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A quantal model description of a discrete localized skyrmion singularity embedded in a ferromagnetic environment is proposed. It allows discussing the importance of various parameters in the appearance of a quantal skyrmion singularity. Analysis of the skyrmion reveals a few specific quantal properties: presence of a whole series of skyrmion states, non-classical nature of the local spins, presence of superposition states and presence of extra-skyrmion states due to the quantization of the central spin of the singularity. The interaction of an electron, tunneling or substrate, with the skyrmion is also described allowing a new view at the origin of the skyrmion stability as well as the possibility to discriminate between ferromagnetic and skyrmion phase in an Inelastic Electron Tunneling Spectroscopy (IETS) experiment, based on the skyrmion quantal properties.

The time evolution of a model quantal magnetic moment proportional to an angular momentum, S, is studied and compared to that of a classical magnetic moment. The presence of an anisotropy quadratic term in the Hamiltonian makes classical and quantal behaviours qualitatively different for small values of the angular momentum, S. When S is increased, the classical behaviour is retrieved for a finite amount of time, even in the presence of magnetic anisotropy terms; however, in general, this limit is only reached for very large S values. The classical vs quantal behaviour is discussed in terms of three characteristic times of the system: the classical precession time, the quantal revival time and the spreading time. This allows showing that the closer to equilibrium the initial conditions are, the easier the classical limit is reached.

The creation of molecule-like structures in which magnetic atoms interact controllably is full of potential for the study of complex or strongly correlated systems. Here, we create spin chains in which a strongly correlated Kondo state emerges from magnetic coupling of transition-metal atoms. We build chains up to ten atoms in length by placing Fe and Mn atoms on a CuN surface with a scanning tunneling microscope. The atoms couple antiferromagnetically via superexchange interaction through the nitrogen atom network of the surface. The emergent Kondo resonance is spatially distributed along the chain. Its strength can be controlled by mixing atoms of different transition metal elements and manipulating their spatial distribution. We show that the Kondo screening of the full chain by the electrons of the nonmagnetic substrate depends on the interatomic entanglement of the spins in the chain, demonstrating the prerequisites to build and probe spatially extended strongly correlated nanostructures., D.J.C. and J.A.J.B. acknowledge postdoctoral fellowship from the Alexander von Humboldt foundation. M.T. acknowledges support from the DFG-SFB 767. J.A.J.B. acknowledges support from the Natural Sciences and Engineering Research Council of Canada. N.L. and R.R. acknowledge support form Spanish MINECO (grant no. MAT2012-38318-C03-02 with joint financing by FEDER Funds from the European Union). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, grant SEV-2013-0295).

Heterogeneous atomic magnetic chains are built by atom manipulation on a ${\mathrm{Cu}}_{2}\mathrm{N}/\mathrm{Cu}$ (100) substrate. Their magnetic properties are studied and rationalized by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) work completed by model Hamiltonian studies. The chains are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the ${\mathrm{Cu}}_{2}\mathrm{N}$ surface. Here, we present results for ${\mathrm{FeMn}}_{x}$ chains $(x=1--6)$ emphasizing the evolution of the geometrical, electronic, and magnetic properties with chain size. By fitting our results to a Heisenberg Hamiltonian we have studied the exchange-coupling matrix elements $J$ for different chains. For the shorter chains, $x\ensuremath{\le}2$, we have included spin-orbit effects in the DFT calculations, extracting the magnetic anisotropy energy. Our results are also fitted to a simple anisotropic spin Hamiltonian and we have extracted values for the longitudinal-anisotropy $D$ and transversal-anisotropy $E$ constants. These parameters together with the values for $J$ allow us to compute the magnetic excitation energies of the system and to compare them with the experimental data.

