Chemical lasers, principles

In principle, after initiation the laser should be operatable purely by chemical reaction, without any external sources of electrical power. In practice, most chemical lasers do use a sustaining source of electrical power. 

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 molecule can store energy in the electronic, vibrational, rotational, and translational degrees of freedom. However, the probability that energy can accumulate in these degrees of freedom and can appear in the form of chemical laser emission differs considerably. Fig. 1 shows the usual form of the reaction profile for an exothermic reaction. It is apparent that a product molecule which has just been released from the activated complex is at some distance from its equilibrium state. It contains excess energy which can in principle be given off in two ways, namely by radiative or collisional processes. There is always competition between these two types of processes. The luminescence quantum yield r]a (4) will be different, depending on the type of excitation. 

Earlier than with pulsed chemical lasers, the first technological breakthrough in chemical lasers occurred for continuous-wave lasers. Almost simultaneously in 1968 two groups successfully operated continuous-wave chemical lasers. One was at the Aerospace Corporation headed by T. A. Jacobs 75>, the other one at Cornell University under T. A. Cool 76>. One of these lasers was an HF laser the other was that is now called a hybrid chemical laser, being pumped by energy transfer rather than by a direct chemical reaction. This laser principle has been described in the context of pulsed chemical lasers in Section 6.5, In addition to these devices, an HF cw laser having millisecond flow duration was also demonstrated in principle in a shock tunnel. The latter employed diffusion of HC1 into a supersonic stream containing F atoms 77>. 

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. 

The principle of time-resolved gain spectroscopy was first applied to a molecular chemical laser by L. Henry and coworkers 119>. The HC1 laser from the flash photolysis of an H2/CI2 mixture was chosen for this study. Initial vibrational population figures have been obtained and rate constants derived for the vibrational deactivation, as given in Table 17. 

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... 

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. 

In principle, the system can also relax to equilibrium ty light emission. Unless the circumstances are unusual, emission of IR photons is too slow a process to significantly compete with collisional relaxation. An extreme case is that of molecules in outer space where the density is very low so the time between collisions is exceedingly long. Another unusual situation is a chemical laser. Section 9.0.3, where the presence of a high density of photons stimulates the emission of other photons. 

The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

Lasers (qv) levels just above the valence band chemical potential are essentially (2) empty and unfilled, but levels just below the conduction band chemical potential are filled, permitting a population inversion. Filled levels above, empty levels below, is the principle by which lasers operate (see also... 

The historical development and elementary operating principles of lasers are briefly summarized. An overview of the characteristics and capabilities of various lasers is provided. Selected applications of lasers to spectroscopic and dynamical problems in chemistry, as well as the role of lasers as effectors of chemical reactivity, are discussed. Studies from these laboratories concerning time-resolved resonance Raman spectroscopy of electronically excited states of metal polypyridine complexes are presented, exemplifying applications of modern laser techniques to problems in inorganic chemistry. 

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