title: Detection of atomic hydrogen in flames using picosecond two-color two-photon-resonant six-wave-mixing spectroscopy.
abstract: 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(alpha) 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.
authors.affiliation: School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: Waruna D
authors.initials: WD
authors.lastname: Kulatilaka
authors.forename: Robert P
authors.initials: RP
authors.lastname: Lucht
authors.forename: Sukesh
authors.initials: S
authors.lastname: Roy
authors.forename: James R
authors.initials: JR
authors.lastname: Gord
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Settersten