**4.1. Lasers operating on first negative system of nitrogen and carbon monoxide**

**N2 +** . When pumping active medium of a high pressure by electron beam with moderate (~3 A/cm2 ) current density was obtained quasi-continuous lasing mode on the first negative system of nitrogen (λ = 391.4 and 427.8 nm) with an efficiency ~1% [114, 115]. In this collision lasers on B2 Σ+ u,v <sup>=</sup> <sup>0</sup> → X2 Σ+ g,v″ <sup>=</sup> 0,1 transitions of nitrogen ion, deactivation of lower level was carried out at low hydrogen concentrations (~0.1%) in He-N2-H2 mixture in the process with proton transfer:

$$\text{N}\_2\text{ }^\ast \text{(X)} + \text{H}\_2 \rightarrow \text{N}\_2\text{H}^\ast + \text{H} \tag{26}$$

In 1996 was obtained laser action at λ = 391.4 nm with excitation of He-N2-H2 mixture by uranium fission fragments on EBR-L [116]. This was the first NPL emitting in the UV spectral region: Laser power was ~10 W, efficiency ~0.01%. To date, it remains to be the most short-wavelength nuclear-pumped laser. On BARS-6 reactor were conducted studies of facility with active core length of 250 cm and volume of 4 liters with the average over the length of the laser element-specific energy deposition to 300 W/cm3 [117]. The lasing threshold on B-X nitrogen ion transitions with λ = 391.4 and 427.8 nm was achieved with low for molecular laser power density of energy deposition 50–60 W/cm3 . It was determined that active medium had non-resonant losses ~5 × 10−5 cm−1. These authors explain the laser efficiency (~0.1–0.2%) was significantly lower than expected. In [118] developed a multicomponent spatially homogeneous model of kinetic processes of NPL active medium on He-N2-H2 mixture. According to the model, the maximum laser efficiency (0.5–0.8%) was achieved at pumping power of 0.5–3 kW/cm3 , for specific pumping power 90–130 W/cm3 the maximum instantaneous efficiency is 0.1%.

Lasing mechanisms of NPLs on ion transitions of cadmium and zinc are very similar. Laser

[110], 60 W output power obtained during pumping by uranium fission fragments at this

Information about cadmium-vapor atomic lasers with ionizing pumping is scarce. During

flux on the lines of 1.648 and 1.433 μm at pumping He-Cd mixture by uranium fission fragments [44]. When pumping He-Cd mixture by an electron beam was obtained laser action on the line 361 nm of cadmium atom [111]. Kinetic model [112] included processes involving excited cadmium atoms and attempted to calculate some laser characteristics for 1.648 μm line.

mercury atoms. A sufficiently high density of cadmium vapor (~3 × 1018 cm−3) was establish‐ ed at a temperature of about 700°C, and such density of cadmium requires consideration of

. When pumping active medium of a high pressure by electron beam with moderate

system of nitrogen (λ = 391.4 and 427.8 nm) with an efficiency ~1% [114, 115]. In this

level was carried out at low hydrogen concentrations (~0.1%) in He-N2-H2 mixture in the

In 1996 was obtained laser action at λ = 391.4 nm with excitation of He-N2-H2 mixture by uranium fission fragments on EBR-L [116]. This was the first NPL emitting in the UV spectral region: Laser power was ~10 W, efficiency ~0.01%. To date, it remains to be the most short-wavelength nuclear-pumped laser. On BARS-6 reactor were conducted studies of facility with active core length of 250 cm and volume of 4 liters with the average over the length of the laser element-specific energy deposition to 300 W/cm3 [117]. The lasing threshold on B-X nitrogen ion transitions with λ = 391.4 and 427.8 nm was achieved with

that active medium had non-resonant losses ~5 × 10−5 cm−1. These authors explain the laser efficiency (~0.1–0.2%) was significantly lower than expected. In [118] developed a multi-

) current density was obtained quasi-continuous lasing mode on the first negative

faster. In addition, cadmium atoms in krypton can be ionized in the Penning processes.

