Kinetics, chemical laser

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. 

We have seen that the limitations of the time characteristics of electronic devices requires the use of optical delays between the pump and probe pulses in ps flash photolysis. There are also indirect ways of using optical properties to measure the kinetics of laser pulses and of fluorescence, known as autocorrelation and up-conversion . These rely on the non-linear properties of certain materials or chemical systems, i.e. they are based on fast biphotonic processes. 

Chemical Thermodynamics Dynamics of Elementary Chemical Reactions Kinetics (Chemistry) Lasers Nuclear Chemistry Photochemistry by VUV Photons Photochemistry, Molecular Process Control Systems Quantum Mechanics... 

Research into chemical lasers is new, fascinating, and in a period of tremendous growth. The object of most of the experiments up to the present time has been the improvement of output powers and efficiencies, but although little quantitative kinetic data have been derived so far from studies of laser systems, it seems unlikely that this will be so for very long. 

The relatively recent development of a new class of chemical laser based on the formation of noble gas-halide exciplexes and producing coherent radiation at a number of different u.v. wavelengths has been quickly adopted by both kineticists and spectroscopists. This Section brings together a few studies which have appeared in the past year dealing with the properties (i.e., kinetics, photophysics, etc.) of complexes important in the noble gas-htilide systems. The numerous articles that have appeared giving details of performance characteristics of such lasers (and their improvement) are deemed to be beyond the scope of this Report and are not included. However, the proceedings of two recent conferences on lasers have been published in which much of this information can be found. 

There is a complex interplay of kinetic processes in these energy transfer lasers. The conditions under which they operate are extreme in terms of the concentrations of transient species and the local gas temperatures. Many of the key reactions are difficult to study in isolation as they involve interactions between pairs of radicals or other transient species (e.g. collisions between electronically and vibrationally excited molecules). The impetus to obtain mechanistic and kinetic data for chemical laser systems (potential as well as demonstrated) has stimulated the development of new experimental techniques and produced a substantial body of kinetic data. Beyond the practical value of this work, the kinetic data that have been obtained are of fundamental scientific interest. They provide insights regarding the underlying principles that govern energy transfer processes and rare examples of reactions involving electronically and/or vibrationally excited species. 

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. 

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. 

The use of accelerated beams, however, raises the old question in chemical kinetics of the relative efficiencies of vibrational and translational energy in supplying the activation energy of a reaction. While vibrational population inversion in a beam can be achieved in selected cases by optical pumping, any beam method in this area will have to compete with chemical laser techniques. In these the decay of emission from the upper vibrational states is monitored in the presence of a quenching gas (i.e. the reaction partner) in the optical cavity itself. 

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. 

As these remarks indicate, chemical lasers employ infrared chemiluminescence. As a method for obtaining kinetic information, they have to be looked at in relation to other spectroscopic techniques having the same goal. The study of spontaneous vibrational-rotational emission has been most fruitfully applied to fast reactions in the gas phase. This method has experimental limitations due to the relaxation processes competing with spontaneous emission. A very authentic discussion of this method has been given in a recent review by J. C. Polanyi 3>. As opposed to this steady-state technique, chemical lasers permit observations in the pulsed mode. 

Kinetic Information through Chemical Laser Studies... 

A great deal of more or less detailed computer modelling has been done to predict operational features of chemical lasers since the first studies of this type by Comeil et al. 144>, Cohen et al. m> and Airey It is beyond the scope of this review to account for all the computational approaches that have been made. One paper of this kind was reviewed in Section 7 in connection with power predictions for an H2/F2 laser oscillator. Here the comprehensive work of Igoshin and Oraevskii 109> on the kinetic processes in an HC1 laser may serve as a reference to show the relevant features. The analysis proceeds from the simultaneous solution of chemical kinetics, vibrational relaxation, and radiational processes. The chain reaction model used here is the following... 

Chemical lasers are complex nonequilibrium molecular systems governed by an intricate interplay between a variety of chemical, radiative, and collisional relaxation processes. Many of their kinetic properties are reflected by the temporal, spectral, and power characteristics of the out-coupled laser radiation. For example, threshold time measurements and other gain experiments have provided detailed information on vibrational distributions of nascent reaction products. Another, more qualitative, example Single-line and simultaneous multiline operation indicate, respectively, whether the lasing molecules are rotationally equilibrated or not. Besides their practical applications, chemical lasers are widely used as means of selective excitation in state-to-state kinetic studies. On the other hand, many experimental and theoretical studies have been motivated by the wish to understand and improve the mechanism of chemical laser operation. 

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. 

The rotational equilibrium assumption implies substantial conceptual and technical simplifications of the kinetic description of chemical lasers. In this section we shall consider the validity and consequences of this assumption and compare the very different lasing mechanisms and efficiencies implied by fast and slow rotational relaxation. 

In this section we shall briefly review the thermodynamic description of chemical lasers. The analysis will be based on the fundamental laws of thermodynamics and the statistical-molecular definitions of entropy and energy. The approach outUned below does not intend to yield new detailed results, these have been supplied adequately in the kinetic analyses. On the contrary, it attempts to compact the detailed data by focusing attention on a few macroscopically significant observables, and by applying general thermodynamic relationships to shed a different light on the phenomena described in the previous sections. 

The different efficiencies of chemical lasers governed by different kinetic coupling schemes can be derived from a general statistical-thermodynamic approach to work processes in nonequilibrium molecular systems " . The two major components of this approach are the maximum entropy principle and the entropy deficiency function. The entropy deficiency is a generalized thermodynamic potential (free energy). That is, it decreases monotonically in time in spontaneous relaxation processes and provides an upper bound to the thermodynamic work performed by the system in a controlled process. For systems of weakly interacting molecules the entropy deficiency DS[X X ] is given by... 

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