Determination of local concentration of H2O molecules and gas temperature in the process of hydrogen – oxygen gas mixture heating by means of linear and nonlinear laser spectroscopy

Employing the methods of linear absorption spectroscopy and nonlinear four-wave mixing spectroscopy using laserinduced gratings we have simultaneously measured the local concentrations of H₂O molecules and the gas temperature in the process of the H₂–O₂ mixture heating. During the measurements of the deactivation rates of pulsed-laser excited singlet oxygen O₂ (b ¹Σ⁺_g) in collisions with H₂ in the range 294 – 850 K, the joint use of the two methods made it possible to determine the degree of hydrogen oxidation at a given temperature. As the mixture is heated, H₂O molecules are formed by 'dark' reactions of H₂ with O₂ in the ground state. The experiments have shown that the measurements of tunable diode laser radiation absorption along an optical path through the inhomogeneously heated gas mixture in a cell allows high-accuracy determination of the local H₂O concentration in the O₂ laser excitation volume, if the gas temperature in this volume is known. When studying the collisional deactivation of O₂ (b ¹Σ⁺_g) molecules, the necessary measurements of the local temperature can be implemented using laser-induced gratings, arising due to spatially periodic excitation of O₂ (X³Σ^-_g) molecules to the b ¹Σ⁺_g state by radiation of the pump laser of the four-wave mixing spectrometer.