Quantum scattering studies of the Λ doublet resolved rotational energy transfer of OH(X 2Π) in collisions with He and Ar

Abstract

Three dimensional potential energy surfaces for the collision systems OH(X 2Π)+He and OH(X 2Π)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate 2Πx and 2Πy states split into two states of 2A′ and 2A″ symmetry, respectively, when the C∞v symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A″ and A′ potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close-coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Λ doublet resolution show, within the same spin–orbit manifold and at low collision energies, a propensity to populate preferentially the e final levels in the F1(2Π3/2) state and an e/f conserving propensity in the F2(2Π1/2) state, while transitions between the two spin–orbit manifolds show a parity conserving propensity. For the v=2 vibrational level kinetic rate coefficients were calculated for a large range of temperatures. The calculated cross sections are in excellent agreement with recent measurements of Schreel, Schleipen, Epping, and ter Meulen.

An analytic model of rotationally inelastic collisions of polar molecules in electric fields

“…It was explained previously in Ref. 27 by a rather involved analysis of the close-coupling matrix elements. The qualitative features of the scattering in an electric field can also be readily explained by the Fraunhofer model.…”

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

“…The X 2 electronic ground state of the OH radical originates from a π 3 electron occupancy, leading to predominantly positive values for V sum and V dif [24,39]. As a result, final states for which the dominant A l and B l factors have equal signs are coupled more strongly to the F 1 (3/2f ) initial state than final states for which A l and B l have opposite signs.…”

“…Excitation into the F 2 (1/2f ) state is governed exclusively by V 10 in combination with a small value for A (A = −0.07), while the V 22 term in combination with a large value for B (B = 0.44) dominates the excitation into the F 2 (1/2e) state [24,39]. As pointed out by Esposti et al the qualitative analysis given above works well for excitation into rotational states with low J only; for excitation to higher J a qualitative analysis based exclusively on the A l and B l coefficients is not straightforward [24]. The measured relative state-to-state cross sections directly yield qualitative information on the expansion coefficients of the potential energy surfaces that govern the scattering between OH and He.…”

“…A general analysis of the scattering of 2 molecules with spherical objects has been developed by Dagdigian et al [39], and can be used to qualitatively understand the inelastic scattering of OH radicals with atomic collision partners [24,40]. From the formal quantum analysis of the scattering, it follows that the coupling between rotational states can be evaluated from…”

Rotational Energy Transfer of SH(X2Π, v′′=0, J′′=0.5–10.5) by Collision with Ar: Λ‐Doublet Resolved Transition Propensity

“…Our discussions are essentially based on their arguments, which are briefly summarized in the following. The coupling matrix elements of potential terms in close-coupling equation are expressed as Equation (2) [32, 34,35] …”