Diode-laser-based sum-frequency generation of tunable wavelength-modulated UV light for OH radical detection.

Greater than 1.5 microW of tunable wavelength-modulated 308-nm radiation was generated by sum-frequency mixing of 90 mW of 835-nm diode-laser output and 1.5 W of 488-nm Ar(+)-laser output in a beta-barium metaborate crystal. Hydroxyl radicals formed in a discharge-flow reactor were detected by use of the generated UV beam by direct absorption, wavelength-modulation absorption spectroscopy, and laser-induced fluorescence. Wavelength modulation with second-harmonic detection yielded an estimated minimum detectable absorbance of 8.5 x 10(-6) at a 1-Hz bandwidth. This absorbance level corresponds to an OH detection sensitivity of 11.7 parts per trillion for a 1-m path length at 1 atm and 298 K.