Br2-CO2-He mixture (2-3). The Br2 acted as a broadband absorber, the CO2 was the lasing medium, and the He acted as a coolant. This laser was an electronic-to- vibrational energy transfer laser, and the low “transfer efficiency” as well as absorption efficiency resulted in an overall efficiency of less than 0.13% Higher overall efficiency might be attained if the transfer efficiency could be eliminated, as in lasers of type [2] above. An example would be photodissociation of a molecule to yield an excited atom, which then lases to the ground state. The population of the lower level might be removed by chemical processes. High pressure working should be possible which would enable more efficient absorption. The objectives of this paper are to study theoretically the IBr solar pumped laser as an example of class [2], to understand the essential lasing features by determining the dominating reactions, and to estimate the efficiency for power conversion. The criteria that have been listed for the selection of candidate lasants will also be assessed. In particular the question of whether steady-state lasing is possible will be discussed. As the solar radiance must be concentrated many times, and since the IBr absorption is broadband near the peak of the solar spectrum, the laser will quickly heat up and lasing inhibited (by mechanisms to be discussed), unless cooling is provided. In the theoretical study it is therefore assumed that the lasant is maintained around room temperature, the purpose being to examine the potential for lasing under favorable operating conditions. Methods for accomplishing cooling are proposed in Sec. IX. While this study was in progress, an experimental investigation of an IBr laser pumped by a xenon lamp was performed by L. Zapata (4). The objective of that experiment was to demonstrate lasing, and not necessarily to provide definitive quantitative data. Hence, only limited comparisons could be made, but the experimental results were useful in assessing the effects of excessive heating in the actual experimental environment. II. CHOICE OF IBr Broadband absorption is essential for high solar efficiency and there are many compounds which can be photodissociated to yield excited atoms X*. Here only halogens are considered for A. They can be divided into three types: diatomic homo- nuclear molecules A2, diatomic heteronuclear molecules YX, and complex molecules of the form RX, where R is a molecular radical. Solar pumped lasing has already been demonstrated for type RX, using a xenon arc to radiate perfluoropropyliodide C3F7I (5-7). Absorption occurred in the ultraviolet (230-320 nm) so that the fraction of the solar radiation absorbed (solar efficiency) was small. (Chemical recycling was also necessary for continuous working.) Types X2 and YX on the other hand can absorb near the peak of the solar spectrum when both X and Y are halogen atoms. The excited atoms F*, Cl*, Br*, and I* have energies of about 0.1,0.2, 0.44, and 1 eV, respectively, above ground; only Br* and I* have values high enough to give acceptable quantum efficiencies. Lasing characteristics depend on the competition between the rate of reduction of the excited species A* by quenching and the depopulation of the lower level A by the exchange reaction A -I- XY —» A2 + Y. Exchange reactions are possible only for heteronuclear molecules. Since the photodissociation of XY always seems to leave the lighter atom in the metastable excited state, and since the quantum efficiency increases with the atomic number of the lasing atom, it
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