Specific Chemical Laser Systems

Contents Population Inversion and Molecular Amplification. Energy-partitioning in Elementary Chemical Reactions Vibrational Relaxation. Requirements for Laser Oscillation. Design Parameters of Pulsed Chemical Lasers. Specific Chemical Laser Systems. Future Chemical Lasers. Present Perspectives of High-Power Chemical Lasers. Kinetic Information through Chemical Laser Studies. 

This article deals with a field of research on the borderline between physical chemistry and laser physics. As it is intended to combine aspects of both areas, molecular amplifiers based on partial or total vibrational inversion are first characterized in general, after which the generation, storage, distribution, and transfer of vibrational energy in chemical processes is reviewed. There is a brief discussion of the experimental requirements for laser oscillation and associated hardware problems. Experimental results for specific chemical laser systems are then surveyed and the prospects for high-power chemical laser operation considered. The concluding sections are devoted to the contribution of chemical lasers to reaction kinetics and their other uses in chemistry. 

Specific Chemical Laser Systems Table 5 (continued)... 

The oxygen-iodine chemical transfer laser, 02( A) + I( / 3/2) 02( 2)+ I( /, /2)> based on the same electronic transition as the iodine photochemical laser, I( 7, /2) I( 3/2). and a few systems operating on pure rotational transitions are among the recent developments in chemical laser research. Other electronic lasers such as the iodine photochemical laser and the large group of excimer lasers are also classified sometimes as chemical lasers. Yet, most chemical laser systems utilize vibrotational transitions, almost exclusively of diatomic molecules. Our discussion will be confined to this type of chemical lasers. To emphasize the nonequilibrium characteristics and the time factor we shall consider only pulsed lasers. We shall not discuss important subjects such as optical properties, gas dynamic factors, and computational methods. As specific guiding examples we shall refer to the well-studied F-l-H2->HF-h H laser and the relatively simple (only one active vibrational band) Cl -I- HBr- HCl -I- Br system. ... 

Several reviews of the chemical laser literature have appeared, but until quite recently no discussion was available which documented the engineering development of specific laser devices. Fortunately the Handbook of Chemical Lasers has recently appeared which reviews the detailed historical development of the chemical laser field and provides specific discussion of virtually every type of chemical laser that had been developed by the end of 1974. This book also provides general discussions of the optical, kinetic, and gas dynamic aspects of chemical laser operation. The present review is restricted primarily to discussions of the use of the chemical laser in the laboratory and of current research directed toward the discovery of new chemical laser systems. No attempt is made here to provide a complete bibliography of the chemical laser literature since 1974 the Handbook of Chemical Lasers should be consulted for references prior to 1974. 

Other methods of excitation are effective or necessary for certain gain media. For example, certain energetic chemical reactions produce molecules in excited states. These excited molecules may then comprise the upper laser level of an inverted-population system. A specific example is the hydrogen fluoride "chemical laser" wherein excitation is provided by the reaction of hydrogen gas with atomic fluorine. Another method of excitation is simply the passage of an electric current through a semiconductor device. This serves as the exciter for diode lasers. 

The observation of the induced emission, its time behavior and threshold conditions allow to study with new techniques details of chemical reactions which lead to specific states of the molecular or atomic reaction product A quantitative study of such laser systems will also yield information about collisional deactivation rates of the excited states (see also Section 111.4). 

The preparation of nonequilibrium level or species populations is the first step in any kinetic experiment. The introduction of lasers to chemical research has opened up new possibilities for preparing, often state-selectively, the initial nonequilibrium states. However, the subsequent time evolution of the molecular populations occurs almost invariably along several relaxation pathways. Some of which, like intra- and intermolecular vibrational energy transfer in infrared multiphoton absorption experiments, may interfere with the exciting laser pulse and/or with the specific process investigated. In such cases, as in chemical laser research, one has to interpret the behavior of complex nonequilibrium molecular systems in which the laser radiation plays of course a major role. This establishes the link between the present article and the general subject of this volume. 

Resonant multi-photon ionization (REMPI). This is a variant of MPI described above, in which one or more photons promote a molecule to an electronically excited state and then additional photons generate ions from the excited state. The power of this method in the study of chemical reactions is its selectivity. In a chemically reacting system, individual reactants and products can be chosen by tuning the frequency of the laser generating the radiation to the electronic absorption band of specific molecules. 

There is a large volume of contemporary literature dealing with the structure and chemical properties of species adsorbed at the solid-solution interface, making use of various spectroscopic and laser excitation techniques. Much of it is phenomenologically oriented and does not contribute in any clear way to the surface chemistry of the system included are many studies aimed at the eventual achievement of solar energy conversion. What follows here is a summary of a small fraction of this literature, consisting of references which are representative and which also yield some specific information about the adsorbed state. 

In photo CVD, the chemical reaction is activated by the action of photons, specifically ultraviolet (UV) radiation, which have sufficient energy to break the chemical bonds in the reactant molecules. In many cases, these molecules have a broad electronic absorption band and they are readily excited by UV radiation. Although UV lamps have been used, more energy can be obtained from UV lasers, such as the excimer lasers, which have photon energy ranging from 3.4 eV (XeF laser) to 6.4 eV (ArF laser). A typical photo-laser CVD system is shown schematically in Fig. 5.14.117]... 

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