was believed that the major cause of this discrepancy was saturation of the transition caused by the 80 mW probe beam. To reduce the effects of probe saturation, the experiment was repeated at lower laser power. Figure 7 shows the measured optical gain coefficients of CO2-He gas mixtures as a function of He concentration ratio using a 15 mW probe beam. Although the basic trends at p = 1 Torr andp = 2 Torr seem to be the same as those predicted by theory, the measured value of the gain coefficient at 1 Torr is about 20% lower than the theoretical prediction for the same He concentration. It was found that the maximum measured optical gain is 2.8 X 10-3 cm*1 for a He concentration of 15% at 1 Torr pressure; the corresponding value predicted by theory is 3.46 X 10-3 cm-1. The value of gain for pure CO2 is substantially lower than the gain for the CO2-He gas mixtures, as may be expected [21]. Various factors determine the effects of pressure on the optical gain. Increased number density of CO2 and decreased saturation effects tend to increase the gain, while the lowered optical mean-free-path and collisional deactivation time decrease the gain. The effect of these factors can be seen in the experimental results (Fig. 7). The values of the measured optical gain coefficients at 2 Torr are about half those at 1 Torr for the same He concentration. The reduction of the mean-free-path of the pumping photons with increased pressure [15] is believed to be the principal cause of this trend. It was found that the measured optical gain at 0.5 Torr for 10% He concentration is about 1.5 X 10-3 cm-1 (Fig. 7), whereas the corresponding measured value at 1 Torr is 2.6 X 10-3 cm*1. This difference is due to the combined effects of reduced concentration of CO, and the saturation effect beinv about four times greater than that at 1
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