**4.1. Lasers operating on first negative system of nitrogen and carbon monoxide**

S1 state by its own atoms. Perhaps krypton ions charge exchange on Cd will be

He-Xe-Cd mixtures in the radiation region of stationary nuclear

s−1), in contrast to the constant of charge exchange on

g,v″ <sup>=</sup> 0,1 transitions of nitrogen ion, deactivation of lower

N X 2 22 ( ) H N H H + + +® + (26)

experiments was registered laser action (1–2 W) with a threshold density of 1016 n/cm2

reactor investigated in [113]. Measured value of rate constant of Xe2

He-Zn mixture observed on transition of 4s22D5/2–4p2

P3/2 (λ = 747.9 nm)

<sup>+</sup> charge exchange on

. It was determined

s neutron

action by pumping <sup>3</sup>

180 182High Energy and Short Pulse Lasers

transition [44].

Luminescence of <sup>3</sup>

quenching 63

**N2 +**

(~3 A/cm2

collision lasers on B2

process with proton transfer:

He-Cd and <sup>3</sup>

**4. NPL active media on molecular transitions**

u,v <sup>=</sup> <sup>0</sup> → X2

Σ+

low for molecular laser power density of energy deposition 50–60 W/cm3

Σ+

cadmium atoms is small (~10−13 cm3

The [119] searched mixture compounds that improve NPL efficiency on 1<sup>−</sup> —nitrogen system, including study of deuterium and neon impact on the laser parameters. Replacement of hydrogen additive deactivating lower level by deuterium provided no improvements to laser efficiency. The use of neon as the buffer gas at partial pressure of 30 Torr resulted in efficien‐ cy decrease for 30–40% for both wavelengths. It has previously been shown [120] that helium replacement with neon impairs laser parameters for B-X transitions of N2 + . At pumping by an electron beam, lasing energy of mixture with addition of 60 Torr of neon was 1.5 times less than for three-component mixture of He-N2-H2 with a total pressure of 6 atm.

In [121], the constants of quenching processes rates of N2 <sup>+</sup> (B) with nitrogen and helium, as well as two- and three-particle charge exchange processes of He2 <sup>+</sup> to H2, D2, Kr, CO, were determined on the luminescence in 0–0 transition of the first negative system of nitrogen in excitation by ∝–particles of polonium-210.

**CO+** . Gain on the first negative system of carbon monoxide by powerful electron beam pumping was obtained in 1975, the same way as in N2 + -laser, by Waller et al. [122]. Quasicontinuous lasing mode can be implemented with addition of H2, D2 or Kr to He-CO [123]. In [123, 124] by spectra of radioluminescence of the gas mixtures with carbon monoxide was determined constants of quenching rate of CO+ (B) with helium, neon, CO molecules, evaluat‐ ed the upper limit of constant of quenching rate with hydrogen, deuterium and krypton.

Kinetic model of nuclear-pumped laser on B2 Σ+ u,v′→X<sup>2</sup> Σ+ g,v″ transition of carbon monoxide ion was developed in [125]. Unfortunately, the only information given on the results of calcula‐ tions of medium gain, efficiency, but there is no information on considered processes, values of constants for processes rate. This provides for the opportunity to discuss the work. In addition, specified calculated wavelength transition for which the calculations were made, 210 nm; the closest wavelength—211.2 nm—corresponds to 1–0 transition [126]. That is, laser action, according to [125], should not take place from the ground vibrational level of CO+ (B), as the wavelength of 0–0 transition—219.0 nm. It should be noted that the gain in [122] was observed at 0-2 transition (242 nm) and Baldet-Johnson's system (391 and 425 nm, B→A transitions) from the ground vibrational level of CO+ (B).

#### **4.2. Excimer lasers operating on halides of inert gases**

Excimer lasers operating on halides of inert gases have been studied for a long time [127]. At present, they are the most powerful lasers that emit in UV region of spectrum. Optimal operation of excimerlasers corresponds to pumping powers of several megawatts per cm3 and a pressure of several atmospheres. Such pumping powers are achieved by electron beams or space discharge. Radiation of nuclear explosions [128] and ion beams [129] also has been used to pump these lasers.

