The scattering of focused beams of hexapole-oriented t-BuCl and CHF3 molecules from a graphite (0001) surface has been studied over the surface temperature range 165 < Ts<730 K. The detected number densities of the surface-scattered beams are compared for parallel vs antiparallel incident orientation of the molecular dipole with respect to the surface normal. From the fractional difference in scattered signals, i.e., the so-called steric effect, and from the scattering angular distributions of the unoriented molecules, the relative difference in scattering probabilities for opposite molecular orientations is obtained. For both molecules the magnitude of the relative difference in scattering (the steric effect) increases as Ts decreases. At the lowest surface temperature of 165 K the relative difference in scattering probability reaches +1.5 for t-BuCl (and −0.8 for CHF3), approaching the largest possible magnitude of 2.0, as compared to +0.3 (and −0.4 for CHF3) at the highest temperatures. At all temperatures the steric effect for directly (near-specularly) scattered molecules depends linearly on the degree of molecular orientation. From the angular distribution measurements, it is found that the inelastic translational energy loss in the ‘‘direct’’ scattering process increases as Ts decreases. Finally, the temperature dependence of the scattering probability of unoriented beams of t-BuCl and CHF3 has been obtained.

Threshold photoionization experiments on beams of rare gas atomic clusters doped with a single metal atom ( A k N ; N = 1-200) reveal pronounced discontinuities in cluster abundances as a function of N. Abundanm at the photoionization thresholds for clusters containing up to 60 atoms and fragmentation probabilities measured on a 5 0 -p~ time scale provide compelling evidence that the neutral clusters, as well as the corresponding ions, pack in icosahedral shells.The optimal packing of a finite number of equivalent spheres follows a noncrystallographic motif characterized by concentric icosahedral shells.' Filled-shell structures consist of N* = 13, 5 5 , 147, 309, ... spheres and are called Mackay icosahedra.2

Time-of-flight distributions of beams of hexapole-oriented CHF3 and t-BuCl molecules scattered from a graphite (0001) surface have been measured for parallel vs antiparallel incident orientations of the molecular dipole with respect to the surface normal, over a range of surface temperatures 170≤Ts≤730 K. The observed difference in arrival times, Δtexp, for opposite initial orientations depends strongly on the degree of orientation of the incident molecules. In the analysis of the Δtexp data, we make use of the two-component model, which assumes that the scattered beams are composed of directly scattered and trapped/desorbed molecules. It is shown that in the common case of short residence times for the trapped molecules, the difference in arrival times for the directly scattered molecules, Δtdir, can be ascertained from the measured Δtexp. The magnitudes of the calculated Δtdir correspond to a strong orientation dependence in the translational energy transfer accompanying the direct scattering of CHF3 and t-BuCl by graphite (0001). The final translational energy of directly scattered molecules E′ is found to be smaller for the collision of the H ‘‘end’’ of fluoroform with the graphite surface; for t-BuCl, E′ is smaller for the Cl ‘‘end’’ collision. These are the orientations that also give rise to higher trapping probability. In the course of the present study, the residence times of t-BuCl on graphite (0001) have been measured over the surface temperature range 170<Ts<240 K.

By employing rotationally state-selected ND3 molecular beams, ‖JKMε≳‖inversion≳ =‖1111≳‖−≳, and REMPI detection, it is found that there is a propensity for inversion symmetry change from antisymmetric to symmetric states, ‖−≳ to ‖+≳, as compared to antisymmetric to antisymmetric states, ‖−≳ to ‖−≳, in the scattering of ND3 on a graphite (0001) surface at 130 K. The total probability of inversion symmetry change summed over the post-collisional rotational states is measured as a function of translational energy. We demonstrate that the observed effect is due to molecular self-diffraction on the surface and that it may serve as a new way to probe the molecule–surface interaction. Relying on the infinite order sudden approximation [Gerber et al., J. Chem. Phys. 73, 4397 (1980)], we suggest a procedure for inverting an approximate, corrugation-averaged molecule–surface interaction potential from experimental data on molecular self-diffraction; measurements of the probability of inversion symmetry change for different incident rotational states as a function of translational energy are required. Since we do not possess sufficient experimental data for a full-fledged analysis, computer simulations of the observed energy dependence are carried out with model interaction potentials.

State-selected molecular beams of deuterated ammonia, ‖JKMε≳‖inversion≳=‖1111≳‖−≳ or ‖222−1≳‖−≳ and ‖3331≳‖−≳ states in the proportion 2.3:1, are produced via hexapole electrostatic focusing and then scattered at near-normal incidence on a graphite (0001) surface at Ts=130 K. The post-collisional rotational distribution of directly scattered molecules is monitored by laser photoionization spectroscopy. It is observed that the state-to-state rotational transitions in the surface scattering of ND3 are strongly coupled to the inversion symmetry change, as predicted by Corey and Alexander [J. Chem. Phys. 89, 790 (1988)]. This coupling may be viewed as a constructive and destructive interference between the ‘‘left’’ and ‘‘right’’ inversion configurations of the molecule on the surface potential. The selection rules prohibiting para to ortho conversion, which have been predicted by Corey and Alexander for rotationally inelastic scattering of ammonia, are observed to be violated, perhaps, due to ND3 distortion at the impact.