The energy flow chart for a laser consisting of molecular absorber M, a molecular lasant Y, and a coolant Z is included in Fig. 2. Material M is a compound containing the halogen X, and dissociates to yield the excited specie X*. The excitation energy of X* is then handed over to the upper level Y, of the lasing medium Y. The lower lasing level is YL. The rate coefficients are defined as shown. The absorption coefficients e (mole-1 /cm-1) for the halogens and halogen compounds (10) are plotted vs wavelength and compared with the solar radiance (W m-2 A-1) (11) in Fig. 6. It can be seen that the three compounds of the perfluoral- kyl halide family have high absorption in the ultraviolet and that several materials can absorb up to 6000 A. The absorbing power of a single gas D = <FAXcr, where <r(cm2) = 1.66 x 10-21 e and D is averaged over wavelength X. The o-(x) can be expressed by a single or the sum of several Gaussian curves vs X, and AX is the full width of the curve at the rms value of a. Values of D are given in the fifth column of Table 1. Only a fraction F of the absorbed photons will yield the excited state X*, and F is between 0.5 and 1 for most halogens. The “absorber storage” is a measure of the density of excited halogen atoms X* present for a given amount of solar radiation. The rate of production of X* is proportional to D, while the loss rate depends on the rate constants k^ k2, and k3 (Fig. 2). If k3 » ki, k2, then X* is essentially determined by D and k3, and a figure of merit for absorber storage is defined: Fs = D/k3. (11) A low value of F, indicates that the absorbing material M is deexciting X* by collisions, and a high value of D does not necessarily guarantee lasing.
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