UV cavity enhanced absorption spectroscopy of the hydroxyl radical

Abstract

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 Q₁₁(2) and Q₂₁(2) absorption lines in the A²Σ⁺(v^′=0)←Χ²Π_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 μW of tuneable radiation at ca. 308 nm by sum frequency generation in a βBaB₂O₄ 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 2.1×10^-7 cm^-1Hz^-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 3.84×10⁹ molecule cm^-3 was estimated in argon at atmospheric pressure. OH collisional broadening in humidified N₂, Ar, Ne and He was determined at atmospheric pressure .