Excimer lasers can operate in a quasi-continuous mode, as in the photon emission, an excimer molecules pass into the lower dissociating or weakly bound state. Much of the research on creation of NPL operating on the inert-gas halides is associated with XeF-laser (λ = 351 and 353 nm). Experiments on SPR-III reactor have shown that in 3 He-Xe-NF3 mixture the gain on the 351 nm band is about 7°10−3 cm−1 at pumping power q≈5 kW/cm3 [130], and at pumping by uranium fission fragments of Ne(Ar)-Xe-NF3 mixture, registered gain of ~2°10−3 cм−1 at q≈2 kW/ cm3 [131]. Experiments in pulsed nuclear reactors with neutron flux up to 1017 n/cm2 s aimed at obtaining lasing at XeF [131] and XeF, KrF [59] did not yield positive result. The work [132] contains theoretically investigated characteristics of laser operating on Ne-Xe-NF3 mixture at 1 atm with nuclear pumping duration 0.1–1 ms at half-height in the near-threshold lasing region. The lasing threshold is 400–500 W/cm3 , and the maximum calculated efficiency of ~1% is achieved at pumping power of 1.5–5 kW/cm3 . Predicting the possibility to create XeF-nuclear pumped laser is mainly limited by the uncertainty of absorption coefficient in active medi‐ um, which greatly affects calculation results.

The luminescence efficiency in chlorine-containing gas mixtures with xenon (to 15% [133], ~11% [134]) is about three times higher than the efficiency of emission at the transition of XeF. The maximum luminescence efficiency can be achieved at low chloride content (<0.7 Torr [134], 0.05 Torr [135]). In [133] reported about the lasing on XeCl molecule (λ = 308 nm) by pump‐ ing Ar-Xe-HCl(CCl4) mixture by uranium fission fragments in experiments on EBR-L reactor. According to the authors, the narrowing in emission spectrum in 308 nm band, the height of film blackening, 1.3–1.5 times less than at other wavelengths, indicates the presence of laser radiation. However, these data are not sufficient for this conclusion [3].

High radiation resistance of <sup>3</sup> He-Xe-CCl4 mixture was noted in [134, 136]. The densities of electrons and negative ions in the plasma of gas mixture of <sup>3</sup> He-Xe-CCl4 and <sup>3</sup> He-Xe-NF3 irradiated with thermal neutron flux 1011–1014 n/cm2 s into the active core of stationary nuclear reactor, measured in [137].

#### **4.3. Lasers operating on vibrational transitions of carbon monoxide and carbon dioxide**

**CO**. Molecular nuclear-pumped laser operating on carbon monoxide was one ofthe first NPLs, the creation of which was reported in the press [64]; the laser action was observed in vibra‐ tional-rotational transitions of the CO molecule with λ = 5.1–5.6 μm. Potential of medium on carbon monoxide was determined by the fact that, unlike lasers on electronic transitions, active medium of CO-laser does not require high pumping selectivity. It is important to get energy into the broad band of vibrational levels of the ground electronic state of CO molecule. Furthermore due to the autonomy of vibrational subsystem and anharmonicity of molecular vibrations, this energy at relatively low translational temperature redistributing in the process of exchange of vibrational molecules to provide full or partial inversion on vibrational levels of CO [138]. This property associated with the anharmonicity of the vibrational levels, as well as the cascade mechanism of lasing in CO, allows directing significant energy share of nuclear reaction products to the laser level.

Experiments have not yet confirmed this finding. In [64], carbon monoxide pumping at a pressure of 0.13 atm and temperature of 77 K was carried out by uranium fission fragments,

and the radiation power was 2–6 W with an efficiency of 0.1–0.3%. In further work, the authors [64] achieved lasing power of about 100 W by using a multiple-pass resonator with an active length of 120 cm. Lasing on vibrational transitions of CO was also obtained in [139] at excitation of 3 He-CO by nuclear reaction products of 3 He(n,p)T. Laser radiation power at the mixture pressure of 3 atm exceeded 200 W from an active volume of 300 cm3 , and the lasing threshold was reached at F = 3×1016 n/cm2 s.

Excimer lasers can operate in a quasi-continuous mode, as in the photon emission, an excimer molecules pass into the lower dissociating or weakly bound state. Much of the research on creation of NPL operating on the inert-gas halides is associated with XeF-laser (λ = 351 and

the 351 nm band is about 7°10−3 cm−1 at pumping power q≈5 kW/cm3 [130], and at pumping by uranium fission fragments of Ne(Ar)-Xe-NF3 mixture, registered gain of ~2°10−3 cм−1 at q≈2 kW/

at obtaining lasing at XeF [131] and XeF, KrF [59] did not yield positive result. The work [132] contains theoretically investigated characteristics of laser operating on Ne-Xe-NF3 mixture at 1 atm with nuclear pumping duration 0.1–1 ms at half-height in the near-threshold lasing

pumped laser is mainly limited by the uncertainty of absorption coefficient in active medi‐

