The spectra of rare-gas atoms have a number of intensive lines belonging to the transitions (n + 1)p-(n + 1)s (where n = 5, 4, 3, 2 for Xe, Kr, Ar, and Ne, respec­tively), which end in metastable states (n + 1)s. The efficiency of populating the upper levels (n +1)p in a recombination-nonequilibrium plasma is very high [80,

81] , so these transitions are promising in the search for lasing. However, in order to obtain lasing on the transitions (n + 1)p-(n + 1)s in cw mode, a high depopulating rate of (n + 1)s-levels is necessary. This can be ensured by two methods: through collisional “quenching” by atoms of the buffer gas (as, for example, in a nuclear — pumped laser at the line of 1.15 qm of the Ar atom [8, 16]), and using additional “quenching” impurities.

The difficulty in choosing the impurity is related first of all to the fact that it must not substantially influence the population of the upper level. This scheme was implemented in a neon NPL operating on transitions 3p-3s of the Ne atom in the lines 585.3-; 703.2- and 724.5-nm [2, 32, 33, 82—91]. Apart from nuclear radiation, electron [92—96] and ion [97] beams were used to pump the neon laser; when electron beams were used, lasing was also observed at the 626.7-; 633.4-; 659.9- and 743.9-nm lines [95]. The quenching impurities in the case of NPLs were М = Ar, Kr, Xe, H2, and with electron beam pumping, NF3 also. Quenching of the lower 3p-levels occurs through the Penning reaction: Ne*(3p) + M! M+ + Ne + e. A diagram of the Ne atom levels with laser transitions is provided in Fig. 3.3.

A neon laser was first pumped with nuclear radiation at VNIIEF in 1985, immediately after the first reports [92, 93] about successful pumping of this laser with an electron beam. These results were published in the public press in 1995 [2,

82] . The basic results of researching NPLs operating on 3p-3s transitions of the Ne atom, obtained in various laboratories, are shown in Table 3.8.

In lasers using binary mixtures Ne-M (M = Ar, Kr, Xe), lasing was observed at two lines—703.2 and 724.5 nm—while with a change in the concentration of the quenching impurity of M, there was restructuring of the lasing spectrum [82­84]. By way of illustration, Fig. 3.4 shows data for the mixture Ne-Kr. The ranges of impurity M pressure at which simultaneous lasing was observed at two lines are diverse. Thus, in the mixture Ne-Ar, simultaneous lasing occurs at PAr«28- 55 Torr, in the mixture Ne-Kr—PKr« 30-50 Torr, while for the mixture Ne-Xe

о

о

cp

О

ON

W

this pressure range is very narrow (close to РХе ~ 28 Torr). Similar results were also obtained in the mixtures He-Ne-Kr (Ar) [89].

The use of hydrogen as the quenching impurity is expedient only when there are sufficiently high specific power depositions (q > 1 kW/cm3) because of competition of the processes of depopulation of 3s-levels of the Ne atom and charge-transfer of the molecular ions Ne2+ on molecules of H2 [83, 85]. Therefore, the report [88] about obtaining lasing at the 585.3-nm line in NPLs using the mixture He-Ne-H2 for q ~ 100 W/cm3 is doubtful.

The ці ~ 0.1 % obtained in experiments for neon NPLs is much lower than in the case of NPLs operating on IR transitions of rare gas atoms, although in experiments by FIAN associates with electron beams for an He-Ne-Kr laser (A = 585.3 nm), ці = 1.6 % [94] was noted. As was shown by the results of theoretical analysis [98], the maximal efficiency of a neon laser at the 585.3-nm line cannot exceed 0.5 %, therefore the high efficiency cited in study [94] is evidently explained by an error in determining the energy deposition. In later studies by FIAN associates, values of ці = 0.1-0.2 % are cited for the neon laser at the 585.3-, 703.2-, and 724.5-nm lines.

By 1980, a report [99] was published stating that lasing had been obtained at transition 5s'[1/2]1°-3p'[3/2]2 of the Ne atom Ne (A = 632.8 nm) while the mixture 3He-Ne was excited by nuclear reaction products 3He(n, p)3H. According to the authors of [99], the laser efficiency was around 0.03 %, and the laser threshold was achieved at a very low thermal-neutron flux density of ФгА = 2 x 1011 cm~2 s_1. The results of these experiments are doubtful, and are the subject of discussion in references [100, 101]. The simple evaluations cited in study [100] show that under the conditions of [99], it is not possible to obtain lasing even in the maximal case, when all power deposited to the active medium is transferred without losses to populate the 5s'[1/2]1°-level and this level is not “quenched” in collisions with atoms. In later experimental investigations, it is noted that attempts at obtaining lasing at the 632.8-nm line with excitation of the mixture He-Ne using an ion beam [102] and radiation of a stationary nuclear reactor [103] yielded negative results. Numerous investigations [60, 103—107] of the luminescence spectra of the mixture He-Ne, excited by various types of nuclear radiation, showed the absence of a

632.8-nm line. The data cited allow us to conclude that the information [99] about creating a low-threshold NPL at the 632.8-nm line was erroneous.

In conclusion, let us examine the possibility of creating NPLs operating on the transitions (n + 1)p-(n + 1)s of other rare gas atoms: Xe, Kr, and Ar. Such explor­atory experiments were carried out at VNIIEF on the LUNA-2M setup in a range of 700-1,000 nm for those transitions (n + 1)p-(n +1)s of Ar, Kr, and Xe atoms at which maximal values of luminescence efficiency were obtained [81]. Studies were performed on Ar, Kr, and Xe, and the mixtures He-Ar (Kr, Xe) and Ar-Xe at pressures of up to 2 atm. For quenching of the lower (n +1)s states, impurities of the molecular gases CO, H2, N2, and NF3 were used, as well as Kr and Xe at partial pressures of 8-60 Torr. The numerous experiments did not yield a positive result, which possibly may be explained by the insufficient rate of quenching of the lower laser levels and/or the essential reduction in the populations of upper laser levels in the presence of impurities. The absence of lasing can be provoked by the fact that the experiments were conducted at low specific power depositions (q < 50 W/cm3), so that the small-signal gain could be deficient for achieving the lasing threshold. It is helpful to conduct such research experiments at higher specific power deposi­tions, for example using beams of fast electrons. This is confirmed by the successful experiments [108] in pumping of a laser using the mixture Ar-NF3 (transition 4p-4s of an Ar atom, X = 750.4 nm) by an electron beam.