Laser chemistry and combustion

LASER DIAGNOSTICS AND COMBUSTION CHEMISTRY FOR PULSE DETONATION ENGINES... 

Laser-induced fluorescence (LIF) depends on the absorption of a photon to a real molecular state, and is therefore a much more sensitive technique, capable of detection of sub-part-per-billion concentrations. Thus, this is the most suitable for measurement of those minor species which are the transient intermediates in the reaction network. Here a tunable laser is required, as well as an electronic absorption system falling in an appropriate wavelength region serendipitously, many of the important transient species have band systems which are suitably located for application of LIF probing. The ability to sensitively detect transitions originating from electronically as well as vibrationally excited levels of a number of molecules offers the possibility of inquiring into the participation of non-equilibrium chemistry in combustion processes. 

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

The recent availability of tunable dye lasers has markedly enhanced our ability to inquire into the chemistry and physics of combustion systems. The high sensitivity, spectral and spatial resolution, and non-perturbing nature of laser induced fluorescence makes this technique well suited to the study of trace chemistry in complex combustion media. A barrier to the quantitative application of fluorescence to species analysis in flames has been the need to take into account or bypass the effects of quenching. The use of saturated fluorescence eliminates quenching as a problem and has the further advantage that fluorescence intensity is insensitive to variations in laser power (1, 2 ). However, the generation of high concentrations of excited states under saturated excitation in an active flame environment opens up the possibilities for laser induced chemistry effects that also must be taken into account or avoided (3,4,5). 

The present study is a computer model of the time evolution of individual level populations of the OH molecule under the influence of laser excitation. The environment simulates that of the burnt gases of an atmospheric pressure methane-air flame at 2000°K. OH is studied because of its importance in combustion chemistry and suitability for LIF, which have made it the most popular molecule for LIF investigations in flames in addition, it has a small enough number of significantly populated levels to be computationally tractable. 

Ghezzi,U., Coghe,A. and Gamma,F., Symposium on Laser Probes for Combustion Chemistry, 178th ACS National Meeting, Washington, U.S.A., 1979. 

The title reaction is also of considerable interest to combustion [48] and laser chemistries [49]. Theoretically it is one of the best known complex-forming reactions from a fundamental point of view [50]. 

The KPS paper stimulated research in several new directions, and ultimately spawned new fields. Many researchers, including Karplus, got interested in the development of QST of chemical reactions, and this led to accurate quantum descriptions of the H + H2 reaction [8] a decade after the KPS paper. There was also significant interest in the application of QCT methods to gas-phase reactions other than H -f- H2, and in fact this approach is now considered to be a standard research tool for studying gas-phase reaction dynamics of relevance to laser chemistry, combustion chemistry, atmospheric chemistry, and other applications. 

Fundamental quantities, such as wavelengths and transition probabilities, determined using spectroscopy, for atoms and molecules are of direct importance in several disciplines such as astro-physics, plasma and laser physics. Here, as in many fields of applied spectroscopy, the spectroscopic information can be used in various kinds of analysis. For instance, optical atomic absorption or emission spectroscopy is used for both qualitative and quantitative chemical analysis. Other types of spectroscopy, e.g. electron spectroscopy methods or nuclear magnetic resonance, also provide information on the chemical environment in which a studied atom is situated. Tunable lasers have had a major impact on both fundamental and applied spectroscopy. New fields of applied laser spectroscopy include remote sensing of the environment, medical applications, combustion diagnostics, laser-induced chemistry and isotope separation. 

Hanson RK, Varghese PL, Schoenung SM, and Falcone PK (1980) Absorption spectroscopy of combustion gases using a tunable IR diode laser. Laser Probes for Combustion Chemistry. ACS Symposium Series 134. Washington DC American Chemical Society. 

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

Emission spectroscopy and, to a lesser degree, absorption spectroscopy have provided considerable information on and insight into the chemistry occurring during the process of combustion. In particular, many of the transient free-radical molecules important in the chain reactions were identified and characterized through their emission spectra in flames. Now, new laser spectroscopic techniques offer the promise of obtaining more detailed and precise information, especially for the ground electronic states of many of the molecules involved in combustion. 

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