We report on a theoretical study of the lifetime of the first excited state of spin chains made of an odd number of Fe atoms on Cu2N/Cu(100). Yan, Choi, Burgess, Rolf-Pissarczyk, and Loth [Nat. Nanotech. 10, 40 (2015)] recently observed very long lifetimes in the case of Fe3 chains. We consider the decay of the first excited state induced by electron-hole pair creation in the substrate. For a finite magnetic field, the two lowest-lying states in the chain have a quasi-Néel state structure. Decay from one state to the other strongly depends on the degree of entanglement of the local spins in the chain. The weak entanglement in the chain accounts for the long lifetimes that increase exponentially with chain length. Despite their apparently very different properties, the behavior of odd and even chains is governed by the same kind of phenomena, in particular entanglement effects. The present results account quite well for the lifetimes recently measured by Yan et al. on Fe3.

Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on CuN/Cu (100) reveal an unsuspected electronic edge state at ∼ 1 eV above the Fermi energy. This Tamm-like state is strongly localised to the terminal Mn atoms of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at ∼ -1 eV is mainly spin unpolarised due to the extended p-states of the N atoms that mediate the coupling between the Mn atoms in the chain. The spin polarisation of the edge state is affected by the antiferromagnetic ordering of the chains leading to non-trivial consequences., We acknowledge the European Union for support under the Marie-Skłodowska Curie Program through IF (DJC) and CIG (JIP) grants. We also acknowledge support from MINECO via grants MAT2015-66888-C3-2-R (NL), MAT2013-46593-C6-01 (JIP) and Severo Ochoa Program grant SEV-2013-0295 (ICN2).

We report on a theoretical study of the escape of confined surface states electrons from quantum corrals made of Cu adatoms on a Cu(1 1 1) surface. This study maps electron transmission through the corral wall and provides an extension of our earlier work focused on confinement in Cu corrals [S. Diaz-Tendero, F.E. Olsson, A.G. Borisov, J.P. Gauyacq, Phys. Rev. B 77 (2008) 205403]. The existence of two decay modes for the confined surface state is stressed: (i) non-resonant tunnelling through the corral wall concentrated on the Cu adatoms and (ii) a resonant-induced decay involving the transient formation of a resonant state localized on top of the corral wall. The present mapping of the electron transmission reveals how the interference between the two decay modes works: there exist regions where the electron leaves the corral, balanced by regions where it enters the corral, though the global behaviour of the quasi-stationary states is electron escape from the corral.

Desorption in the Na/Cu(1 1 1) system induced by an electronic excitation is studied using a quantal approach. The system is excited by a laser pulse in the fs range to the Na∗ state corresponding to the transient capture of an electron by the alkali adsorbate. The present quantal approach describes on an equal footing the laser-induced vibrational excitation of the adsorbate in the adsorption well and the photo-desorption process. It confirms earlier results using a semi-classical input. It also allows a discussion of the photo-desorption probability with the photon energy: the maximum of the desorption probability per absorbed photon occurs off-resonance in the high-energy wing of the electronic transition. This feature is related to the dynamics of the laser-induced process.

The Cs/Cu(1 1 1) and Na/Cu(1 1 1) systems exhibit a transient excited electronic state localized on the adsorbate. Photo-excitation of this state triggers a motion of the alkali adsorbate away from the surface, leading to vibrational excitation of the adsorbate and possibly to desorption. A theoretical study of these photo-induced processes in the case of an exciting fs laser pulse is reported, based on a time-dependent approach of the adsorbate motion. The mean energy transfer from the laser photon energy to the adsorbate motion is shown to be weak, about 1% of the photon energy. Correspondingly, the vibrational excitation to high lying levels is very weak as well as the desorption process. The initial electronic state of the photo-induced process belongs to a continuum and vibrational excitation and desorption are found to vary rapidly with the energy of the initial electronic state. Initial vibrational excitation of the alkali adsorbate is also found to efficiently favour the desorption process, leading to a drastic variation of the desorption probability with the vibrational temperature of the adsorbate. The present results for the two systems are discussed and compared, in connection with available experimental data on these systems and on similar ones.

