UV cavity enhanced absorption spectroscopy of the hydroxyl radical

摘要

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~2Σ~+(υ′ = 0) ← X~2Π_(3/2) (υ″ = 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 μW of tuneable radiation at ca. 308 nm by sum frequency generation in a β-BaB_2O_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 (1σ) absorption sensitivity of 2.1 x 10~(-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 ～ 2 cm~(-1) scanning range at a UV power of ～ 0.5 μW. An OH detection limit (1σ) of 3.84 x 10~9 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.