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 Omega M-J] rotational states. The radicals were produced in a corona discharge, supersonic molecular beam source by dissociating H2O (D2O) 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 A-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.