Analysis of 205-nm photolytic production of atomic hydrogen in methane flames

We investigate the 205-nm photolytic production of atomic hydrogen in methane flames. This process represents a significant interference in two-photon, laser induced-fluorescence (TP-LIF) detection of atomic hydrogen in flames. Relative TP-LIF profiles of the photolytically produced H atoms were measured using a pump-probe technique in atmospheric-pressure, premixed CH4/O-2/N-2 flames. A high-fluence, non-resonant, nanosecond pump laser created H atoms by photodissociating flame constituents, and a copropagating, non-perturbing picosecond laser probed the photolytically produced Hatoms via TP-LIF. Spatial profiles of photolytically produced H atoms indicate that both intermediate and product species contribute to the interference in all flames. Excellent agreement between simulated and measured interference signals is observed in the product region of the flames. Vibrationally excited H2O is the dominant source of interference in the product region, but an additional contribution is attributed to vibrationally excited OH radicals. In the flame-front region, CH3 is the dominant precursor, and photodissociation of C2H2 becomes increasingly important in rich flames. Mechanisms for sequential photodissociation of CH3 and C2H2 are presented, indicating that complete dissociation at 205 nm of both precursors is feasible.