HF/DF chemical laser

Fluorine reacts with ammonia in the presence of ammonium acid fluoride to give nitrogen trifluoride, NF. This compound can be used as a fluorine source in the high power hydrogen fluoride—deuterium fluoride (HF/DF) chemical lasers and in the production of microelectronic siUcon-based components. 

There has been a great revival of interest in nitrogen fluoride over the last decade because of its potential applications as a high-energy oxidizer for HF-DF chemical lasers, where it can replace elemental... 

Nitrogen trifluoride, first synthesized in 1928 by electrolysis of molten NH4F-HF, is the only nitrogen fluoride which has attained some commercial importance. It is presently produced by the direct fluorination of ammonia with fluorine in the presence of molten ammonium fluoride. Since about 1975, NF3 has been used as the fluorine source in high-power HF/DF chemical lasers. 

NF3 H2(D2) mixtures have gained importance in HF (DF) chemical lasers for more details, see Fluorine Suppl. Vol. 3, 1982, p. 128ff. 

The formation of F Pj atoms from the bimolecular reaction of NO with F2 is extensively used in HF, DF chemical lasers ... 

The designs for several small-scale cw HF(DF) chemical lasers have been given in the literature. " These devices deliver power outputs of a few watts when operated with mixtures of H2(D2), SFe, and helium. Operation as an HCl chemical laser is also possible in this type of laser with either the Cl + HI > HCl + I or H + CI2 -> HCl + Cl reactions, albeit with somewhat lower power outputs than the HF(DF) lasers. Figure 3.8 illustrates one such device. An electrical discharge of about 1 kW(DC, RF, or micro-wave) is commonly used for dissociation of SFg to provide a source of F atoms. Hydrogen is injected by means of small orifices in a direction transverse to the primary flow of partially dissociated SFg in a helium diluent. The optical cavity is aligned transversely to the flow direction as shown in Figure 3.8. The output of such lasers usually consists of several P-branch transitions in the 1 ->0, 2-> 1, or 1 ->0, 2-> 1, and 3->2 bands for HF or DF, respectively. Operation as an HCl laser produces P-branch transitions in the... 

A repetitively pulsed HF or DF chemical laser is used to excite either HF or DF to the V = 1 level in a fluorescence cell. A sufficient quantity of an inert diluent gas such as argon is employed in the cell to ensure rotational thermal-ization and to provide a buffer against diflFusive deactivation at the cell wall surfaces. Fluorescences from HF(v = 1) or DF(v = 1) levels, and fluorescences from the vibrational bands of admixed gases, are isolated by narrow-band interference filters. The temporal behavior of the fluorescence from a... 

This experiment demonstrated the possibihty of a purely chemical laser. The new system exhibited large gain and considerable output power. About 500 watts continuous power at 10 jum have been achieved with the DF-CO2 system and about 1 Watt for the HF-CO2 laser 409a)... 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

Efficient, purely chemical laser operation is possible in hydrogen halide-CO2 transfer lasers, as developed by Cool and coworkers 83>. In these lasers no external energy sources are required. The systems, which operate on the mixing of commercially available bottled gases, are HC1—CO2, HBr—COg, DF— CO2, and HF—CO2. The pumping scheme for a DF—CO2 laser, for instance, is as follows ... 

The molecular beam technique already has been discussed in great detail, as have the principles of laser operation. With the chemical laser, the light emitted from the excited product molecules, HF or DF in this case, is monitored as a function of time. The F atoms are usually produced by flash photolysis of a fluorine containing compound such as F2 or CF3I. By varying the tuning, the details of the time dependence of the vibrational energy level populations can be studied. The rotational relaxation time is usually too short to be studied by this technique. With the chemiluminescence method. 

VibrationaOy Excited Hydrt en Halides.—Because it is relatively easy to construct pulsed chemical lasers which emit lines in the (1,0) bands of HF, DF, HCl, DCl, HBr, and DBr, collisional processes involving these molecules in their (v — 1) states have already been investigated quite extensively. An unusually elegant experiment has been carried out by Odiome, Brooks, and Kasper. " They compared the production of KCl in the slightly endothermic reaction... 

Continuous-wave HF/DF lasers are designed with elements best illustrated by considering a simple combustion-driven supersonic diffusion laser as an example. In this case, no external power supply is required the laser operates purely on chemical energy. Figure 5 presents the main elements that make up this device. The laser combustor... 

