from 300 to 800 K reduced the peak value of a at 4200 A by 26%, but the AX increased correspondingly and hence the temperature had little effect on D. The absorbed photons cause an electronic transition to a higher upper energy level (3) (Fig. 1), and in Br2 there are two potential curves associated with such transitions in the visible. The transitions obey the Franck-Condon principle, and only those electrons which arrive in the upper level above the line AB can cause dissociation, Br2 + hv Br* + Br, corresponding to only the shaded part of the absorptivity curve (3). Below AB, the molecule ends in an electronically excited vibrational state. The transitions into the 3IL state all result in dissociation, but the cross section is probably lower. It will be assumed only a fraction Fof the absorbed photons creates Br*, while the remainder causes heating of the gas, with F somewhere between 0.5 and 0.9. It is convenient to summarize the reactions in an energy flow diagram (see Fig. 2). The Br* can hand over energy to CO2 to raise it to an asymmetric oscillation mode (001) at about 0.3 eV: CO2 + Br* —> CO2 (001) + Br + A£\, with ~ 0.2 eV, and a rate coefficient « 6 x 10 12 cm3s'' (4). The Br* can also be deactivated by collisions with CO2 which end up in levels other than (001) [rate coefficient X2 10-11 cm3s-1 (5)], and by collisions with Br2 [X3 = 4.7 x IO13 cm3s-1 (6)]. The rate coefficient for deactivation by He is negligible. The CO2 (001) can be deactivated by collisions with CO2 [X4 = 10-14 cm2s"‘ (7)], He [X5 = 10-14 cm3s-1 (7), Br and Br2 [A'e ~ 8 x 10“15 cm3 s-1 (8)]; however, it will be seen that Br << Br2 under usual conditions. The CO2 (001) can then lase into the lower level, CO2 (100).
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