Fig. 4. Efficiency of IBr solar pumped laser plotted vs length I. at different pressures, and concentration factors C. The reflectivities are r, = 1, r2 = 0.95, respectively. An understanding of why the efficiency is low can be obtained if the overall efficiency is considered to be the product of four efficiencies (3): r, = VsVaVkVo- Here ■qs, the solar efficiency depends on the absorption bandwidth of the total solar radiance, and its value for IBr is 0.12. The absorption efficiency r)a depends on the IBr gas pressure and depth of absorption J; for low pressures = o-„(IBr)J = 10-2pt/, where p is the pressure in torr, d the depth in cm. The expression is valid up to pd = 50; for higher pd most of the photons are absorbed and 1. The kinetic efficiency pk depends on the number of absorption events producing Br* and also on the competition between stimulated emission and quenching. Approximately 0.7 Br* atoms are produced for every absorbed photon (Table 1). The fraction of Br* atoms which yield a stimulated photon can be obtained by examining Eq. 6, and comparing the value of T with the rest of the loss processes, as all eight variables vs time were known. The ratio T/(all loss processes) was 0.82, when p was a maximum, the major competitors with T being quenching by IBr and I. The quantum efficiency is rja = 0.44 eV/2.5 eV = 0.18. Hence forpd < 50 torr-cm, p = 6 x 10-4 at 5 torr, d = 1 cm, agreeing with 5 x 10-4 in Fig. 4. With pd > 50 torr-cm, r)a —» 1 and the maximum efficiency would be about 1 2 x 10~2. Pulsed working would further reduce the efficiency because of the duty cycle, unless there was adequate storage of energy when the laser was not emitting. VIII. POSSIBILITY OF CONTINUOUS LASING The computer runs showed that IBr was depleted, while the quenchers I2, Br2, and
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