The luminescence efficiency in chlorine-containing gas mixtures with xenon (to 15% [133], ~11% [134]) is about three times higher than the efficiency of emission at the transition of XeF. The maximum luminescence efficiency can be achieved at low chloride content (<0.7 Torr [134], 0.05 Torr [135]). In [133] reported about the lasing on XeCl molecule (λ = 308 nm) by pump‐ ing Ar-Xe-HCl(CCl4) mixture by uranium fission fragments in experiments on EBR-L reactor. According to the authors, the narrowing in emission spectrum in 308 nm band, the height of film blackening, 1.3–1.5 times less than at other wavelengths, indicates the presence of laser

**4.3. Lasers operating on vibrational transitions of carbon monoxide and carbon dioxide**

**CO**. Molecular nuclear-pumped laser operating on carbon monoxide was one ofthe first NPLs, the creation of which was reported in the press [64]; the laser action was observed in vibra‐ tional-rotational transitions of the CO molecule with λ = 5.1–5.6 μm. Potential of medium on carbon monoxide was determined by the fact that, unlike lasers on electronic transitions, active medium of CO-laser does not require high pumping selectivity. It is important to get energy into the broad band of vibrational levels of the ground electronic state of CO molecule. Furthermore due to the autonomy of vibrational subsystem and anharmonicity of molecular vibrations, this energy at relatively low translational temperature redistributing in the process of exchange of vibrational molecules to provide full or partial inversion on vibrational levels of CO [138]. This property associated with the anharmonicity of the vibrational levels, as well as the cascade mechanism of lasing in CO, allows directing significant energy share of nuclear

Experiments have not yet confirmed this finding. In [64], carbon monoxide pumping at a pressure of 0.13 atm and temperature of 77 K was carried out by uranium fission fragments,

radiation. However, these data are not sufficient for this conclusion [3].

electrons and negative ions in the plasma of gas mixture of <sup>3</sup>

irradiated with thermal neutron flux 1011–1014 n/cm2

[131]. Experiments in pulsed nuclear reactors with neutron flux up to 1017 n/cm2

He-Xe-NF3 mixture the gain on

, and the maximum calculated efficiency of ~1%

. Predicting the possibility to create XeF-nuclear

He-Xe-CCl4 and <sup>3</sup>

s into the active core of stationary nuclear

He-Xe-CCl4 mixture was noted in [134, 136]. The densities of

s aimed

He-Xe-NF3

353 nm). Experiments on SPR-III reactor have shown that in 3

region. The lasing threshold is 400–500 W/cm3

is achieved at pumping power of 1.5–5 kW/cm3

um, which greatly affects calculation results.

High radiation resistance of <sup>3</sup>

reactor, measured in [137].

reaction products to the laser level.

cm3

182 184High Energy and Short Pulse Lasers

Excitation of vibrational levels of CO can occur due to molecular collisions with plasma electrons. In [140] based on calculation of electrons spectrum formed in a molecular gas under the action of ionizing radiation, it was shown that the efficiency of CO nuclear-pumped laser cannot exceed 0.5%. Further work [141] showed the main mechanism of molecular forma‐ tion in the form of dissociative recombination of cluster ions with formation of electronically excited molecules and subsequent collision of these molecules with molecules in ground state. According to the authors [141] plasma chemical processes in active medium can make a significant contribution to the energy pumping into vibrational modes of molecules and allow achieving the efficiency pumping up to 18%. The use of argon as a buffer gas instead of helium should increase 1.5-fold the efficiency of pumping in vibrational levels and reduce by an order the threshold energy for active medium pumping. It should be noted that a record efficiency of electron beam-controlled laser operating on carbon monoxide—63% [142]—was achieved due to Ar:CO = 10:1 mixture.

Presently achieved low parameters of NPL operating on CO, the need to cool down an active medium reaching cryogenic temperatures, apparently, makes carbon monoxide medium insufficient for creation of nuclear-pumped lasers.

**CO2**. The possibility of CO2-laser pumping (λ = 10.6 μm) by nuclear radiation was consid‐ ered in the earliest stages of NPL study [2], and gas-discharge laser operating on carbon dioxide had the highest output parameters for that time. Numerous attempts to create NPL on CO2 have yield negative results. Experiments on pumping <sup>3</sup> He-CO2 and <sup>3</sup> He-CO2-N2 mixtures with products of <sup>3</sup> He(n,p)T reaction showed no gain in band of 10.6 μm at a wide variation of pressure (0.28–0.8 atm) and composition of mixture [143]. Moreover, these experiments showed probe laser radiation absorption, which indicates preferential population of the lower laser level of CO2 when excited by ionizing radiation. Calculations of kinetic processes in CO2- N2-He mixture also support the conclusion on ineffectiveness of direct pumping of CO2-laser with nuclear radiation [144].

