Laser power and vibrational energy transfer in CO2 lasers

Abstract

The decay of the laser power in the afterglow of a CO2 laser discharge is shown to be exponential when the upper laser level is excited directly by electrons. The decay time is shown to be a function of the rate coefficients describing the relaxation of the lower laser level. From power decay measurements it is found that the deactivation of the lower laser level to the ground state occurs in two steps. The rate coefficient for the transfer of vibrational energy from the 1000 level to the 0110 level of CO2 is found to be 5× 10−13 cm3 sec−1. This is an order of magnitude smaller than a previously reported value. The rate coefficients for the relaxation of the 0110 level of CO2 by H2O and D2O are found to differ by a factor of 1.8. These rate coefficients increase with increasing temperature in agreement with calculated results using the theory of Marriott. The measured dependence of the relaxation time of the lower laser level on the H2O vapor density explains the observed maxima of the laser power and small signal gain in conventional and gas-dynamic CO2 lasers. The power decay for a CO2 laser, when the upper laser level is additionally excited indirectly via the N2 (ν = 1) level, is shown to be described by the sum of two exponential functions. The two time constants depend on the rate coefficients for the relaxation of the lower laser level as well as for the transfer of energy from the N2 (ν = 1) level to the upper laser level. The decay time of the laser power is found to increase with increasing N2 density.