The mechanism of a laser operating on the transition 7p2P3/2-7s2S1/2 of the Hg+ ion (A = 615.0 nm) was considered in studies [4, 106, 122]. Populating of the upper laser level 7p2P3/2 occurs as a result of the charge-transfer process He+ + Hg! (Hg+)* + He. For low concentrations of Hg atoms, populating of the level 7p2P3/2 can also occur by means of the Penning process with the participation of excited mercury atoms: He* + Hg* ! (Hg+)* + He + e [122]. The basic quenching channels of the upper laser level are the Penning reaction on its own atom (Hg+)* + Hg! Hg+ + Hg+ + e and collisions with electrons [4, 106]. Calculations in [4, 106] show that at the 615.0-nm line, even in optimal modes, the energy laser characteristics are not great (see Table 5.10).

Significantly higher output energy parameters were obtained for NPLs operating on the transition 73S1- 63P20 of the Hg atom Hg (A = 546.1 nm) when the mixture He-Xe-Hg-H2 was used (see Chap. 3, Sect. 3.3). The lasing mechanism and the possibility of pumping this laser with nuclear radiation were examined in the studies [122, 123] before lasing was obtained in experiments with the EBR-L pulsed reactor [116].

The most detailed kinetic models of the mercury laser at the 546.1-nm line are presented in studies [88, 124, 125]. The main populating channel of level 73S1 is dissociative recombination Hg. j~ + e! Hg * (73S1) + Hg, and the populating effi­ciency of level 73S1 by means of this process may reach 80 % [123,124]. Formation of Hg2 ions occurs in a sequence of processes: Xe^ + Hg! Hg+ + 2Xe, Hg+ + Xe + M! XeHg+ + M, XeHg+ + Hg! Hg^ + Xe (Xe is the buffer gas, M is a third particle). The presence of helium in the active medium leads to a reduction in the electron temperature and an increase in the rate of dissociative recombination of Hg2 molecular ions with electrons. The use of an additional admixture (in this case Н2) for selective quenching of the lower levels 63P20 is a specific feature of this laser. We note that even before the appearance of the first lasers, the experiments of [14] were carried out on quenching of the levels 63P0012 of the Hg atom during collisions with H2 molecules, in which amplification was obtained at all lines of the mercury triplet: 546.1, 435.8, and 404.7 nm.

The lasing mechanisms of NPLs based on metal vapors, in which the necessary concentration of vapors (~ 1 Torr) was created by means of thermal evaporation of metals inside the laser cell, were examined above. A number of studies by MIFI associates (see [126, 127], for example), examine a different lasing mechanism, based on the formation of excited ions or atoms directly as a result of bombarding the metallic layer with charged particles. In the opinion of the authors [126, 127], this “atomization” mechanism of laser operation, which does not require a buffer gas, and forms excited particles directly in the upper laser states, makes it possible to increase laser efficiency by a factor of five to seven. To confirm this hypothesis, studies [126, 127] cite data that was obtained in experiments with the VIR-2 M pulsed reactor at irradiation by a neutron flux of two laser cells that have thin cadmium layers deposited to the walls and are filled with 3Не at a pressure of

1.1 atm. As the authors of [126, 127] state, the output laser power at the 441.6-nm line was < 3 mW, with anomalously low temperatures of laser cells of 460 and 510 °K. At such temperatures, the concentration of cadmium vapors due to thermal evaporation was very low, <2 x 10~4 Torr.

Based on a very limited number of experiments, one cannot draw an unequivocal conclusion regarding the presence of lasing effect in studies [126, 127], because signals from the photodetectors were too low and scarcely exceeded the level of electrical and radiation noise. If one assumes that lasing did indeed occur, then as the calculations in [128] show, the results of this experiment can be entirely explained based on the traditional kinetic model, if one allows for the effect of atomization of the cadmium layer by products of the nuclear reaction 3He(n, p)3H and subsequent injection of atomized cadmium atoms into the active medium.