Future Chemical Lasers

For the exploitation of chain reactions, we follow the discussion of Basov et al. l . With reference to the discussion of population inversions in Section 4 (14), we consider first the relationship between chemical pumping P(t) and relaxation L(t) which determines whether the reaction proceeds with or without an inversion. The temporal dependence of the level populations is given by the set of balance equations (16). Various types of temporal dependence of the pumping function P ( ) now have to be investigated. If the initial external energy input produces a certain concentration of active centers n, the pumping rate in the case of a linear chain reaction is given as 

The rate of chemical pumping in the case of branched chains can be written as 

The chemical laser systems suggested by this reaction and other reactions of the same type are numerous, and more work along these lines is to be expected. 

The list of unsuccessful attempts to find new chemical laser reactions is very long and will not be discussed in detail here. The reader is referred to the discussion of prospects at the first chemical laser conference which appeared as a supplement to Applied Optics 32 . A new approach of a more general nature is the photorecombination laser first suggested by R. A. Young 85 as early as 1964 and treated in detail by Kochelap and Pekar 86 . In describing this principle, we follow in part the argument of A. N. Oraevskii 87 . A number of chemical processes give rise to the emission of a photon such that this emission is not a consequence but a necessary condition of the elementary act. 

Alternatively the radiative lifetime rraa may be decreased by stimulated emission to 

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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. 

A considerable data base of quenching and energy transfer rate constants for NF(a) and NF(6) has been established. Many of the reactions of importance for chemical laser systems have been determined, although the greater majority of these measurements have been made at room temperature. As noted in Sec. 7.1, it will be of interest in future studies of NF kinetics to examine the temperature dependencies of the rate constants. 

Thus the paper serves three goals to provide an introduction to chemical lasers to review the literature up to spring 1972 and to examine some current concepts and perspectives in order to point out possible directions for future developments in this field. 

The gas reactions listed in Table 2 have high rates at room temperature and emission occurs not too far in the infrared. These restrictions are due to limitations of the experimental method which may be overcome in the future. The table could be considerably enlarged by including alkali-metal reactions which have largely been studied by molecular beam methods. 21> Though much discussed, chemical lasers on alkali halides have not yet been realized experimentally. Other results, obtained for instance by flash photolysis/absorption studies, or by the study of combustion, are less detailed and axe not included here. But even in this limited form. Table 2 indicates that nonequilibrium distributions which can lead to molecular amplification are often found and are perhaps the rule rather than the exception in simple chemical reactions. 

The future prospects for selective chemistry research as an aspect of laser development are not as bright as several years ago. With the exception of the quest for a visible chemical laser, most new laser research is oriented towards systems that are patently nonmolecular. The leading candidate for very high power applications is the free electron laser (PEL). In this device, the interaction of a relativistic electron beam with a periodic magnetic structure produces coherent radiation. Such devices on paper can have substantially higher power and efficiency than electric molecular lasers. For moderate power applications, advances in solid state lasers, nonlinear optical conversion processes, and tunable solid state media offer the prospect of broadly tunable compact sources. At low powers, diode lasers and diode laser arrays are gaining increasing application and hold out the promise, when used with solid state media, of versatile tunable sources. 

An extremely metastable excited state is not itself a suitable laser candidate because the optical gain is directly proportional to the radiative rate. One is faced with a dilemma If the only states diat can be efficiently produced are extremely long lived, how can one hope to build a laser A possible solution to this problem is found in the only electronic transition chemical laser yet demonstrated, the chemical oxygen iodine laser (COIL). In the COIL chemically produced, highly metastable 02( A) resonantly transfers energy to atomic iodine, and an inversion is produced between the Pi/2 and P3/2 iodine levels and atomic lasing occurs at 1.315 im. This laser was predicted by Derwent and Thrush [11] in 1972 and was demonstrated by McDermott et al in 1977 [12]. This device has been described in numerous papers [12-16], and we do not discuss it further. It clearly demonstrates the concept of a transfer-laser and may serve as a model for future visible lasers using this two step approach. 

The examples given above represent only a few of the many demonstrated photochemical appHcations of lasers. To summarize the situation regarding laser photochemistry as of the early 1990s, it is an extremely versatile tool for research and diagnosis, providing information about reaction kinetics and the dynamics of chemical reactions. It remains difficult, however, to identify specific processes of practical economic importance in which lasers have been appHed in chemical processing. The widespread use of laser technology for chemical synthesis and the selective control of chemical reactions remains to be realized in the future. 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

The development of IR kinetic spectroscopy has been challenging. Organometallic chemists have had to learn about lasers and electronics, while chemical physicists have learned about organometallic chemistry. However the final apparatus has turned out to be relatively uncomplicated and not difficult to use. We therefore anticipate that such equipment will become more widely available in the near future. 

To summarize the state of technology for the chemist wishing to practice laser chemistry the laser devices exist with the capability one would like, but they are expensive. We may expect that cheaper pulsed laser systems based upon excimer, Nd YAG, N2, alexandrite, etc. may be in the offing in the near future. This has already begun to happen with a new generation of N2 pumped dye lasers from two manufacturers. No such prospects presently exist for c.w. lasers in the visible and ultraviolet, but one may hope that the ion laser will be radically improved or supplanted soon. For chemical applications which can use infrared excitation, satisfactory devices presently exist and the price is right. 

Finally, we note that future instrument for lifetime-based sensing and imaging can be based on laser diode light sources. At present it is desirable to develop specific probes which can be excited from 630 to 780 nm, the usual range of laser diodes. The use of such probes will allow us to avoid the use of complex laser sources, which should result in the expanded use of fluorescence detection in the chemical and biomedical sensors. 

Microdialysis is a sampling technique that must be coupled with an analytical method to identify and quantify chemical components of the dialysate. The samples can be analyzed immediately upon collection (i.e., online), or they can be stored (—80°C) for future analysis. Only analytical techniques sensitive enough to measure both small sample volumes and low concentrations of substances can be used to measure compounds in dialysate samples. High-performance liquid chromatography (HPLC) or capillary electrophoresis (CE) combined with ultraviolet (UV), electrochemical (EC), or laser-induced fluorescence (LIF)... 

The dominating method of ion formation in metabolic flux analysis is electron impact. It might be supplemented in the future by novel methods, such as matrix assisted laser desorption and electrospray. Additional techniques such as chemical ionization, fast atom bombardment or inductively coupled plasma ionization are only of minor importance and not further discussed in this context. 

When extending the quenching studies to larger molecules, a guideline hopefully may be obtained from statistical predictions since certainly ab initio calculations will not be possible in the near future. On the other hand, E-V-R transfer from Na to larger molecules may be of interest for a number of practical reasons such as laser applications or the enhancement of chemical reactivities. 

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