In quasi-CW NPLs operating on transitions nd-(n +1) p of Xe, Kr and Ar atoms, the depopulating of the lower (n +1) p levels occurs as a result of collisional quenching in collisions with atoms. The efficiency of quenching, which depends on the pressure and composition of the gas mixtures, significantly influences not only the energy characteristics of the NPLs, but also the laser spectrum. The laser characteristics can be influenced not only by the quenching processes of the nd and (n +1) p levels by atoms of the medium, but also by the quenching and mixing of these levels by plasma electrons, the influence of the latter being especially appreciable for q > 103 W/cm3.

Let us examine the spectral characteristics of the most studied NPLs operating on transitions of the Xe atom (see Fig. 3.1a). Experiments showed that the laser spectra depend not only on the parameters of the cavity mirrors and the specific power depositions, but also on the type of buffer gas, the pressure, and the ratio of mix components. The list of laser lines and transitions for the different mixtures compiled on the basis of Sect. 3.1 in Chap. 3 is given in Table 5.4. The spectra of the Xe laser in experiments with electron beams were basically similar [57]; in addi­tion, the 3.43 pm laser line, belonging to the transition 7p [5/2]2-7 s[3/2]10 [24], was observed.

The level 5d [3/2]10, from which the most intensive laser lines 1.73; 2.03 and

2.65 pm begin, is a resonance level. The radiation time of the isolated Xe atom in this state is equal to 0.58 ns [55], but with taking into account the imprisonment of the resonance radiation, about 200 ns [36].

From the data shown in Tables 3.1, 3.4, and 3.5 (see Chap. 3, Sect. 3.1), it follows that when a He-Xe mixture is used, the 1.73, 2.03, 2.65, and 3.65 цш lines are the most intensive. In the Ar-Xe mixture, lasing occurs primarily at the 1.73 and

2.65 цш lines, in the mixture Kr-Xe at the 2.63 цш line, and in pure Xe at the 3.51 цш line. All of the laser lines noted above (except for 3.65 цш) belong to the 5d-6p transitions. In the Ne-Xe mixture, low-power lasing was observed at the 1.73, 2.03, and 2.65 цш lines [58].

According to the model proposed in study [33], regardless of the type of buffer gas, the level 5d[3/2]10 of the Xe atom is populated initially; it is the upper level for the three most intensive lasing lines: 1.73, 2.03, and 2.65 цш. Populating of this level occurs selectively with an efficiency close to 100 %, through the dissociative recombination process Xe2+ + e. The absence of 1.73, 2.03, and 2.65 цш lines in the Kr-Xe mixture and pure Хе is explained by the high rates of collisional quenching of the 5d[3/2]10 level by Kr and Xe atoms.

The weaker 2.48, 2.63, 3.37, and 3.51 цш laser lines are also present in Не-Хе, Ar-Xe, and Kr-Xe mixtures and pure Хе; these begin with the levels 5d[5/2]30, 5d [5/2]20, and 5d[7/2]30.

Populating of these levels occurs as a result of collisional intra-multiplet transi­tions during collisions with He, Ar, Kr, and Xe atoms in ground states. In the He-Xe mixture, the 3.65 цш laser line, belonging to one of the 7p-7s transitions, was also observed [59]. It is possible that the appearance of this line is related to populating

of the 7p level as a result of the processes of collisional-radiative (three-body) recombination and subsequent cascade transitions. Indeed, experimental data [59] show that the optimal concentration of Xe for this line is much lower than for the 2.03 and 3.51 ^m lines. The absence of the 3.65 ^m line in mixtures with heavy buffer gases (Ar, Kr) may be explained by the fact that in this case, processes of the three-body recombination were suppressed because of the high electron temperature.

There are no reliable data in the literature on the rate constants for processes of collisional quenching of the 5d levels of the Xe atom, which makes analysis of experimental results and calculation of laser characteristics difficult. Such infor­mation exists for lower 6p levels (Table 5.5).

Laser spectra depend on the rates of processes of collisional quenching of lower 6p levels. From a comparison of the NPL energy characteristics shown in Sect. 3.1 of Chap. 3 and the data of Table 5.5, it is clear that the most powerful lasing is observed for lines with high rate constants of the processes of collisional quenching of lower 6p levels: the 2.03 and 2.65 ^m lines for the He-Xe mixture and 1.73 and

2.65 ^m lines for the Ar-Xe mixture. The addition of small amounts of He to the Ar-Xe mixture, which is of little consequence on the specific power deposition, leads to the appearance of powerful lasing at the 2.03 ^m line and elimination of competition of the 1.73 and 2.03 ^m lines (see Fig. 3.2), which may be explained by efficient quenching of the 6p[3/2]1 level by He atoms.

In the Ne-Xe mixture, almost all of the 6p levels were quenched with inadequate efficiency, so the energy characteristics for NPLs operating on this mixture are not great. As was shown in study [58], small additions of He or Ar to the Ne-Xe mixture led to a sharp increase in output power at the 2.03 and 2.65 ^m lines.