Despite the very large cross section of I2 for photoabsorption, the high rate of deexcitation of I* by I2 (k3) reduces the F„ value to one-tenth that for Br2, implying that for equal conditions the value of I* is an order of magnitude less than Br*. The coefficient k, is much less for I2-HF than any of the Br2 systems, implying poor transfer of energy to HF, (FT), and also low retention in the upper level because of deexcitation (Fu). The highest value of kt is that of Br^HjO, which also has the highest transfer efficiency FT. Unfortunately, it has a very high deexcitation rate of the tipper laser level (low Fu). The Br2-HCN-He system has values of FT and Fu which are intermediate between the combinations of CO2 and H2O. The overall efficiency of the laser can be expressed as tj = where it is assumed that complete absorption occurs. The constant a = 1.4 x 10-18De/cr and for Br2 is 1.32 x 10-2 and for I2 is 1.4 x 10-1 units. The value of F, the fraction of excited species formed, is taken as 0.5. Plots of rj vs a, the ratio M/ Y (Fig. 7), show ?) = 5 x 10”3 for Brz-HjO-He and Brz-HCN-He, provided a > 10-1 and for Iz-HF-He if a > 102. Unfortunately, in the latter case the high value of a will probably result in the gases becoming overheated, thus filling the lower laser level. The Br2-CO2-He laser has an efficiency of around 10-3 for a > 0.1.
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