Vibrational relaxation of OH (X 2Πi, v=2)

Abstract

Vibrational relaxation rates for the v=2 level of the X ²Π_i state of the OH radical have been measured in a low pressure flow system, using a novel two‐laser pump‐and‐probe technique. The OH is prepared in the v=2 level by overtone pumping (2←0) and monitored by ultraviolet laser‐induced fluorescence in the (1,2) band of the A–X system. Scanning the time delay between the lasers at a given collider pressure produces exponential decay whose rate as a function of collider pressure yields the rate constant. We determine values (all cm³ s⁻¹ units) for NH₃: (1.20±0.15)×10⁻¹⁰; CH₄: (2.3±0.2)×10⁻¹²; CO₂: (6.7±1.1)×10⁻¹³; N₂O: (4.6±0.6)×10⁻¹³; O₂: (2.6±0.54)×10⁻¹³; N₂ and H₂: ≤10⁻¹⁴. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation of v=1 in the A ²Σ⁺ excited state of OH, where attractive forces appear to play a role.