Measurements of ground-state OH rotational energy-transfer rates

We have studied rotational energy transfer (RET) in collisions of OH with t he bath gases Ar, N-2, O-2, and H2O at 293 K. Rotationally hot OH(X (2)Pi(3 /2), upsilon "=0, N "=1-12) was generated by photolysis of H2O2 at 266 nm, and collisional relaxation of the nascent rotational distribution was monit ored by laser-induced fluorescence. The data are remarkably well described by an exponential-gap model for the matrix of state-to-state RET rate const ants. For Ar, N-2, and O-2, RET rates are significantly faster at low N " t han high N "; for H2O, RET is approximately an order of magnitude faster th an for the other bath gases, and the rate is not as strongly dependent on N ". The rates of rotationally inelastic energy transfer are similar in the X and A states, but the X-state depopulation rate constants (including near ly elastic, h-doublet-changing collisions) are faster than the A-state valu es. By comparing the depopulation rates derived from the present experiment with previous linewidth measurements, we conclude that RET is the dominant source of pressure broadening for OH microwave transitions and makes a sig nificant contribution for ultraviolet A-X transitions. While generally good agreement is found between the present results and previous OH RET studies for both the ground and excited electronic states, some significant discre pancies are noted. (C) 1999 American Institute of Physics. [S0021-9606(99)0 1701-8].