Lasers based on transitions of C, N, O, and Cl atoms (see Chap. 3, Sect. 3.4) have certain common features. First of all, in the active media of these lasers, consisting of the buffer gases He, Ne, and Ar, and the molecular additives CO, CO2, N2, O2, Cl2, CCl4, etc., lasing occurs at IR transitions of the atoms making up the belonging to molecules. Second, the optimal concentrations of molecular additives are very low, ~ 10~2 Torr, which is comparable with the concentration of molecular impurities in highly pure rare gases. We note that lasing was earlier observed at many transitions of the C, N, O, Cl atoms in low-pressure gas-discharge lasers [129].

Lasing mechanisms of NPLs based on C, N, O, and Cl atoms have not as yet been finally established owing to the variety of plasma processes in active media which contain a large number of neutral particles (atoms, molecules, radicals), various types of positive and negative ions, as well as to the lack of information regarding the rate constants of many of them. The processes that are suggested as populating for laser levels include dissociative excitation during collision of a metastable atom of buffer gas with molecules [130, 131], collisional-radiative recombination of atomic ions [131, 132], as well as recombination of positive and negative ions [133].

Despite the sizeable lack of information noted above, IOFAN associates pro­posed quite detailed kinetic models for NPLs based on transitions of Cl [134], C, N [135], and O [136] atoms. It was proposed that the basic channels of populating of levels in lasers operating on C, N, and O atoms are the reactions of three-body recombination C+(N+,O+) + e + M! C*(N*,O*)+ M (M is buffer gas atoms or electrons). In a laser operating on transitions of the Cl atom, along with the three — body recombination reactions, processes of dissociative recombination of molecu­lar ions ClJ and CCl^, and ion-ion recombination of an excited ion (CClJ)* with a negative ion Cl2 were suggested. Unknown constants were determined during adjustment of calculation results to experimental data.