State-to-state cross sections for rotational excitation of OH by collisions with He and Ar

Abstract

Parity resolved state-to-state cross sections for rotational excitation ofOH(X 2n) colliding with He and Ar, have been obtained in a crossed molecular beam experiment. The OH radicals were produced in a pulsed dc discharge in a H 2 0/ Ar gas mixture. Adiabatic expansion into vacuum resulted in an effective rotational cooling yielding a 94% population of the lowest A-doublet (X 2n3/2' J=~). Further state preparation could be achieved via electrostatic state selection in a hexapole electric field, resulting in a 93.5% population of the upper A-doublet component (f, + ). Experiments were performed both with and without the state selector to provide detailed information about the rotational excitation from both A-doublet states. The OH rotational state distribution was probed, before and after the collision event, by means of laser-induced fluorescence (LIF) spectroscopy of the A -X electronic band at 308 nm. The OH-Ar and OH-He scattering behave very similar with the exception of the J=~ A-doublet transition which is induced much weaker by the He collisions.

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

“…At high collision energies, the relative state-to-state cross sections and the propensities are consistent with the observations by Schreel et al [20]. In that experiment, however, the strong propensities were partially concealed due to a sizable initial population in the F 1 (5/2f ) state [20].…”

“…The (mass scaled) collision cross sections for the theoretically more tractable OH-He system are often used to model the collision dynamics in these environments. In a crossed beam experiment, Schreel et al prepared the OH radicals in the upper -doublet component of the X 2 3/2 ,J = 3/2 level by hexapole state selection [20]. Accurate state-to-state inelastic scattering cross sections were obtained at a collision energy of ∼400 cm −1 .…”

State-to-State Scattering of Oriented OH

“…6 The potential surfaces for the two OH orientations show a larger anisotropy for Ar than for He. This seems to be in direct contradiction with our observation that He shows on the average larger steric asymmetry factors, whereas the state-to-state cross sections are smaller than for Ar.…”

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

“…In a series of crossed beam experiments, ter Meulen and coworkers prepared the OH radicals in the upper L-doublet component of the X 2 P 3/2 , J = 3/2 level by hexapole state selection. 27,28 Accurate parity-resolved integral state-to-state cross sections for rotational excitation up to the X 2 P 3/2 , J = 9/2 and the X 2 P 1/2 , J = 5/2 states were obtained at high collision energies. Preferred excitation to one of the L-doublet states of the final rotational and spin-orbit state was observed, in agreement with the general propensity rules that follow from a formal quantum analysis.…”