Λ-doublet substate specific investigation of rotational and fine structure transitions in collisions of OH with H2 and D2

The selective population of rotational, spin, and Λ-doublet states of OH(2Π,v=0,1) by inelastic collisions with H2 and D2 is investigated in two experiments. In the first experiment OH radicals are generated by photolysis inside a pulsed nozzle beam source and prepared in the ground state j=1.5, v=0, 2Π3/2 with equal amounts in both Λ-doublets by rotational cooling in the subsequent expansion. The collisional excitation of OH by the secondary beam is probed via laser-induced fluorescence selectively for the Λ-doublet states for the different rotational levels in both spin manifolds of v=0. Integral cross sections σ(j=1.5,Ω̄=1.5→j ′,Ω̄′,ε′) averaged over the Λ-doublet substates in the input channel, but Λ-doublet substate resolved in the output channel are obtained in this case. In contrast to some previous predictions the Π(A′) Λ-doublet substate is preferentially populated. This implies a population inversion in the Λ-doublets for 2Π1/2 and an anti-inversion for 2Π3/2, which rules out previously proposed pump mechanisms for astronomical OH masers. In the second experiment OH is generated also by photolysis, in this case however in a flow system. Single Λ-doublet states are prepared by infrared excitation of the thermally relaxed OH in 2Π3/2, v=1 for two rotational states (j=1.5,4.5). The redistribution in v=1 induced by collisions with H2 is probed by laser-induced fluorescence. Rate constants are obtained for transitions from the initially prepared Λ-doublet states to the other Λ-doublet of the same j and also for transitions to other rotational states.