Combustion diagnostic by quantitative two-photon polarization spectroscopy of atomic hydrogen

The aim of this paper is to reveal the high potential which two-photon polarization spectroscopy can provide for the investigation of atomic species in combustion processes. As a first demonstration, we applied this technique to determine number densities and kinetic temperatures of atomic hydrogen in flames at atmospheric pressure. Our method, developed for laser-aided plasma diagnostics, renders possible absolute optical measurements in such dense combustion environments characterized by the presence of many kind of reacting species and a plasma state far off thermodynamic equilibrium. The measured polarization signal is caused only by the 1S-2S two-photon resonance of hydrogen atoms and not perturbed by the strong quenching rates or by the complicated radiative transfer. We performed the experiment with pulsed UV radiation of the highest spectral quality possible, i.e. single-mode laser radiation tunable around 243 nm with ns pulse duration close to the Fourier transform limit.