Hexapole-Oriented Molecule Beams Scattered by Single Crystal Surfaces.

A newly constructed machine capable of producing beams of spatially oriented molecules is described in detail. Beam molecules are focused and state-selected by a 2.85 m electrostatic hexapole. The machine consists of seven differentially pumped chambers with an overall length from nozzle to final collimator in the surface scattering configuration of 3.78 m. For symmetric top molecules pure (>95%) {mid}JKM> rotational state selection has been achieved. The distribution of orientations among CH_3I beam molecules has been quantitatively measured using the photodissociation/multiphoton ionization time of flight technique. Results accord with simulated ion time of flight distributions using theoretical orientational probability distribution functions which include the nuclear hyperfine interaction. Oriented molecule beams of seven different molecules have been scattered by a graphite (0001) surface. The results show a large diversity in the sign and magnitude of the steric effects (i.e., "heads" vs. "tails"). The steric effects have been quantitatively measured, and have been analyzed in terms of a two component model: a trapping/desorption component and a direct scattering component. Analysis of the scattered angular distribution data yields estimates for the anisotropy of the trapping probability (e.g., for CHF_3 there is 25% higher trapping probability when the H "end" of the molecule is incident on the graphite surface than for the F_3 "end"). The magnitude of the steric effect is found to be a linear function of the degree of orientation of the beam molecules for all systems studied. Over the limited range of the present data, the steric effect increases with incident kinetic energy. A null steric effect result was observed for the scattering of oriented CH_3Cl by a W(110) surface. However, the initial sticking probability for randomly oriented CH_3Cl was measured to be unity. It is not surprising that there is no observable steric effect in the scattering of CH _3Cl from this surface. From the body of experimental data on the orientation dependence of the scattering (both direct and trapping/desorption) of polyatomic molecules by graphite, it appears that the origin of the steric effect is the anisotropic molecule -graphite interaction potential, which is, in turn, governed by the charge density distribution of the molecule.