$ fundamental transition of HCl. The theoretical model also requires as input data the photolysis flash intensity profile, the cavity geometry, and the losses. A discussion of the model is given, and a comparison is made between experiment and theory. The model is able to predict quantitative variations of the laser pulse characteristics as a function of flash intensity, cavity losses, and reagent partial pressures. The quantitative calculations are in reasonable agreement with experiment. It is concluded that (a) the most important single contribution to deactivation in the system is $$\mathrm{HCl}\text{(V\hspace{0.17em}=\hspace{0.17em}1)\hspace{0.17em}+\hspace{0.17em}}\mathrm{HBr}\text{(V\hspace{0.17em}=\hspace{0.17em}0)\hspace{0.17em}→\hspace{0.17em}}\mathrm{HCl}\text{(V\hspace{0.17em}=\hspace{0.17em}0)\hspace{0.17em}+\hspace{0.17em}}\mathrm{HBr}\text{(V\hspace{0.17em}=\hspace{0.17em}1),}$$ and (b) the rate of reaction to produce $\mathrm{HCl}\text{(V\hspace{0.17em}=\hspace{0.17em}1)}$ is at least as large as that to produce $\mathrm{HCl}\text{(V\hspace{0.17em}=\hspace{0.17em}0)}$ .