No abstract

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

The orientational dependence of the scattering of hexapole-focused, oriented molecule beams of six symmetric-top molecules (CH3Cl, CH3F, CHCl3, CHF3, t-BuCl, CH3CN) and CH3OH from the (0001) surface of a graphite crystal is reported. Experimental angular distribution data are well represented by a two-component model, consisting of a ‘‘trapped/desorbed’’ and a ‘‘direct’’ scattered contribution. The steric effect, defined as the difference (‘‘heads’’ vs ‘‘tails’’) divided by the average of the scattered signals, has been measured as a function of the scattering angle and the degree of orientation of the molecules. There is considerable diversity among the different molecules with respect to the direction and magnitude of the steric effect of the scattering (and trapping). In all cases, however, the magnitude of the steric effect is essentially a linear function of the degree of orientation. Limited data on the incident energy dependence of the angular distributions and the steric effect are also presented. A model which deconvolutes the steric effect for the trapped/desorbed and directly scattered components is introduced. It also provides an independent estimate of the trapping probability of the incident molecules.

The degree of orientation of CH3I in rotation-state-selected, oriented-molecule beams has been measured via the asymmetry of the angular distribution of the I( 2 Pm) photodissociation product from the linearly-polarized-laser-induced photofragmentation of CH3I in (parent) rotational states | JKM) = 1111), I 212), I 211), and | 222). For the latter state the up/down asymmetry is nearly 4/1 (less for the other states). Partial disorientation of the CH3I due to hyperfine coupling in a weak electric field has also been demonstrated.