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) ← Χ²Π_3/2 (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.