A new interferometric multi-photon photoemission scheme has been recently introduced by Gudde et al. [1] and applied to a study of the n=1 image state on Cu(100). We report on the results of a theoretical modelling of this photoemission scheme based on a wave function representation. It incorporates both resonant transitions and two-photon non-resonant transitions. In the case of the n=1 image state on Cu(100), the initial state of the photoemission belongs to a continuum. Summation of the contributions to the photoemission yield from the various initial states is shown to drastically blur the interference structure, acting as an efficient dephasing process. The sensitivity of the new scheme and of the one-colour TR-2PPE scheme to the system dynamical properties is discussed.

When a core-excited Ne ∗ (1s −1 n l) atom is formed surrounded by other Ne atoms, the excited orbital interacts with the surrounding dielectric medium. This problem is studied for Ne ∗ (1s −1 n l) atoms located at the surface of a Ne crystal using a microscopic description of the Ne surface that incorporates the dielectric character of the Ne crystal. The environment-induced modifications of the excited orbital can be qualitatively described in terms of confinement and polarization effects. The vacuum/crystal asymmetry in the environment of the excited orbital leads to a considerable asymmetry of the excited orbital in particular for high lying states. Though the lowest lying 3s and 3p states mostly retain their atomic character. Higher lying states are much hybridized and cannot be assigned to a single atomic orbital. The present results compare very well with available experimental data. They lead to a reassignment of the various peaks in the excitation spectra of Wiethoff et al. [P. Wiethoff, H.-U. Ehrke, B. Kassuhlke, C. Keller, W. Wurth, D. Menzel, P. Feulner, Phys. Rev. B 55 (1997) 9387].

The low coverage adsorption of alkalis on metal surfaces induces excited states localised on the adsorbate. In the case of noble metal substrates, these excited states can exhibit a very long lifetime, up to tens of fs in the Cs/Cu(1 1 1) system. We review recent experimental and theoretical investigations of alkalis adsorbed on noble metal surfaces, with emphasis on the characteristics of the alkaliinduced excited states, the origin of their long lifetimes, and the consequences for the adsorbate dynamics. The possibility of long-lived resonances in other adsorbate/substrate systems is also discussed.

Due to the quantum evolution of molecular magnetic moments, the magnetic state of nanomagnets can suffer spontaneous changes. This process can be completely quenched by environment-induced decoherence. However, we show that for typical small supported atomic objects, the substrate-induced decoherence does change the magnetic-moment evolution but does not quell it. To be specific and to compare with experiment, we analyze the spontaneous switching between two equivalent magnetization states of atomic structures formed by Fe on Cu2N/Cu (1 0 0), measured by Loth et al (2012 Science 335 196-9). Due to the substrate-induced decoherence, the Rabi oscillations proper to quantum tunneling between magnetic states are replaced by an irreversible decay of long characteristic times leading to the observed stochastic magnetization switching. We show that the corresponding switching rates are small, rapidly decreasing with system's size, with a 1/T thermal behavior and in good agreement with experiments. Quantum tunneling is recovered as the switching mechanism at extremely low temperatures below the ¿K range for a six-Fe-atom system and exponentially lower for larger atomic systems. The unexpected conclusion of this work is that experiments could detect the switching of these supported atomic systems because their magnetization evolution is somewhere between complete decoherence-induced stability and unobservably fast quantum-tunneling switching., NL acknowledges financial support from Spanish MINECO (Grant No. MAT2012-38318-C03-02 with joint financing by FEDER Funds from the European Union)

The energy of core-excited Ne ∗ (1s −1 n l) atoms inside bulk Ne is computed using a joint pseudopotential-wave packet propagation approach. Special care is taken to ensure the proper dielectric properties of bulk Ne together with its 3D-geometric structure. Results are compared with available X-ray absorption experimental results on the (1s −1 n p) excitons. Analysis of the state energy and of the wave-function of the excited n l electron allows to define two different regimes for the perturbation of the outer n l electron by the neighbouring Ne atoms in the crystal: confinement in the cage formed by the first Ne neighbours for low-lying states and Rydberg states in a screened Coulomb potential for the higher-lying states. An effective screened spherically symmetric potential representing well the electron–hole interaction in the Ne ∗ (1s −1 n l) atoms inside bulk Ne is proposed, based on the analysis of the present 3D-modelling.

