Chemical laser methods

The behaviour of the product vibrational distributions in the reaction F + (H/D>2 -> (H/D)F + (H/D) has been studied, using chemical laser methods, as a function of reactant temperature. This study provides the first quantitative evidence that the distributions are temperature-dependent. 

The formation of CO from O 4- CS has been studied by infrared chemiluminescence (analysis of steady-state populations), by the chemical laser method and by laser probe techniques. Good agreement exists for the relative populations in v = 6-15. The disagreement over the populations of the lower levels recently was resolvedby the discovery that the O 4- CSg reactions, which in most systems accompanies the O 4- CS reaction, yields CO in the lower levels. Following the suggestion of Kelley, the lower levels in Table 2.13 were obtained from extrapolation of the linear... 

For the analysis of the chemical structure of flames, laser methods will typically provide temperature measurement and concentration profiles of some readily detectable radicals. The following two examples compare selected LIF and CRDS results. Figure 2.1 presents the temperature profile in a fuel-rich (C/O = 0.6) propene-oxygen-argon flame at 50 mbar [42]. For the LIF measurements, 1% NO was added. OH-LIF thermometry would also be possible, but regarding the rather low OH concentrations in fuel-rich flames, especially at low temperatures, this approach does not capture the temperature rise in the flame front [43]. The sensitivity of the CRDS technique, however, is superior, and the OH mole fraction is sufficient to follow the entire temperature profile. Both measurements are in excellent agreement. For all flames studied here, the temperature profile has been measured by LIF and/or CRDS. 

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. 

Chemical Conversion Methods. Laser-Induced and Resonance Fluorescence of HO. Considerable effort has been applied to the measurement of HO in the stratosphere and troposphere. Ultraviolet fluorescence techniques based on lasers or resonance lamps have received a great deal of attention and study. Because HO concentrations are typically factors of one-tenth to one-hundredth those of H02 in the atmosphere, the difficulties associated with making HO measurements by using fluorescence [low signal-to-noise ratio, laser-generated HO, background fluorescence, etc. see the... 

Synthesis of metallic nanoparticles proceeds in many ways they can be divided into physical and chemical. Physical methods include inert gas condensation, arc discharge, ion sputtering, and laser ablation. The main idea behind these methods is condensation of solid particles from the gas phase, the substrate for nanoparticle generation being pure metals (or their mixtures/alloys in the case of complex particle composition). Chemical methods, in turn, include various methods utilizing... 

The arrested-relaxation method has been applied [227,228] to the reactions of F + H2 and F + D2, and the measured-relaxation technique to F + H2 [229, 230]. These values of / ., and Rv are particularly important since they can be compared with the results of molecular beam and chemical laser experiments (see Table 1.4), and the agreement is satisfactory. The HF vibrational and rotational degrees of freedom absorb approximately 67% and 6% of the total energy and once again there is a marked parallelism between the results for a reaction and its isotopic analog. Preliminary measurements on other reactions producting HF have been reported by Jonathan et al. [230]. 

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 above process is a key element in the operation of the gain medium in the HF chemical laser [57]. It is also thought to explain the remarkable persistence and exceptionally high rotational energy (n < 32) of OH emission in Earth s airglow which has been detected some 12 h after sunset and therefore cannot be the result of direct solar excitation [58]. In the final part of this contribution, the AM method is used to demonstrate how such effects might come about in a multicollision environment that represents a rudimentary model of Earth s atmosphere. 

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. 

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. 

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. 

In principle, there exist several techniques that are specifically conceived to obtain measurements of chemical species [6-9]. Their physical origin has a direct consequence for the experimental strategy and, as will become apparent shortly, it is preferable to classify these laser methods in reference to the characteristics of their... 

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. ... 

Among other methods for determining trace and toxic elements in the soil, there are also electro-chemical analytical methods, mainly polarogra-phy and in the case of nuclear analytical methods, activation analysis and radionuclide X-ray fluorescence analysis are employed. Mass spectrometry, laser emission spectral microanalysis and other instrumental methods can also be used. 

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