An electrostatic hexapole was used to state-select OH and OD radicals in single, low-lying, |JΩM 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 > 1013 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.

Communication: Feshbach resonances in the water molecule revealed by state-selective spectroscopy

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Recently, Hain, Toby D.; Weibel, Michael A.; Backstrand, Kyle M.; and Curtiss, Thomas J. J. Phys. Chem. A 1997, 101, 7674 reported the production of intense, rotationally state-selected, supersonic beams of hydroxyl radicals via electric hexapole focusing. Here, a detailed description of the lab frame orientation of selected radicals is provided. The distribution of orientations can be systematically varied with the electric field strength in a post-hexapole scattering region. This control of orientation results from the field-dependent mixing of the different parity states comprising the OH Λ-doublets. Calculated fluorescence yields show polarization-dependent LIF measurements probe the alignment terms of the orientation distribution, and field-dependent measurements probe the parity state composition.

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