UV cavity enhanced absorption spectroscopy of the hydroxyl radical

We present the application of a continuous-wave ultra-violet tuneable light source for detection of the hydroxyl radical (OH) using cavity-enhanced absorption spectroscopy of the Q(11)(2) and Q(21)(2) absorption lines in the A(2)Sigma(+)(v'=0)<--X(2)Pi(3/2)(v"=0) band at ca. 308 nm. A tuneable infra-red diode laser operating at 835 nm and either an Ar+ laser or a single frequency continuous-wave intracavity frequency-doubled diode laser, both operating at ca. 488 nm, were used to produce 0.1-0.5 muW of tuneable radiation at ca. 308 nm by sum frequency generation in a betaBaB(2)O(4) crystal. Cavity enhanced absorption spectroscopy was used to detect OH generated by UV photolysis of water vapour in argon, nitrogen, neon and helium at atmospheric pressure. A noise-equivalent (1sigma) absorption sensitivity of 2.1x10(-7) cm(-1)Hz(-1/2) measured over 128 scans in a time of 1.16 s was demonstrated with mirrors of reflectivity 0.9963 in a cavity of length 58.5 cm for a similar to2 cm(-1) scanning range at a UV power of similar to0.5 muW. An OH detection limit (1sigma) of 3.84x10(9) molecule cm(-3) was estimated in argon at atmospheric pressure. OH collisional broadening in humidified N-2, Ar, Ne and He was determined at atmospheric pressure .