Effects of CO2, He, and N2 on the Lifetimes of the 00°1 and 10°0 CO2 Laser Levels and on Pulsed Gain at 10.6 μ

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Abstract
Relaxation times of the 00°1 and 10°0 CO2 vibrational levels and the afterglow gain at 10.6 μ were studied by a pulsed‐gain technique, for pure CO2, CO2:He, and CO2:N2 gas mixtures in 22‐mm‐ and 34‐mm‐bore nonflowing laser amplifiers. Measurements of the exponential rise, τr, and decay, γd, times of the afterglow gain pulse were made using a cw 10.6‐μ CO2 laser as the amplifier input radiation source. Evidence is presented to support the interpretation of τr and τd as measures of the effective lifetimes of the 10°0 and 00°1 CO2 laser levels, respectively. For the case of a pure CO2 gas in the pressure range from 1 to 8 Torr, the measured τr and τd values vary from 50 to 250 μsec and 0.3 to 2.5 msec, respectively. Both 1/τ00°1 and 1/τ10°0 increase almost linearly with increasing CO2 pressure for P>4 Torr. Collision cross sections for volume quenching in pure CO2 are computed to be σ00°1=2.86×10−19 cm2 and σ10°0=1.62×10−18 cm2, assuming Tgas=300°K. Addition of helium to CO2 causes essentially no change in τ00°1 but results in a substantial decrease in τ10°0. The measured collision cross section for quenching of the 10°0 level population by He is (σ10°0)He=1.04×10−18 cm2, about two‐thirds of that obtained for CO2−CO2 collisions. Nitrogen, on the other hand, causes an increase in both τ00°1 and τ10°0 by much as a factor of two at P>5 Torr. The peak values of amplifier gain in the afterglow, Gp, in general, are higher and depend more critically on tube bore than those obtained for cw discharges. The relative values of optimum Gp for CO2, CO2:He, and CO2:N2 are consistent with those obtained from cw gain measurements.