We have performed high resolution XPS experiments of the Ru(0001) surface, both clean and covered with well-defined amounts of oxygen up to 1 ML coverage. For the clean surface we detected two distinct components in the Ru 3d 5/2 core level spectra, for which a definite assignment was made using the high resolution Angle-Scan Photoelectron Diffraction approach. For the p(2 × 2), p(2 × 1), (2 × 2)-3O and (1 × 1)-O oxygen structures we found Ru 3d 5/2 core level peaks which are shifted up to 1 eV to higher binding energies. Very good agreement with density functional theory calculations of these Surface Core Level Shifts (SCLS) is reported. The overriding parameter for the resulting Ru SCLSs turns out to be the number of directly coordinated O atoms. Since the calculations permit the separation of initial and final state effects, our results give valuable information for the understanding of bonding and screening at the surface, otherwise not accessible in the measurement of the core level energies alone.

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We report the first evidence for trajectory-dependent neutralization of hyperthermal energy alkali ions scattered from a clean metal surface. We have scattered 7.5 and 50 eV Na from Cu(001) and found that, for atoms leaving the surface with the same velocity and direction, the measured neutral fraction varies by a factor of 7 depending on the type of surface collision. The collision types are identified with scattering simulations.We suggest that coupling of the charge exchange and collision dynamics is responsible for the variations in neutralization. PACS numbers: 79.20.Rf, 34.70.+e, 79.90.+b As charge exchange plays a key role in many dynamical processes at surfaces, such as trapping, molecular dissociation, and stimulated desorption, a better understanding of charge exchange mechanisms may eventually lead to advances in surface processing and catalysis. Additionally, a number of analytical techniques including secondary ion mass spectroscopy (SIMS) and low energy ion scattering spectroscopy (LEIS) depend on the detection of ions leaving a surface at low velocities. The quantitative accuracy of these techniques can rely on knowledge of the charge state distribution of the atoms leaving the surface.More fundamentally, charge exchange provides a way to probe the evolution of the electronic states of an atom approaching a surface. For these reasons charge exchange has received extensive experimental and theoretical investigation.In general, charge exchange is a very complex process involving a number of different mechanisms for electronic rearrangement. Resonant [1] and Auger [2] mechanisms of charge exchange, and electron promotion in the collision with the surface [3], possibly followed by autoionization of excited neutrals [3], must all be considered. However, for alkalis scattered from metals at hyperthermal energies (a few to several hundred eV) a picture which includes only resonant charge exchange has been sufficient to explain measured charge state fractions [1,4]. For an alkali scattered from a metal surface at hyperthermal energies, the final charge state is thought to be determined on the outgoing portion of the trajectory by resonant charge exchange between the electronic states of the scattering atom and the surface [1]. Close to the surface, very high electron transition rates between the atom and surface force the electronic states of the atom and metal towards equilibrium occupancies independent of their initial fillings;i.e. , the atom loses memory of its initial charge state. As the electron transition rates decrease along the outgoing trajectory, the electronic system evolves nonadiabatically. The evolution continues until the atom is far from the surface where the electron transition rates approach zero.The final charge state depends on the velocity and angle of the outgoing trajectory. The same final velocity and angle can be produced by different types of collisions.Since the scattering collision occurs at the surface and the determination of the final charge state occurs on the outgoing ...

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