Rotational State Selection and Orientation of OH and OD Radicals by Electric Hexapole Beam-Focusing

An electrostatic hexapole was used to state-select OH and OD radicals in single, low-lying, | J Ω M J 〉 rotational states. The radicals were produced in a corona discharge, supersonic molecular beam source by dissociating H 2 O (D 2 O) seeded in Ar or He. Beam velocities ranged from 650 to 1850 m s -1, and translational temperatures were less than 10 K for all expansion conditions. Measured beam flux densities, J, of selected states were high (e.g., J > 10 13 radicals cm -2 s -1 for the | 3 / 2 ± 3 / 2 ∓ 3 / 2 〉 states of OH seeded in He). Classical trajectory simulations reproduced the well-resolved rotational state structure of experimental beam-focusing spectra. Simulations were based on a Stark energy analysis of the rotational energy levels, including significant effects due to Λ-doubling and spin−orbit coupling. Orientational probability distribution functions were calculated in the high-field limit for all selectable states and demonstrate exceptional experimental control over collision geometry for scattering experiments.