Hydrogen atoms are important in flame kinetics, and efficient methods for measuring the concentration of such atoms are of considerable interest. Laser excitation and optical detection are very attractive for monitoring flame constituents since no intrusive probes are needed. Lucht et al.1 have performed such measurements for hydrogen atoms in a low pressure flame employing two-photon excitation at the rather cumbersome 205-nm wavelength. A three-photon excitation scheme for the n = 2 level was recently used in optogalvanic experiments on low pressure flames by Tjossem and Cool.2 We experienced problems in extending this type of measurement to atmospheric flames due to molecular interference. By three-photon excitation of the n = 4 level and detection of Balmer-β photons we succeeded in monitoring hydrogen atoms in an atmospheric pressure C2H2/O2 flame using readily available laser radiation of 291 nm.3 In Fig. 1 recordings of excitation and fluorescence spectra are shown.
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