Lasers on Infra-Red (IR) transitions of Xe, Kr, and Ar atoms have the highest energy parameters among the NPLs (see Chap. 3, Sect. 3.1). Apart from high efficiency (n < 2-3 %), the chief merits of these lasers are the low laser thresholds, the full restoration of the medium after radiative influence, and the possibility of obtaining lasing over a wide range of the spectrum (1-3.5 ^m). The record holder in terms of advantages is the laser operating on transitions 5d-6p of the Xe atom (A = 1.73, 2.03, and 2.65 ^m).

From the survey of results of experimental investigations provided in Sect. 3.1 of Chap. 3, one can draw the following conclusions:

1. All the most intensive laser lines belong to the transitions nd-(n + 1)p of atoms Xe, Kr, and Ar (n = 5, 4, and 3 for Xe, Kr, and Ar, respectively). Analogous results in laser spectrums are also obtained with electron beam excitation.

2. Maximal energy parameters were registered for an Xe laser at the 1.73, 2.03, and 2.65 qm lines, which start from the common upper laser level 5d[3/2]10 ; these lines are the most powerful when using various buffer gases (He, Ar, or He-Ar and Ne-Ar mixtures).

3. For lasers operating on transitions of the Xe, Kr, and Ar atoms excited by nuclear radiation and electron beams, one observes a qualitative similarity of experi­mental dependencies of energy parameters on the mixtures’ pressure and composition.

4. For the most intensive laser lines at excitation by nuclear radiation and electron beams, efficiencies were obtained that are close to ultimate values, which testify to the high populating selectivity of the upper nd levels.

Considering these circumstances, as well as the similar structure of energy levels of the Xe, Kr, and Ar atoms (Fig. 3.1), one can conclude that the basic processes of populating nd levels are similar or even identical, so that NPLs operating on IR transitions of Xe, Kr, and Ar atoms should be viewed as one family.

The characteristics of IR NPLs operating on transitions of rare gas atoms were investigated in detail for roughly 40 years in a wide range of experimental condi­tions (see Chap. 3, Sect. 3.1), and a large number of studies [19—39] were dedicated to their theoretical modeling. The processes leading to depopulating of the lower laser (n +1) p levels can be considered well established: collisional quenching during collisions with atoms of the active medium and electrons (at high specific power depositions). However, to date the discussion on the mechanisms for popu­lating the upper laser levels is not yet concluded. The variety of proposed mecha­nisms for populating nd levels (see Table 5.3) is explained by difficulties in registering and researching radiation in the IR range of the spectrum, uncertainties in the rate constants of many plasmochemical reactions, and by significant differ­ences in the experimental conditions in which these lasers were studied.