appears that IBr should be the best laser candidate of type [2]. It absorbs near the solar peak with a high probability of dissociating into I + Br*. III. PHYSICAL MECHANISMS The processes occurring in IBr are assumed to be photodissociation with the formation of excited and ground state atoms, quenching of the excited atoms, recombination and exchange reactions. The species are assumed to be eight in all, namely, IBr, I, Br, I2, Br2, Br*, and I*, and photons created by spontaneous and stimulated emission. Vibrational excitations are neglected at room temperature because of very rapid relaxation. The reactions and rate constants are summarized in Table 1. The absorption of photons results in reactions 1 through 6. The photodissociation rates 5 are equal to C<t>(X)AXcr„(X)GV), where C is the number of times the solar radiation is concentrated, <F(X)AX is the number of photons arriving per unit area per second (8), cr„(X) the absorption cross-section in question, and (N) the number density of absorbers. As the ratio Br*/Br initially produced by the photodissociation of IBr is critical to attaining an inverted population, care must be taken in evaluating 5] and 54 in Table 1. The potential curves for the ACS*), A3nH and B3H0+ levels for IBr were plotted by computer on the energy diagram (Fig. 1) using data from Huber and Herzberg (9). The B'3H+ is included, and is drawn in approximately. The lowest vibrational level (v = 0) and the Franck-Condon transitions are shown. The horizontal dashed lines B and C are based on the transitions from the lower to the upper A3Hj and B3n(l levels and indicate the peaks and widths of the absorption curves. The absorption cross-section for IBr as a function of wavelength has been measured (10-11), and the function can be represented by three Gaussians whose peaks are at 268, 477, and 507 nm, respectively. These are plotted on the left. The peak of Gaussian F coincides almost exactly with the line C, but the peak of the Gaussian G is displaced from B. We believe the Gaussians to be accurate as well as the asymptotes A and D. The integral is a measure of what fraction of absorption events are caused by transitions to the level in question and au(\) is the respective Gaussian. Thus Gaussian H produces Br, but can be neglected as </>(X) is small here (8). The part of Gaussian F above asymptote A produces Br*; the part below A does not produce dissociation. The B'3H^ and B3IV curves cross and possibly absorption into the latter could result in Br. However, the translational energy at the crossing is sufficiently high and the nature of the crossing is such that the probability is near unity that all parts of the absorption curve above A in Fig. 1 correspond to dissociation into I + Br*, as confirmed experimentally (13). The Gaussian G, corresponding to absorption into the A3I1, level produces 1 -I- Br. Performing the integration of <(>(x)cr„(x) dX then yielded the fractions of absorption events resulting in the production of Br*, Br, and I (Table 1). The total absorption rate for IBr was found to be G X 1.25 X 1015 (IBr),
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