The largest number of studies were dedicated to analysis of the lasing mechanisms and calculations of the characteristics of NPLs using a He-Cd mixture radiating at transitions of the Cd+ion (A = 441.6, 533.7, and 537.8 nm) [4,106—112]. A diagram of levels of the Cd ion and atom with laser transitions is shown in Fig. 3.5 (see Chap. 3, Sect. 3.3).

At present one can consider the basic populating processes of upper laser levels to be established:

• Levels 4/2F05/2j7/2, which are the upper levels for transitions with X = 533.7 and 537.8 nm, are populated by means of charge-transfer processes

He+ + Cd! (Cd+) * + He (5.17)

with the formation of higher levels 6f, 6 g, 8d, 9 s, etc. [113] and subsequent cascade transitions in 4f states;

• Levels 5s22D5/23/2, which are upper levels for transitions with X = 325.0 and 441.6 nm, are populated by means of charge-transfer and Penning processes

He2+ + Cd! (Cd+) * + 2He (5.18)

He * (21:3S) + Cd! (Cd+) * + He + e (5.19)

As a result of processes (5.17)-(5.19), various states of the Cd+ ion can arise. In a number of studies, for example [108, 109], it was proposed that by means of these processes, all energy-allowed states are populated with a probability proportional to the statistical weight of the given state. This approximation is inapplicable to charge-transfer processes, the probability of which is maximal if the potential energy of the starting ion, with an accuracy up to kTg, is equal to the excitation energy of the final state of the Cd+ ion. At present there is some information about partial rate constants of processes (5.17)-(5.19) for various states of the Cd+ [ПО— 115] ion. Study [112], based on examination of the populating kinetics of individual levels of the Cd+ ion, concluded that the contribution of the charge-transfer reaction (5.18) to populating of levels 5s22D5/23/2 and 6s2S1/2 is roughly an order of magnitude greater than the Penning reaction (5.19).

Relaxation or quenching of laser levels can occur as a result of the ion conver­sion process

(Cd+) * + Cd + He! Cd2+ + He, (5.20)

of radiative transitions, inelastic collisions with electrons, and (for levels above state 6s2S1/2), the Penning process on its own atom

(Cd+) * + Cd! Cd+ + Cd+ + e. (5.21)

Reaction (5.21) was not studied experimentally. In models [4, 106], its cross­section was assumed to be equal to the cross-section of polarization capture.

Among the transitions of the Cd+ ion, much attention was given to the transition with X = 325.0 nm, which is similar to the transition with X = 441.6 nm [112,116] in terms of its luminescence and other parameters. However, lasing under the conditions of nuclear pumping was obtained only in the second of them [116, 117], although for pumping with electron beams, lasing was observed at the 325.0-nm line both in the pulsed mode [118] and in the CW mode [119, 120], while the minimal specific power deposition at the laser threshold was just 10 W/cm3 [120]. The reasons for the lack of lasing at the 325.0-nm line in nuclear pumping conditions have not as yet been finally ascertained. Possible reasons that have been considered include absorption of radiation at the 325.0-nm line by excited atoms Cd*(3P2), quenching of level 5s22D3/2 by helium atoms and plasma electrons and the influence of uncontrolled impurities [107, 110, 117, 120].

Neutralization of plasma in the He-Cd mixture occurs basically by means of dissociative recombination processes of Cd2 molecular ions with electrons. As a result of recombination processes, the formation of an inverse population is possi­ble between certain excited levels of the Cd atom. In experiments, lasing was registered at the lines 1.43 and 1.65 ^m [116] and 361.0 nm [118] with pumping of the He-Cd mixture by uranium fission fragments [116] and an electron beam [118]. The kinetic model [107] included processes with the involvement of excited Cd* atoms, and an attempt was made to calculate some laser characteristics for the 1.65 ^m line. In the opinion of the authors [107], the two-humped laser pulse observed at the 1.65 ^m line may be explained by quenching of the upper laser level by electrons.

Differences in the results of calculations for various models for NPLs based on a He-Cd mixture are generally explained by the shortage of data on rate constants of many important processes. The maximal computed efficiency was obtained at the 441.6-nm line and is < 1 % [106, 111].

Lasing during pumping of a He-Zn mixture by uranium fission fragments was observed at transition 4s22D5/2-4p2P3/2 (A = 747.9 nm) (see Table 3.8), and during pumping by an electron beam, it was observed at transition 5d2D5/2-5p2P3/2 (A = 610.2 nm) [121]. Lasing mechanisms of NPLs based on the transitions of Zn+ and Cd+ ions largely coincide. A kinetic model of NPLs based on a He-Zn mixture was examined in studies [4, 106, 121].

Just as for the He-Cd mixture laser, the main processes of populating the levels 4s22D5/2 3/2 of the Zn+ ion are the charge-transfer and Penning reactions of He2 ions and metastable atoms He*(213S) during interaction with Zn atoms, while for the higher states, it is the process of charge-transfer of He+ ions on Zn atoms. Depopulating of lower laser levels with A = 747.9 nm occurs due to radiative transitions, and that of the transition with A = 610.2 nm by means of the Penning reaction on its own atom (Zn+)* + Zn! Zn+ + Zn+ + e and collisional quenching by electrons. The results of calculation of characteristics of He-Zn mixture NPLs in optimal modes are given in Table 5.10.