Laser-induced fluorescence instrument for the detection of OH and HO2
LIF is used for detection because of its sensitivity and selectivity. Tunable UV light is produced by a narrow bandwidth tunable dye laser pumped by a high repetition rate, solid state Nd:YAG laser. Ground state OH molecules are promoted to the first electronically excited state via the Α2Σ+ (ν’ = 1) ← Χ2Π3/2 (ν” = 0) transition at 282 nm. Some of the vibrationally excited molecules relax to ν’ = 0 via vibrational energy transfer and subsequently fluoresce near 309 nm. Air flow is introduced to the detection region of the instrument in a boundary layer free manner using a two-ducted system. OH radicals are measured by laser induced fluorescence (LIF) at the center of the sampling duct, avoiding the edges where ambient OH is lost to the walls. HO2 is measured as OH after chemical conversion to OH with nitric oxide via the reaction: HO2 + NO → OH + NO2.