THE APPLICATION OF RESONANCE ENHANCED MULTIPHOTON IONIZATION TO THE DETECTION OF HYDROGEN-ATOMS DURING THE OSCILLATORY COMBUSTION OF HYDROGEN

The relative concentrations of hydrogen atoms were measured during the oscillatory ignition of hydrogen in a well stirred flow reactor. Comparisons were made with the experimental concentration-time profiles of the hydroxyl radical obtained previously under similar experimental conditions. The predicted concentration profiles obtained from numerical analysis of a thermokinetic model were also compared with the experimental results. Experiments were performed in a 600 cm3 Pyrex glass, jet-stirred reactor with the reactants, 2H-2 + O2, at a total pressure of 16 Torr (ca. 2132.8 Pa) and at a vessel temperature of 753 K. The mean residence time was 1.2 s. Oscillatory ignition was established at a period of 3 s in which high radical concentrations were attained and in which the temperature rise was almost adiabatic. The concentration-time profile of hydrogen atoms was obtained by a resonance enhanced multiphoton ionization (REMPI) which was induced by a laser pulse at energies in the vicinity of 364 nm, with ion collection at a stainless steel probe inserted into the reactor. Supplementary studies were made to characterize the signals and to identify effects of the probe within the reaction volume. A measurement of the relative concentrations of hydrogen atoms was obtained from an integration of the area of the REMPI spectrum determined over the laser wavelength range 363.8-364.6 nm. The spectrum was measured at successive times in the oscillatory cycle by imposing a variable delay on the laser firing signal. The results show that, during oscillatory ignition, the maximum concentration of hydrogen atoms was reached and a sharp decay was already well advanced before that of the hydroxyl radicals was attained. The numerical analysis was in very good quantitative accord with this experimental result. The phase difference of the cyclic variation in the H atoms relative to that of OH radicals is a key feature of the kinetic mechanisms which control the oscillatory oxidation of hydrogen.