Rotationally inelastic collisions of OH(X 2Π)+Ar. II. The effect of molecular orientation

Abstract

Orientation effects in inelastic collisions of OH(X 2Π)+Ar at an energy of 746 cm−1 have been studied in a crossed molecular beam experiment. The OH(X 2Π) radicals were prepared in the v=0, Ω=32, J=32, f state by hexapole state selection and oriented with their O-end or H-end towards the Ar atom by a static electric field in the collision zone. The orientation-dependent probability density function has been determined by laser induced fluorescence spectroscopy using a narrow band dye laser and the Stark mixing induced P1′(1) transition. A degree of orientation of 〈cos θ〉=0.55 has been obtained. The steric asymmetry factor has been determined for rotational excitation up to the Ω=32, J=92 and Ω=12, J=52 states. Differences up to a factor of 8 in cross section were found between O-end and H-end impact. In general a slight preference for O-end impact was found for low rotational excitation, while a strong preference for H-end impact was found for excitation to high rotational states. The experimental results are compared to quantum scattering calculations on ab initio potential energy surfaces developed by Degli-Esposti and Werner [J. Chem. Phys. 93, 3351 (1990)] and Klos et al. [J. Chem. Phys. 112, 4952 (2000)]. The agreement between experiment and calculations based on the Klos and Chalasinski potential is excellent for transitions to states of A″ symmetry and good for transitions to states of A′ symmetry.

Near-Threshold Inelastic Collisions Using Molecular Beams with a Tunable Velocity

“…Collisions populating the F 1 (3/2e) level were most likely. Within the F 1 (5/2) level, collisions populating the lower Λ-doublet component of e parity were favored, consistent with findings of other 2 Π-rare gas systems (28,29 ). For the F 1 (3/2e) and the F 1 (5/2e/f ) levels, the transition probabilities were almost constant at higher collision energies.…”

“…In a crossed beam set-up, rotationally inelastic scattering between state-selected OH radicals (X 2 Π 3/2 , v = 0, J = 3/2, f (27 ), referred to hereafter as F 1 (3/2f )) and Xe atoms is studied throughout the 0.14 to 1.14 kcal/mol (50 to 400 cm −1 ) region, with an overall energy resolution of ∼ 0.03 kcal/mol (13 cm −1 ). We chose the OH-rare gas system because, at higher collision energies, rotationally inelastic collisions have been studied for this system in great detail, both experimentally and theoretically; state-to-state cross-sections and the effects of molecular orientation have been determined (28,29 ). The energy range covered in the present study encompassed the energetic thresholds for inelastic scattering down to the lowest rotational levels of OH.…”

Resonances in rotationally inelastic scattering of OH(X2Π) with helium and neon

“…Early calculations [17,18] on rotationally inelastic scattering of N 2 molecules with He atoms have shown that resonances occur at low collision energies, but the experimental verification of these predictions was not yet possible. Collisions of OH(X 2 Π) with rare gases have emerged as paradigms of scattering of an open-shell molecule with an atom [13,15,[19][20][21][22][23][24]. The OH-rare gas systems are good candidates for the observation and analysis of resonances in rotationally inelastic collisions because the collision energy can be reduced by Stark deceleration of the OH beam.…”

State-to-state inelastic scattering of Stark-decelerated OH radicals with Ar atoms

“…29,30 Steric asymmetries of the inelastic cross sections were measured by orienting the OH radicals with either the O-end or the H-end towards the Ar atom by a static electric field in the collision zone. 31 The collision induced reorientation of the OH radicals was measured by probing the Stark-split states of the products corresponding to different orientations. 32 Under thermal bulk conditions, the evolution of oriented or aligned OH (X 2 P) radicals was studied in collisions with argon by polarization spectroscopy.…”