High-resolution degenerate four-wave-mixing spectroscopy of OH in a flame with a novel single-mode tunable laser

The first application of a novel single-mode tunable laser system to nonlinear spectroscopy is reported. The device uses a modeless dye laser, pumped by a single longitudinal mode (SLM) Q-switched Nd:YAG laser, as a narrow-bandwidth amplifier of the output of a SLM diode laser. The system provides pulses of 5-ns duration, 30-mJ energy and 165-MHz spectral line width tunable in the range 632–639 nm at 10-Hz repetition rate. The frequency-doubled output of the laser is used to record spectral line shapes of degenerate four wave mixing (DFWM) signals from the P₁(15) line of the A²–X² (0,0) band of OH in a methane/oxygen flame. Pressure broadening of the DFWM line shape is studied for the first time in a low-pressure flame and a pressure-broadening rate of 1.31±0.09×10^-4 cm^-1/Torr is derived from the data. Power-broadening effects are measured and compared with predictions of the standard perturbative model and of an analytical solution derived from a non-perturbative treatment of DFWM with arbitrary pump and probe intensities. PACS 42.55.Px; 42.62.Fi; 42.65.-k