The influence of the surface projected electronic structure on resonant charge transfer is investigated on the Cu(1 1 1) and Cu(1 0 0) surfaces using core hole clock spectroscopy and wave packet propagation computations. The charge transfer time of core excited Ar ( 2 p 3 / 2 5 4 s 1 ) adsorbed in a dense Ar monolayer is 5.6 ± 0.1 fs and 3.5 ± 0.1 fs for the Cu(1 1 1) and Cu(1 0 0) surfaces. A two times longer residence time on Cu(1 1 1) than on Cu(1 0 0) is also obtained from theory and is attributed to the differences in the projected electronic band structure of the two metal surfaces, especially to the different energies of the surface projected band gap.

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of defects (Cu adatoms and Cu vacancies) is presented for low defect density at the surface. A wave packet propagation approach is employed for the electron scattering calculations, where the defect induced potentials are obtained from an ab initio density functional study. Scattering of the image state electron by a defect induces inter-band and intra-band transitions leading, respectively, to the population decay and to the dephasing of the image states. Comparison of the respective effects of adatoms and vacancies shows that Cu adatoms are much more efficient in inducing population decay and dephasing of the image potential states. Present results for the case of Cu adatoms are compared with available time-resolved two-photon photoemission data.

The characteristics of negative ion states formed by temporary electron attachment to adsorbed molecules depend on the environment of the molecule. Adsorption site effects on the energy positions and widths of the N 2 - 2 Π g resonance are theoretically investigated for a nitrogen molecule adsorbed on an Ar monolayer covering a Cu metal substrate. The molecule is shown to preferentially adsorb above the valleys between Ar atoms, with its axis parallel to the Ar plane. The N 2 - characteristics are determined by a scattering approach with a 3D microscopic description of the adsorbed Ar layer. The anisotropy of the potential around the molecular adsorption site lifts the resonance degeneracy. For both resonance components, the energy is lowered and the lifetime is increased by the interaction with the Ar layer. The importance of the effect of the Ar–vacuum interface reflectivity observed in an earlier study using a dielectric continuous medium description of the Ar layer is confirmed. Different adsorption sites and orientations are studied, revealing an adsorption site dependence of the resonance characteristics.

Finite size effects are investigated in the case of image states localised on Ar islands on Cu(1 0 0) surfaces. A wave-packet propagation approach associated to a microscopic description of the Ar layer is used to compute the energies and widths of the quasi-stationary image states confined on the Ar islands. Electron transmission through the island edges is found to induce a very efficient decay process for the confined image states. The possibility of observing these finite size effects, together with an estimate of the minimum island size for which the image states properties converge to those on a full ML is also presented.

We use the wave packet propagation approach to study the dynamics of resonant charge transfer in hydrogen back-scattering from Cs adsorbates on Cu(1 1 1). The affinity level of the projectile strongly interacts with an adsorbate-localized state leading to an important modification of the projectile–substrate resonant charge transfer. At very low collision energies (

Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.

We present a 'virtual detector' method which allows the extraction of the momentum distributions in continuum states from time-dependent wave packet propagation studies with explicitly time-dependent Hamiltonians. It is based on the approach developed by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741) for quantum chemistry applications and consists in measuring a signal in the asymptotic region. In that sense, it is close to the semiclassical method proposed recently in this journal (Feuerstein and Thumm 2003 J. Phys. B: At. Mol. Opt. Phys. 36 707). The advantage of the present approach is that it is based on a rigorous quantum mechanical treatment, robust, easy to handle and requires a much smaller grid size. The method is illustrated with the 3D example of electron detachment from the H− ion colliding with a free-electron metal surface.

