The proposed work is divided into two parts. The first part consists in the development of density functional models and their efficient numerical implementation with the goal of providing effective surface potentials for at and nano-structured surfaces that reproduce to the extent possible the known surface electronic structure, including surface states, image states, and the position of band gaps for specific crystal orientations. In the second part of the proposed work, these potentials will serve as input for the dynamical investigation of electron exchange between a projectile and the surface. Technically, this part will be based on the direct numerical integration of the one electron time dependent Schreodinger equation on a three dimensional numerical grid, using wavepacket propagation
Introduction: We developed models and carried out quantum mechanical calculations that quantify the transfer and emission of localized core and delocalized conduction electrons from metal surfaces during their interaction with incident particles or ultrashort laser intense laser pulses.Our research examined the dependence of the energy and life time of transient electronic states near and in metal surfaces on the morphology and electronic structure of the substrate and may contribute to the better understanding of surface--chemical (catalytic) reactions. This work was interdisciplinary: The modeling of i) the electronic structures of projectile and substrate, ii) the electron-transfer and emission dynamics, and iii) the projectile motion all provided new insights into atomic collision, solid state, and surface physics. The visualization of the evolving electronic probability distributions during the particle--surface interaction, promotes an intuitive understanding of the basic underlying physics in terms of classical electronic motion and electronic confinement, scattering, and diffraction. The investigation of electron transfer and resonance formation during particle--surface interactions is of critical importance for various areas of science and engineering. The knowledge gained from this work may be used in applied fields of physics, electronics, and engineering. Examples are the development of ion sources, nano--technology, surface analytical methods, the control of ion-wall interactions in fusion plasma, ion etching, sputtering, thin-film deposition, and, in a very broad sense, surface chemistry, including catalysis and corrosion prevention. The enabling character of basic light or ion--surface interaction studies is due to the critical intermediate role of electron--transfer processes in complex surface--chemical reactions. Specific outcomes: 1. For the reflection of hydrogen atoms off unreconstructed atomically flat silicon surfaces, we showed how the evolution of the outgoing hydrogen anion charge fraction is determined by electronic transitions from the valence-band levels of the substrate into the affinity level of the projectile. Our numerical results are in good qualitative agreement with available experimental data. 2. Silicon (100) surfaces are known to reconstruct into pairs of Si atom that are aligned on the surface. We calculated the yield of outgoing hydrogen negative ions after the reflection of neutral hydrogen atoms from (2x1)-reconstructed Si(100) surfaces and found that the electron-transfer dynamics very sensitively depends on both, the substrate electronic structure and the projectile trajectory. In particular, we showed that electron capture is more likely for scattering trajectories that are directed perpendicular to rows of silicon-dimers compared with trajectories that are oriented parallel to the dimer bonds. Our theoretical results are in good quantitative agreement with measured outgoing hydrogen-negative-ion fractions. 3. The emission of surface electrons by ultrashort laser pulses with photon energies in the extended ultraviolet (XUV) regime into the electric field of a delayable infrared (IR) laser pulse allows the measurement of relative time delays between the detection of photoelectrons that originate in different electronic states of the surface. Our calculated emitted electron spectra for XUV-induced and IR-assisted ("streaked") photoemission from a tungsten surface reproduce the recently measured relative delay between core-level and conduction-band electrons for a tungsten surface. Our model suggests the interpretation of the observed temporal shift in terms of the interference of photoreleased 4f-core-level electrons that originate in different layers of the substrate. 4. Our time-dependent calculations also reproduce the relative sideband heights and thus the Auger lifetime of a recent time-resolved experiment on photoelectron emission from adsorbed-covered surfaces. Furthermore, our calculations allows for a systematic search for the most suitable laser parameters (pulse lengths, wave lengths, and intensity) for the observation of electronic processes at surfaces with a time resolution equal to the natural time scale of the motion of electrons in matter. 5. We showed that the survival probability of negative hydrogen ions scattered from an atomically flat metal surface is not monotonic, as a simple ion-surface interaction-time model would suggest, but structured with band-gap-confinement and image-state-recapture peaks. The band-gap-confinement peak emerges from the spectral confinement of the ion's affinity level inside the metal band gap as the interaction becomes non-adiabatic in character. The image-state-recapture peak is due to a boost in the recapture probability as image states are dynamically populated by charge transfer from the ion. We expect this result to be generic for any metal surface which has a band gap closely below or across the vacuum level.
