Effect of vibrational and rotational relaxation mechanisms in pulsed H_2 + F_2 lasers

Abstract

A comprehensive model of the H2 + F2 pulsed chemical laser is used to investigate mechanisms important to laser performance. The model employs vibrational-to-rotational energy transfer and rotational nonequilibrium in an attempt to explain experimental observations of rotational lasing and P-branch lasing from high J states. The effect of partitioning energy between vibrational–translational (V–T) and vibrational–rotational (V–R) in the V–R, T mechanism was considered. Increased V–T produced smaller predicted pulse duration and laser power. The major effect of V–R is to populate rotational levels above J = 12; hence, it did not produce major changes in predicted P-branch lasing spectra. Increasing the dependence of rotational relaxation rate coefficients on rotational quantum numbers was effective in sustaining larger nonequilibrium populations at high J levels, but the effect was not as pronounced as an overall decrease in the rotational relaxation rate. The prediction of excessive energy in the hot bands of HF appears to be the result of insufficient vibrational deactivation from these levels. Both increasing the endothermic cold pumping reactions and the vibrational dependence of V–R rates were effective in decreasing hot-band lasing. Vibrational scaling of the V–R rate coefficients larger than V2.3 may be required to properly describe vibrational deactivation. Although the model compares favorably with experimental studies for the lower three P-branch bands, excessive P-branch lasing is predicted for high rotational levels in the hot bands. Insufficient experimental data on the pulsed H2 + F2 chemical laser exist to reconcile theory with experiment.