Vibrational relaxation rates for thev=2 level of theXthinsp;2Pgr;istate of the OH radical have been measured in a low pressure flow system, using a novel twohyphen;laser pumphyphen;andhyphen;probe technique. The OH is prepared in thev=2 level by overtone pumping (2larr;0) and monitored by ultraviolet laserhyphen;induced fluorescence in the (1,2) band of theAndash;Xsystem. 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 cm3thinsp;sminus;1units) for NH3: (1.20plusmn;0.15)times;10minus;10; CH4: (2.3plusmn;0.2)times;10minus;12; CO2: (6.7plusmn;1.1)times;10minus;13; N2O: (4.6plusmn;0.6)times;10minus;13; O2: (2.6plusmn;0.54)times;10minus;13; N2and H2: le;10minus;14. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation ofv=1 in theAthinsp;2Sgr;+excited state of OH, where attractive forces appear to play a role.
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