Chemical laser techniques

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. 

The microscopic product energy disposal has been characterized for thermal F atom hydrogen abstraction reactions through chemiluminescence and chemical laser techniques. 

The chemical laser technique resembles chemiluminescence, but stimulated, rather than spontaneous emission, is observed. One advantage, relative to the chemiluminescence technique, is that the vibrational population for the r = 0 level can be measured. However, unlike the chemiluminescence experiments most laser experiments are done in the 10-100 torr pressure range hence, the initial rotational populations are not measured and in fact rotational equilibration with the bath gas is assumed in obtaining relative vibrational populations. The chemical laser techniques have been recently reviewed, and, further details are not given here. 

The F + H2 reaction has been characterized thoroughly by both chemiluminescence and chemical laser techniques, and the results are shown in Table 2.2. Arrested relaxation chemiluminescence work from... 

The first line corresponds to experimental measurements the second includes allowance for the contribution from t = 0, which was estimated from the extrapolation of the surprisal plot. The chemical laser technique, ref. 51, measured the r = 0 population. 

Once vibrational population ratios for the reaction product molecule of interest have been determined with one or more of the methods just described, ratios of rate constants for specific vibrational levels can be deduced, provided the effects of collisional redistribution are negligible, or at least not so large that correction for these effects cannot be made with some confidence. An important advantage of the chemical laser techniques is that the ratio of rate constants, kijkoy to the y = 1 and t = 0 vibrational levels can be obtained. This type of information is not directly obtained from chemiluminescence studies. 

Chemical laser techniques have been applied to the study of many types of reactions including three-atom-exchange reactions, abstraction reactions, and elimination reactions. Table 3.2 summarizes some of the reactions... 

Examples of the use of chemical laser techniques to provide unique information concerning reaction product distributions are the studies of Pimentel and Berry and co-workers of unimolecular photoelimination reactions/ " A number of these reactions are listed in Table 3.2. A very detailed study of photoelimination of vibrationally excited HCl from chloro-ethylene was reported by Berry. > Here again, relative times-to-threshold were used to determine relative vibrational populations with a grating selection technique. Figure 3.14 illustrates measured and calculated vibrational distributions of HCl product molecules formed in the t < 4 levels by... 

Very recently the use of chemical laser techniques for the measurement of rotational relaxation times for individual rotational levels has been demonstrated by Hinchen in an elegant series of double-resonance experi-ments/ The apparatus consisted of two HF chemical lasers as the schematic diagram of Figure 3.18 indicates. A pulsed HF laser operated on a single P-branch transition of the t (l 0) band was employed to permit selective excitation of a single rotational state in the t = 1 level of HF. The temporal variation in the absorption of the output of a cw HF laser operated on the t (2 1) band was used as a probe for monitoring of the populations... 

Bernstein R B (ed) 1982 Chemical dynamics via molecular beam and laser techniques The Hinshelwood Lectures (Oxford, 1980) (Oxford Oxford University Press)... 

Up to the present, a number of conventional film preparation methods like PVD, CVD, electro-chemical deposition, etc., have been reported to be used in synthesis of CNx films. Muhl et al. [57] reviewed the works performed worldwide, before the year 1998, on the methods and results of preparing carbon nitride hlms. They divided the preparation techniques into several sections including atmospheric-pressure chemical processes, ion-beam deposition, laser techniques, chemical vapor deposition, and reactive sputtering [57]. The methods used in succeeding research work basically did not... 

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. 

The following case study contains examples of several topics discussed in previous sections, including some aspects of laser technology, laser spectroscopy and laser chemistry. A variety of lasers and laser techniques are applied in a straightforward manner to the problem of ascertaining structural and dynamical information on an excited electronic state of wide chemical interest. This information is obtained rather simply, illustrating the potential of laser techniques in the resolution of problems in solution chemistry. 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

Bernstein, R.B. (ed) (a) Chemical Dynamics via Molecular Beam and Laser Techniques, Clarendon Press, Oxford, New York (1982). (b) Atom-Molecule Collision Theory A guide to experimentalist, Plenum Press, New York (1979). 

Reaction dynamics is the part of chemical kinetics which is concerned with the microscopic-molecular dynamic behavior of reacting systems. Molecular reaction dynamics is coming of age and much more refined state-to-state information is becoming available on the fundamental reactions. The contribution of molecular beam experiments and laser techniques to chemical dynamics has become very useful in the study of isolated molecules and their mutual interactions not only in gas surface systems, but also in solute-solution systems. 

Bernstein, R. B. Chemical Dynamics via Molecular Beam and Laser Techniques Oxford University London, 1982, p. 45. 

Today, ultrafast pulsed-laser techniques, high-speed computers, and other sophisticated instrumentation make it possible to measure the time evolutions of reactants, intermediates, transition structures, and products following an abrupt photoactivation of a starting material. Detailed theoretical calculations, experienced judgments based on the literature, and newly accessible femtosecond-domain experimental data providing observed intensities of chemical species versus time can provide insights on the atomic-scale events responsible for overall reaction outcomes. 

LEE, YUAN T. (1936-). Awarded the Nobel prize in chemistry in 19X6 jointly with John C. Polanyi and Dudley R. Herschbach for their contributions concerning the dynamics of chemical elementary processes. A former student of Herschbach. Lee relined molecular-beam and laser techniques, comhining them with theory to perform definitive studies of reactions of individual complex molecules. Lee received his Doctorate from the University of California at Berkeley in 1965. 

On the experimental side, the chemical dynamics on the state-to-state level is being studied via molecular-beam and laser techniques [2]. Alternative, and complementary, techniques have been developed in order to study the real-time evolution of elementary reactions [3]. Thus, the time resolution in the observation of chemical reactions has increased dramatically over the last decades. The race against time has recently reached the ultimate femtosecond resolution with the direct observation of chemical reactions as they proceed along the reaction path via transition states from reactants to products. This spectacular achievement was made possible by the development of femtosecond lasers, that is, laser pulses with a duration as short as a few femtoseconds. In a typical experiment two laser pulses are used, a pump pulse and a probe... 

In fuel-rich flames, the CO should be in local chemical equilibrium, and hence the partial pressure of CO can be calculated from the local temperature and the measured fuel and air flowrates. Thus, a comparison between measured and calculated CO levels can serve as a validation of the diode laser technique for flame measurements. Such a comparison is shown in Figure 5 for equivalence ratios in the range <(> = 1.04 - 1.37. The data points shown represent the average of several observations on separate lines including ground state (v" = 0) and excited state (v" = 1) transitions. The agreement is consistently within the experimental uncertainty of 5%. 

With the advent of picosecond and subsequently femosecond laser techniques, it became possible to study increasingly fast chemical reactions, as well as related rapid solvent relaxation processes. In 1940, the famous Dutch physicist, Kramers [40], published an article on frictional effects on chemical reaction rates. Although the article was occasionally cited in chemical kinetic texts, it was largely ignored by chemists until about 1980. This neglect was perhaps due mostly to the absence or sparsity of experimental data to test the theory. Even computer simulation experiments for testing the theory were absent for most of the intervening period. 

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