The local density of states of two model systems formed by an individual adsorbate (Cs or Ar) on a Cu(111) surface is investigated using a wave-packet propagation approach. In both cases a sharp peak appears at the threshold of the 2D Shockley surface-state continuum. This peak is attributed to quasi-stationary states located just below the 2D continuum. Following the existence criterion for a bound state in two dimensions by Simon, such states splitting off the surface-state bottom should appear whenever the surface-state continuum is perturbed by an attractive potential, independently of the existence of atomic-like states bound by the perturbing potential. These findings are discussed in connection with recent observations of peaks located at the surface-state threshold in scanning tunnelling microscopy experiments.

A model is developed to describe the adsorbate coverage dependence of excited state (image states and adsorbate-localised states) on a metal surface. It is applied to the case of Na adsorbates on a Cu(1 1 1) surface. It allows to define in a consistent way (i) an effective potential, which is only function of the electron-surface distance representing the average perturbation introduced by a large set of alkali adsorbates on the surface and (ii) the local perturbation induced around each adsorbate. It can be applied up to moderate coverages, typically for Na-induced work function changes below 1.5 eV. In this range, its predictions reproduce the experimental findings of Fischer et al. [Surf. Sci. 314 (1994) 89] for the energy of the image states and Na-localised state.

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of adsorbates is presented. Scattering of the image state electron by an adsorbate induces inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. We compare results obtained with low coverage (typically 1 adsorbate atom per 1000 surface atoms) Cs, Ar, and a model electronegative adsorbates. As follows from our results, Cs adsorbates lead to both appreciable dephasing and decay, while electronegative adsorbates mostly affect the dephasing rate. The effect of low coverage Ar adsorbates is small, consistent with their neutrality.

A theoretical study of the effect of an atomically thin rare gas layer on the dynamics of excited electronic states at metal surfaces is presented for the case of a few mono-layers of Ar on a Cu(1 0 0) surface. We develop a 3D-microscopic model with predictive capabilities of the interaction of an electron with an Ar layer physisorbed on a metal surface. It takes into account the 3D structure of the Ar layer as well as its dielectric character. The dynamics of the excited electron on the surface is treated within a wave-packet propagation approach. The calculations show that two different types of excited states are present at the Ar/Cu(1 0 0) surface. (i) Image states that are repelled into vacuum as compared to their position on clean Cu(1 0 0) surfaces, leading to a decrease of their binding energies and to an increase of their lifetimes. (ii) Quantum-well resonances, corresponding to quasi-stationary states localised inside the Ar layer; they are associated with the quantisation of the conduction band in the finite size Ar layer. The present results on image states nicely agree with very recent time-resolved two-photon-photo-emission experiments by Berthold, Feulner and Hofer.

The charge transfer between an H− ion and a free-electron metal surface with a single alkali adsorbate (Li and Cs) is studied with the wave packet propagation approach in the back-scattering geometry. Both the static problem for a fixed projectile–surface distance and the problem of charge transfer during a collision are considered. The three body (projectile–adsorbate–surface) aspect of the charge transfer process in this case results in an avoided crossing between the projectile and adsorbate-localized quasi-stationary states. We analyze and discuss the local effect of the adsorbate on the resonant charge transfer and the applicability of the rate equation approach.

Excited electronic states on a free-electron metal surface are investigated in the case when an atomically thin Ar layer is adsorbed on the surface. It is shown that the dielectric character of the Ar layer allows the existence of image states as well-defined resonances in this system. The image states are confined in vacuum by the Ar layer and decay by electron transmission through the Ar layer. A 3D microscopic description of the Ar layer associated to a wave-packet propagation approach is used to determine the characteristics, energy and lifetime, of the image states in a model Ar/free-electron metal system. The possibility of the observation of these states in a time-resolved experiment is discussed.

Image states at surfaces are very sensitive to the presence of impurities on the surface. A theoretical study of the effect of alkali adsorbates on the image state dynamics is performed for the Cs/Cu(1 0 0) and Na/Cu(1 1 1) systems using a wave-packet propagation approach. Scattering of the image state electron by an alkali adsorbate is shown to induce large inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. Even very low alkali coverage in the 10−3 monolayer range can effectively affect the image states, reducing their lifetime and broadening their photoemission peaks. The present theoretical results are discussed and compared with available experimental results on the perturbation of image states by various adsorbates.

