Vib-rotational energy distributions and relaxation processes in pulsed HF chemical lasers

The rate equations governing the temporal evolution of photon densities and level populations in pulsed F+H₂→HF+H chemical lasers are solved for different initial conditions. The rate equations are solved simultaneously for all relevant vibrational-rotational levels and vibrational-rotational P-branch transitions. Rotational equilibrium is not assumed. Approximate expressions for the detailed state-to-state rate constants corresponding to the various energy transfer processes ( V-V, V-R, T, R-R, T) coupling the vib-rotational levels are formulated on the basis of experimental data, approximate theories, and qualitative considerations. The main findings are as follows: At low pressures, R-T transfer cannot compete with the stimulated emission, and the laser output largely reflects the nonequilibrium energy distribution in the pumping reaction. The various transitions reach threshold and decay almost independently and simultaneous lasing on several lines takes place. When a buffer gas is added in excess to the reacting mixture, the enhanced rotational relaxation leads to nearly single-line operation and to the J shift in lasing. Laser efficiency is higher at high inert gas pressures owing to a better extraction of the internal energy from partially inverted populations. V-V exchange enhances lasing from upper vibrational levels but reduces the total pulse intensity. V-R, T processes reduce the efficiency but do not substantially modify the spectral output distribution. The photon yield ranges between 0.4 and 1.4 photons/HF molecule depending on the initial conditions. Comparison with experimental data, when available, is fair.