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 of OH(X 2Π) 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 H2O/Ar gas mixture. Adiabatic expansion into vacuum resulted in an effective rotational cooling yielding a 94% population of the lowest Λ-doublet (X 2Π3/2, J=3/2). Further state preparation could be achieved via electrostatic state selection in a hexapole electric field, resulting in a 93.5% population of the upper Λ-doublet component (f,+). Experiments were performed both with and without the state selector to provide detailed information about the rotational excitation from both Λ-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=3/2 Λ-doublet transition which is induced much weaker by the He collisions. For both collision systems the experimental results are in agreement with theoretical cross sections obtained from quantum scattering calculations of Werner et al. [J. Chem. Phys. (submitted)].