This paper presents an investigation of how the charge transfer between an H − ion and an Al surface is influenced by the presence of a point defect on the surface. We consider the case of a single Al adatom on an Al(1 1 1) metal surface, and we study the resonant charge transfer process when the projectile is in the vicinity of the adatom. The adatom-induced electron potential is calculated using a density functional method. The coupled angular mode method is used to determine the shift and broadening of the H − level. The results show that the Al adatom introduces an attractive potential which, at large H − -metal distances, causes a downward shift of the H − level and an increase of its width. At close distances, the calculation also shows a strong coupling between the H − ion level and the Al(3p) and Al(3s) resonances located on the adatom. A diabatic modeling including the effect of the mixing of the projectile and adatom levels is developed which allows the discussion of the non-adiabatic transitions between projectile and adatom-localised levels when the H − passes through the region near the adatom.

A theoretical study of the electron dynamics in image potential states on Cu(1 1 1) and Cu(1 0 0) surfaces with alkali adsorbates is presented. It is shown that scattering of the image potential state electron by the adsorbates yields a sizable contribution to the decay of the image state population. Even for extremely low coverages (1 adsorbate atom per 1000 surface atoms) the decay rates due to the adsorbate induced scattering might be comparable to the decay rates due to the electron–electron interactions. We demonstrate that the inter-band transitions to other 2D continua of image or surface states is an efficient decay channel close to the Γ point.

The resonant rotational excitation of physisorbed H 2 molecules by low energy electron impact is studied using the rotational sudden approximation. The rotational excitation efficiency is analysed as a function of the constraint imposed on the molecular rotation by the adsorption. This allows the description of the variation of the energy loss spectrum corresponding to rovibrational excitation as a function of the constraint on molecular rotation. This model study is then used to discuss the recent results by Svensson et al. [Phys. Rev. Lett. 83 (1999) 124] on the rovibrational excitation of H 2 molecules adsorbed at steps on Cu(5 1 0), in terms of quasi-2D rotor and of constrained 3D rotors.

Neutral one-hole, one-electron excitations converging to the $\mathrm{He}1s$ ionization limit of He bilayers on a metal surface are studied with linearly polarized synchrotron radiation and time-of-flight techniques. Comparing signals of emitted electrons, emitted He atoms in metastable excited states, and fluorescence photons as a function of photon energy and polarization with theoretical results, a detailed picture of the photoexcitation is obtained. We show that the width of the photoabsorption peaks is governed in an anisotropic way by the large zero point motion of the He atoms; broadening directly correlates with the shape of the electronic clouds of the absorbing states.

Inelastic electron tunneling spectra (IETS) are evaluated for spin-1/2 Heisenberg chains showing different phases of their spin ordering. The spin ordering is controlled by the value of the two different Heisenberg couplings on the two sides of each of the chain's atoms (bond-alternating chains). The perfect anti-ferromagnetic phase, i.e. a unique exchange coupling, marks a topological quantum phase transition (TQPT) of the bond-alternating chain. Our calculations show that the TQPT is recognizable in the excited states of the chain and hence that IETS is in principle capable of discriminating the phases. We show that perfectly symmetric chains, such as closed rings mimicking infinite chains, yield the same spectra on both sides of the TQPT and IETS cannot reveal the nature of the spin phase. However, for finite size open chains, both sides of the TQPT are associated with different IETS spectra, especially on the edge atoms, thus outlining the transition.

A model study of the (2π*) resonance in the CO/Cu(1 1 1) and CO/Cu(1 0 0) systems is presented. Although the CO−(2π*) resonance is located inside the surface projected band gap in both systems, it is found to be very short-lived. These results are at variance with the apparently similar alkali/Cu systems where a strong reduction of the resonance decay by one-electron transfer to the substrate as compared to a free-electron metal surface is observed and attributed to the presence of the surface projected band gap. The differences between the two systems are analysed, allowing a discussion of the situations where a projected band gap induced stabilisation can be expected. The present results are compared with recent experimental results obtained by time resolved 2-photon photo emission.