Lasers with CO as an active medium can also be constructed. This laser type yields a large number of lines in the region 5.1-5.6 /zm. In order to achieve efficient operation, the discharge tube must be cooled to low temperatures which complicates practical use. It is simpler to use HF or DF lasers, which give lines in the 2.8-4.0 /zm region. Such lasers are examples of chemical lasers for which the active molecules are formed in the discharge tube from the supplied gases H2/D2 and SF0. 

Two very different techniques (a flow system with mass spectrometric detection [17] and a measurement of relative gain on HF vs. DF lines in a chemical laser [18]) gave very similar results (a) (b) 1.45 0.03 [17] and 1.42 0.1 [18]. The "prior" statistical expectation would be (a) (b) = 0.88. The large difference could be explained [19] on the basis of toe information theory of Bernstein and Levine. 

Current Interest in fluorine ased chemical lasers extends from HF and DF lasers t>ased on F atom reactions into the area of elementary processes involving electrcmically excited radicals such as NF. Also, the chemistry of NF cuid OF radicals is of considerable Interest, and we sumnarize here selected aspects of the kinetic behaviour of these species. 

The experimental results which we will describe are primarily those obtained in this laboratory but a few experimental data exist which have been collected elsewhere. Our experimental program in fluorine atom chemistry has been motivated primarily by two facts which have also been important to studies performed by other methods (1) atomic fluorine abstraction of hydrogen atoms from appropriate molecules has been demonstrated to be an important class of reactions for chemical lasers W. In particular, the reactions of F + H2 HF + H and F + D DF + D have been investigated in great detail by various theoretical and experimental approaches (3-11) the latter reaction provides us with an example from the general class of reactions of fluorine atoms with diatomic molecules. (2) Substitution reactions of fluorine atoms with unsaturated hydrocarbons Involving the formation of C-F bonds frequently are observed to proceed through a "complex" which... 

This review is organized as follows Section 3.2 presents a brief discussion of the HF(DF) and CO chemical lasers which are at present the most highly developed systems for practical applications Section 3.3 outlines recent uses of small chemical lasers in laboratory research and Section 3.4 is devoted to a general discussion of problems in the search for new chemical lasers, with particular emphasis on electronic transition lasers capable of operation at visible wavelengths. 

There are, at present, two major types of chemical lasers which are capable of the continuous efficient conversion of chemical energy into laser radiation. These are the HF(DF) lasers based on the reaction... 

The design of present large scale HF(DF) and CO chemical lasers exemplifies the use of gas dynamic techniques for the achievement of a favorable kinetic environment for laser operation. Besides the aforementioned importance of rapid gas dynamic mixing, flow control is essential for other reasons. Supersonic expansion can provide the means to freeze large concentrations of active atoms or radicals initially created by thermal dissociation. Moreover, such a flow can permit precise control of the translational and rotational temperatures within the reaction zone. Supersonic flows also provide a high power per unit cavity volume and minimize flow pumping requirements through exhaust gas pressure recovery. 

The kinetics of the chemical reaction and energy transfer processes in HF(DF) has been extensively studied and reviewed. Elaborate computational programs have been developed to permit optimization of high power lasers. Present models include the essential aspects of reaction and deactivation kinetics, diffusive mixing, and the interaction with the cavity... 

Just as in the case of HF(DF) lasers, detailed computer codes have been successfully used for laser performance prediction. One unsolved problem in the development of large purely chemical CO lasers has been the lack of suitable means for the production of O atoms and CS radicals which are as convenient as those employed for F-atom generation in the HF(DF) lasers. 

Although present development of these lasers is directed toward high powers, necessitating relatively sophisticated gas dynamics, both the HF(DF) and CO chemical lasers were operated as purely chemical lasers of great simplicity at an early stage in their development. Figure 3.6 illustrates the concept of the purely chemical HF, DF, HF-C02, and DF-CO2 chemical... 

J. A. Shirley, R. N. Siieo, R. R. Stephens, and T. A. Cool, Purely Chemical Laser Operation in the HF, DF, HF-CO2, and DF-CO2 Systems, presented at the AIAA Ninth Aerospace Sciences Meeting, New York, paper 71-27, January, 1971. 

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