Resonant Charge Transfer of Hydrogen Rydberg Atoms Incident on a Cu(100) Projected Band-Gap Surface

Abstract

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...

Rydberg-state ionization dynamics and tunnel ionization rates in strong electric fields

“…The results presented are of importance in the optimization of state-selective electric field ionization schemes employed in Rydberg-atom−polar-molecule collision studies [7,8]. They are also of interest for experiments in which helium atoms in low-Rydberg states are used to study charge transfer processes in collisions with surfaces [11]. The general observations made suggest that the Rydberg states studied here can act as a valuable testing ground for calculations of tunnel ionization in strong electric fields.…”

Experimental progress in positronium laser physics

“…Similar methods have been used in neutral atom lithography with metastable atoms [653,654], with some improvements suggested if Rydberg atoms are used [655]. As an extension, the Rydberg atoms themselves could be used as surface probes via resonant charge transfer processes [426,427]. Neutralization of laser cooled ions in Penning or Paul traps has also been suggested as a method to produce cold atoms or molecules that are not amenable to laser cooling [435].…”

“…This may occur because in most Rydberg systems an electron is promoted to a highly excited state, leaving the rest of the atom or molecule as a slow moving spectator, whereas in the case of Ps the electron and positron can both interact with external systems in similar ways. This may, for example, affect the way that Rydberg Ps atoms interact with surfaces [421][422][423][424]; such interactions can be exploited to study various processes, for example charge transfer [425][426][427] or measurements of electric fields [428,429], and may also be of relevance to possible Ps interferometry experiments.…”

Handshake electron transfer from hydrogen Rydberg atoms incident at a series of metallic thin films

“…The experimental setup is shown in figure 3 and has been described in more detail elsewhere 2,7 . In outline, a velocity-and quantum-state-selected beam of Rydberg H atoms is incident on the surface of an iron thin film deposited in vacuum on a Muscovite substrate.…”