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

Abstract

Vibrational relaxation rates for the v = 2 level of the X 2Πi 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. © 1989 American Institute of Physics.