Optogalvanic Spectroscopy-Application To Combustion Systems

Abstract

This paper reviews the application of optogalvanic spectroscopy to flames. Optogalvanic spectroscopy is a method of obtaining absorption spectra of atomic and molecular species in flames and electrical discharges by measuring voltage and current changes upon laser irradiation. The technique alleviates problems associated with optically monitoring either small absorptions or weak fluorescence in the presence of strong laser light. Optogalvanic spectroscopy in discharges has been useful in characterizing laser linewidths as well as provid-ing a convenient frequency calibration for tunable dye lasers. In addition, opto-galvanic signals have been used to frequency stabilize continuous-wave dye lasers. Optogalvanic spectroscopy has also been possible on some molecular species which exist only in flame or discharge environments. Analytical flame spectrometry utilizing the optogalvanic effect for trace metal detection shows significant promise for many metallic elements. The optogalvanic effect can also serve as a probe of ionization effects in flames. Ionization cross sections and ion mobilities may be determined from an analysis of optogalvanic signals. This technique has been extended to include determination of mobilities for soot precursor species. Flow velocities may also be determined in laminar flames by following the residual depletion of neutrals due to laser-enhanced ionization.