Apparently, the most promising method of nuclear energy conversion into radiation on vibrational transitions of CO or CO2 is the creation of nuclear power plant with electroioniza‐ tion laser based on thermionic converter reactor [145, 146].

#### **4.4. Radiation of heteronuclear ionic molecules of inert gases**

Molecular bands in radiation spectra of pairinert gas mixtures were first discovered more than half a century ago. The [147] recorded the band of 507–550 and 496–508 nm in Ar-Xe mixture, and the authors attributed the presence of these bands with emission of heteronuclear molecules or ions. When pumping Ar-Xe mixture by electron beam, the molecular band at 510 nm was detected and the band of 495–460 nm in Kr-Xe was registered for the first time [148]. Two emission systems were observed, in 600–670 and 670–685 nm regions, whenXe was added to Kr flowing afterglow at a pressure of 30 Pa [149]. Kugler [150] obtained similar results for Ar-Xe mixture and discovered new band in Ar-Kr in the region of 605–642 nm. He explained these bands as transitions of neutral heteronuclear molecule formed in the processes of metastable atoms of argon.

In 1975, Tanaka et al. [151] have published data on radiation spectra of 10 pair mixtures of inert gases in the region of 100–700 nm. Molecular bands observed in radiation spectra in the discharge were identified as transitions between states of heteronuclear ionic molecules:

$$\mathbf{M}^\*\mathbf{N} \to \mathbf{M}\mathbf{N}^\* + \mathbf{h}\mathbf{v} \tag{27}$$

where molecular states of M+ N asymptotically correspond to states of M+ +N, and MN+ to the state of M+N+ ; here M, N—atoms of inert gases, and N—a heavier atom. If the plasma of low pressure in an electric discharge in paired mixtures of inert gases has up to 5 similar bands [151], there are no transitions from levels corresponding to the states of atomic ions 2 P3/2 [152, 153] when excited by ionizing radiation of medium- and high-pressure mixtures.

Kinetics of Ar-Kr, Ar-Xe, and Kr-Xe mixtures' excitation by low activity 241Am alpha parti‐ cles was studied in [152, 154]. Constants of processes rates in these mixtures were identified; however, constants' values of a number of processes are underestimated: ~10−15 cm3 /s for twoparticle and ~10−34 cm6 /s for three-particle processes. Emission of Ar-Xe and Kr-Xe mixtures when excited by 210Po alpha particles with activity of ~0.5 Cu investigated in [153, 155–157] determined the rate constants of processes of formation and destruction of levels of hetero‐ nuclear ionic molecules. In [153] includes first noted high luminescence efficiency of (ArXe)+ , (KrXe)+ at pumping by ionizing radiation. Luminescence of Ar-Xe mixture pumped by powerful electron beam was studied in [158], attempts to obtain lasing on transitions of (ArXe)+ at pumping by an electron beam had yield negative results [155, 158].

In [42] was built kinetic model of Ne-Ar mixture relaxation pumped by a hard ionizer with regard to the possibility of lasing on transition Ne+ Ar→NeAr<sup>+</sup> . When using typical rates of plasma chemical reactions, calculations show that lasing is only possible at high pressure (above 16 atm) and powerful pumping (1 MW/cm3 ), and lasing efficiency should not exceed 0.05–0.25%. In this work were considered the triple (with Kr) instead of the binary mixtures of inert gases, as the authors suggested that the constant of deactivation rate of the lower level in exchange processes:

$$\text{NeAr}^{\ast} + \text{Ar} \rightarrow \text{Ar}\_2^{\ast} + \text{Ne} \tag{28}$$

may occur negligible. It was assumed that the lower laser level will be deactivating in the processes with Kr atoms:

$$\text{NaAr}^+ + \text{Kr} \rightarrow \text{Kr}^+ + \text{Ne} + \text{Ar} \tag{29}$$

$$\text{NeAr}^{\circ} + \text{Kr} + \text{M} \rightarrow \text{Kr}^{\circ} + \text{Ne} + \text{Ar} + \text{M} \tag{30}$$
