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

Abstract

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.

Low-energy inelastic collisions of OH radicals with He atoms andD2molecules

“…The interaction between OH radicals and D 2 molecules has been studied less extensively. In crossed beam experiments, inelastic as well as reactive scattering processes have been studied at high collision energies [27][28][29]. The system has recently been treated theoretically using the quasiclassical trajectory method [30].…”

The interaction of OH(X2Π) with H2: Ab initio potential energy surfaces and bound states

“…6 Subsequently, Lester and co-workers have observed and analyzed the rotational structure of these complexes. [7][8][9] Andresen et al 10,11 first determined relative inelastic cross sections of OH j = 3/2 F 1 (with equal populations in both -doublet levels) in collisions with H 2 and D 2 . Later, Schreel and ter Meulen 12 measured -doublet resolved inelastic cross sections for OH-H 2 , using an electrostatic hexapole as a state selector for OH.…”

Rotationally inelastic scattering of OH by molecular hydrogen: Theory and experiment

“…6 Andresen and co-workers were the first to measure relative state-to-state integral cross sections for collisions of OH with H 2 . 7,8 They employed a supersonically cooled OH beam, which prepared an equal population in the two Λ-doublet states of the lowest rotational level ( j = 3/2 F 1 ). Schreel and ter Meulen 9 measured Λ-doublet resolved integral cross sections for OH-para-/normal-H 2 collisions with the help of an electrostatic state selector to prepare the j = 3/2 F 1 f initial level.…”