One-electron and multielectron contributions to the decay of transient states in the Cs/Cu(111) and (100) systems are studied by a joined wave-packet propagation and many-body metal response approach. The long lifetime of these states is due to the Cu $L$ and $X$ band gaps which reduce the electron tunneling between Cs and Cu. In the (111) case, the decay is mainly by inelastic $e\ensuremath{-}e$ interaction, whereas in the (100) case, electron tunneling is dominating. This accounts very well for the experimental findings [Bauer et al., Phys. Rev. B 55, 10 040 (1997) and Ogawa et al., Phys. Rev. Lett. 82, 1931 (1999)].

D. Teillet-Billy, J. P. Gauyacq, and M. Persson Laboratoire des Collisions Atomiques et Moleculaires, Unite mixte de recherche CNRS-Universite Paris-Sud UMR 8625, Bâtiment 351, Universite Paris-Sud, 91405 Orsay CEDEX, France Department of Applied Physics, Chalmers University of Technology and Goteborg University, Goteborg, S-41296, Sweden ~Received 7 June 2000; revised manuscript received 25 September 2000!

Inelastic electron scattering data are used to study the geometry of molecular adsorbates on metal surfaces. The analysis of the vibrational excitation of N2 molecules physisorbed at 15 K on Ag(110) by electron impact provides information on the orientational ordering of the molecules on the surface. The incident electron energy (19 eV) is in the range where vibrational excitation is dominated by the N − 2 ( 2 Σ u ) resonance. The angular distributions (polar angle) of the inelastically scattered electrons, determined at monolayer, bilayer and multilayer N2 coverages, reflect the orientational distribution of the molecules on the surface. Using the inelastic angular distributions of the free molecule, it is shown that the experimentally observed distributions for physisorbed N2 molecules on Ag(110) are consistent with the theoretical ordering predicted in a previous paper [Ramseyer et al., Phys. Rev. B 58 (1998) 4111]: herringbone for the monolayer and a dense hexagonal pinwheel structure for the bilayer.

Electron detachment from ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions scattered at a grazing angle from a LiF(001) surface is studied by three-dimensional wave-packet propagation. Electron detachment is shown to be due to binary interactions between the projectile ion and the surface ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ anions, resulting primarily in electron ejection into vacuum. The importance of the detachment rate confirms the interpretations, proposed for various collisional phenomena at ionic crystal surfaces, involving repeated electron-capture/loss cycles in grazing scattering.

The neutralisation by resonant charge transfer (RCT) of Li + ions back-scattered from a Cs/Al(100) surface is studied theoretically. The case of a single Cs adsorbate on a free-electron substrate is modelled and used to determine the size of the region around the Cs adsorbate where the RCT process is deeply modified. The perturbation introduced by the Cs adsorbate has a very long range character. For back-scattering in the keV energy range, the Li + neutralisation cross-section is typically around 700 a 2 0 . Back-scattering from the region around the adsorbate is associated with large yields of excited Li 2p atoms. The size of the perturbation zone around each Cs adsorbate is strongly dependent on the process that is considered and on the projectile energy. The possibility of and difficulties in describing the case of a low coverage Cs/Al(100) surface using the results obtained in the case of a single adsorbate on the substrate are discussed.

The electron impact vibrational excitation of H2 molecules physisorbed on a free-electron metal surface is studied theoretically at collision energies of a few eV. The role of the short-lived low-lying 2Σu resonance is investigated. The electron scattering by a free H2 molecule is described by the R-matrix method and the corresponding results are used to model the electron scattering by the physisorbed molecule with the coupled angular mode (CAM) method. The strength of the vibrational excitation and, in particular, the overtone vibrational excitation ratio are found to be smaller for the physisorbed molecule than for the free molecule. However, the energy dependence of the vibrational excitation process is found to be weakly influenced by the physisorption, as observed experimentally by Demuth et al. [Phys. Rev. Lett. 47 (1981) 1166].

