Rotational temperature determinations in molecular gas lasers

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Abstract
The small‐signal gain expressions for vibrational‐rotational transitions are examined in detail to determine possible methods of extracting the rotational temperature from experimental gain measurements in molecular gas lasers. Approximate values of Tr can be deduced from the rotational quantum numbers for which the P‐ and R‐branch gains are maximum. Quite accurate values of Tr and the population inversion density (nv′−nv″) can be determined by fitting data to suitably linearized gain relationships, or by performing least‐squares fits of the P‐ and R‐branch experimental data to the full gain expressions. Experimental gain measurements for 15 P‐branch and 12 R‐branch transitions in the 10.4‐μm CO2 band have been performed for pulsed uv‐preionized laser discharges in CO2 : N2 : He=1 : 2 : 3 mixtures at 600 Torr. These data are subjected to the several gain analyses described herein, yielding a rotational temperature of 401±10 °K and an inversion density of (3.77±0.07) ×1017 cm−3 for conditions of maximum gain. These techniques provide accurate values of the gas temperature in molecular gas lasers with excellent temporal and spatial resolution, and should be useful in extending the conversion efficiency and arcing limits of high‐energy electrically excited lasers.