A wave-packet propagation study is presented of the ionization dynamics of xenon and hydrogen Rydberg atoms interacting with a metal surface in the presence of an external field. The calculations are performed using a Coulomb-wave discrete variable representation, which allows an efficient extension of previous calculations to a higher principal quantum number. The wave-packet calculations include nonadiabatic effects at avoided energy level crossings. Ionization probabilities as a function of distance from the surface are compared with complex-scaling calculations, which assume purely adiabatic traversal of the avoided crossings. A comparison is made between the dynamics calculated for the "normal" experimental situation, where the applied field is oriented so as to repel positive ions away from the surface, versus the dynamics for the reversed field situation, in which electrons are repelled from the surface. Overall it is clear that reversing the field direction has a pronounced effect on the ionization dynamics for any given starting level and that the nonadiabatic effects are most pronounced in the reversed field case. For certain field ranges, electron flux is found to be "backscattered" away from the surface in the reversed field configuration. Preliminary mean-field calculations are also presented to evaluate the effect of the acceleration of the atom on the ionization dynamics.

The charge transfer of Rydberg hydrogen atoms at a metal surface is investigated for the first time. The surface ionization of Stark states with various electron density distributions with respect to the surface is examined. Unlike the nonhydrogenic species studied previously, genuine control over the orientation of the electronic wave function in the surface-ionization process is demonstrated. A comparison of the results for a range of collisional velocities for the most redshifted Stark state with principal quantum numbers n=20-36 with the classical over-the-barrier approach shows a good agreement for the onset of the ion signal, but the shallow rise in signal is not accounted for. An excellent fit of the experimental results can be achieved using a simple semiempirical model.

The charge transfer (ionization) of hydrogen Rydberg atoms (n = 25 − 34) at a Cu(100) surface is investigated. Unlike fully metallic surfaces, where the Rydberg electron energy is degenerate with the conduction band of the metal, the Cu(100) surface has a projected bandgap at these energies, and only discrete image states are available through which charge transfer can take place. Resonant enhancement of charge transfer is observed for Rydberg states whose energy matches one of the image states, and the integrated surface ionization signals (signal versus applied field) show clear periodicity as a function of n as the energies come in and out of resonance with the image states. The surface ionization dynamics show a velocity dependence; decreased velocity of the incident H atom leads to a greater mean distance of ionization and a lower field required to extract the ion. The surface-ionization profiles for 'on resonance' n values show a changing shape as the velocity is changed, reflecting the finite field range over which resonance occurs.The collision of a Rydberg atom in the gas phase with a solid surface typically leads to transfer of the Rydberg electron to the surface at distances less than 5n 2 a 0 , where n is the Rydberg electron principal quantum number. This is especially true for metallic surfaces, where the Rydberg electron energy is degenerate with the conduction band so that resonant charge transfer (RCT) can occur. Experimental and theoretical studies of this phenomenon have focused on the effects of varying the n quantum number, the parabolic quantum number k, the velocity of the incoming particle and the applied fields [1,2], and observing how the rate of ionization varies as a function of distance from the surface [3]. For nonhydrogenic atoms, adiabatic and non-adiabatic passage through surface-induced energy level crossings leads to behavior that varies with the Rydberg species [4]. Thus, such studies reveal important information about the Rydberg states and their dynamics near surfaces.An equally important question for such studies is what they reveal about the nature of the surface. Experimental studies have been primarily conducted with flat-metal surfaces for which the ionization dynamics are almost independent of the material because of the generic behavior of RCT to the conduction band. However, there have also been some experimental and/or theoretical investigations of the effects of adlayers and thin insulating films [5], interaction with doped semiconductor surfaces [6] and dielectric materials [7], effects of corrugation and of patch charges [8,9]. Related theoretical calculations were used to investigate the variation of ionization rate of ground state H -with the thickness of a metal film substrate [10]. All these studies point to a degree of sensitivity of the charge transfer process to the surface characteristics. The mean radius of a hydrogenic Rydberg orbit is of order n 2 a 0 (e.g., ∼ 20 nm for n = 20) and charge transfer typically occurs at a Rydberg-surface distance of 3 − 5...

Wavepacket propagation calculations are reported for the interaction of a Rydberg hydrogen atom (n = 2 − 8) with Cu(111) and Cu (100) surfaces (represented by a Chulkov potential), in comparison with a Jellium surface. Both copper surfaces have a projected band gap at the surface in the energy range degenerate with some or all of the Rydberg energies. The charge transfer of the Rydberg electron to the surface is found to be enhanced for n values at which there is a near-degeneracy between the Rydberg energy level and an image state or a surface state of the surface. The enhancement is facilitated by the strong overlap of the surface image-state orbital lying outside the surface and the orbital of the incoming Rydberg atom. These calculations point to the possibility of using Rydberg-surface collisions as a probe of surface electronic structure.

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