caused by photodissociation (it would require C 105). High temperatures coqld also increase the ratio of reverse to forward rates of the exchange reactions, thus tending to neutralize the mechanisms necessary to depopulate the lower laser level (30). Also, the quenching reaction Br* + I —> Br + I is enhanced in the heated gas because of the greater number of I atoms. The calculations assumed no heating. On changing the pressure p both experiment and theory showed the output laser pulse duration was roughly independent ofp for pumping pulses of 200 p.s duration. Experimentally the output light signal amplitude increased with p up to about 20 torr, where it reached a maximum and then decreased; as p increased, absorption in the outer layers decreased the pump power on axis. The theory showed the amplitude proportional to p because constant pumping energy density was assumed throughout the gas. Both experiment and theory showed that increasing C increased the laser amplitude proportionally. There was also agreement between the measured and calculated output power of the laser. In an experiment with a tube length of 1 m and 2.22 cm radius, and an output mirror of 95% transmittance, the measured output was approximately 2 kW for an IBr pressure of 4 torr, with C ~ IO4 (4). The calculated peak output power P is V. INTERPRETATION By varying the parameters in the program one at a time, it was established that the dominant mechanisms were quenching of Br* by IBr and I and the exchange reaction Br + IBr = I + Br2. In particular, the program indicated lasing would not occur if the exchange reaction was removed. Three-body recombinations made little difference to the shape of the light output for pumping pulses of 200 p.s duration. Fortunately the rate coefficients most accurately known turned out to be dominant ones. However, for computer runs of longer duration pulses, it was found essential to include all the terms. Evidently the exchange reactions can overcome quenching, and high quenching cross-sections per se may not be a valid selection criterion for rejecting otherwise promising gases. VI. THRESHOLD CONDITION The threshold condition is rp2 exp(2aL) = 1, where r^2 are the reflectivities of the laser mirrors, a = a^N is the gain per unit length, and L is the length of the laser (29). The threshold condition is contained in Eq. 9, and occurs when Our results showed n = 1.2 x 1012 cm 13 for C = 1 X 104, corresponding to 1.7 kW which is in reasonable agreement.
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