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

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, τ_r, and decay, γ_d, 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 τ_r and τ_d 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 τ_r and τ_d values vary from 50 to 250 μsec and 0.3 to 2.5 msec, respectively. Both 1/τ_00°1 and 1/τ_10°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 σ_00°1=2.86×10⁻¹⁹ cm² and σ_10°0=1.62×10⁻¹⁸ cm², assuming T_gas=300°K. Addition of helium to CO₂ causes essentially no change in τ_00°1 but results in a substantial decrease in τ_10°0. The measured collision cross section for quenching of the 10°0 level population by He is (σ_10°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 τ_00°1 and τ_10°0 by much as a factor of two at P>5 Torr. The peak values of amplifier gain in the afterglow, G_p, in general, are higher and depend more critically on tube bore than those obtained for cw discharges. The relative values of optimum G_p for CO₂, CO₂:He, and CO₂:N₂ are consistent with those obtained from cw gain measurements.