Detection of atomic hydrogen inflames using picosecond two-color two-photon-resonant six-wave-mixing spectroscopy

We report an investigation of two-color six-wave-mixing spectroscopy techniques using picosecond lasers for the detection of atomic hydrogen in an atmospheric-pressure hydrogen-air flame. An ultraviolet laser at 243 nm was two-photon-resonant with the 2S(1/2) <-<- 1S(1/2) transition, and a visible probe laser at 656 nm was resonant with H, transitions (n = 3 <- n = 2). The signal dependence on the polarization of the pump laser was investigated for a two-beam polarization-spectroscopy experimental configuration and for a four-beam grating configuration. A direct comparison of the absolute signal and background levels in the two experimental geometries demonstrated a significant advantage to using the four-beam grating geometry over the simpler two-beam configuration. Picosecond laser pulses provided sufficient time resolution to investigate hydrogen collisions in the atmospheric-pressure flame. Time-resolved two-color laser-induced fluorescence was used to measure an n = 2 population lifetime of 110 ps, and time-resolved two-color six-wave-mixing spectroscopy was used to measure a coherence lifetime of 76 ps. Based on the collisional time scale, we expect that the six-wave-mixing signal dependence on collisions is significantly reduced with picosecond laser pulses when compared to laser pulse durations on the nanosecond time scale. (C) 2007 Optical Society of America.