A theoretical study of the electronic processes occurring in collisions of H2+ molecular ions on an Al surface is presented, with an emphasis on the molecular dissociation induced by electronic transitions. The H2+ neutralization proceeds through two different channels: the H2 (b 3Σu+) dissociative state (resonant process) and the H2 (X 1Σg+) ground state (Auger process). The Auger deexcitation process of the b state, mediated by the H2− (2Σu) resonant state is studied in detail and shown to result in an efficient quenching of the excited state and a reduction in the molecular dissociation probability. Its efficiency depends on the competition between the molecular dissociation and the electronic process. The importance of this process can explain the experimental observation of significant amounts of bound H2 molecules surviving the electronic processes.

A brief review of the results obtained recently with the wave packet propagation method in the problem of resonant charge transfer (RCT) in collisions of atomic particles with metal surfaces is given. This method can be equally applied to the case of free-electron metal targets and to metal surfaces with a projected band gap. It provides a very transparent understanding of the one-electron processes occuring in such collisions. The new features of the RCT process due to the presence of a projected band gap for the electron motion along the normal to the surface are described for Li and H − projectiles colliding with a Cu(1 1 1) surface. It is shown that the decay of the resonance states located inside the band gap is strongly dominated by the 2D surface state continuum, while the decay into bulk Cu is strongly reduced. The resonance widths obtained for the metal with the projected band gap are, as a rule, significantly smaller than the widths in the corresponding jellium metal problem. These results provide new opportunities for theoretical investigations of the resonant charge transfer.

In this paper an investigation of how the presence of a vacancy on a metal surface influences the energy shift and broadening of negative ion states near the surface is presented. Results for H− and F− ions in the vicinity of a surface vacancy defect on an Al(1 1 1) surface are included. The vacancy-induced electron potential is calculated using a density functional method and the Coupled Angular Mode (CAM) method is used to determine the shift and broadening of H− and F− levels. The results show that the vacancy induces a repulsive potential, shifting the negative ion levels up, while narrowing the width. These two effects have opposite influences on the resonant charge transfer process. Their approximate cancellation makes the RCT process in these systems relatively insensitive to vacancies in the surface layer.

The formation of the metastable (1D and 1S) negative ion levels associated with the 2p4 electronic configuration of nitrogen is theoretically investigated in the course of collisions on an aluminium surface with alkali adsorbates. The multi-electron and multi-state aspects due to the presence of a few quasi-equivalent electrons in nitrogen are shown to lead to a complex structure of the resonant charge transfer (RCT) process. The RCT between the metal target and the nitrogen is deeply influenced by the presence of alkali adsorbates on the surface. The interaction between the N− ion levels and the adsorbate level leads to the possibility of non-adiabatic transitions induced by the collision movement. A diabatic modelling is presented, allowing an easy treatment of these non-adiabatic transitions and an evaluation of their importance. These theoretical results are then used for a discussion of the experimental results by Muller et al. [H. Muller, R. Hausmann, H. Brenten, V. Kempter, Surf. Sci. 303 (1994) 56]. They can account for the unexpected experimental observation of a sizeable electron emission due to the decay of free N−(1D) ions formed by collisions on alkali-covered metal surfaces.

Metastable N 2 ( A 3 Σ + u ) molecules colliding with a metal surface can be de-excited via an Auger process in which the excitation energy of the molecule is given to an electron of the metal. We report on a theoretical study of this process for N 2 ( A 3 Σ + u ) molecule collisions on a free electron metal, Al. This de-excitation process is efficiently mediated by the formation of a transient negative molecular ion, in the present case, the well-known resonance N − 2 of 2 Π g symmetry. A modelling of the electron–N 2 molecule interaction in the intermediate 2 Π g symmetry allows the computation of the Auger de-excitation process characteristics: transition rate, emitted electron spectra. The present results are used to discuss the experimental results on the N 2 ( A 3 Σ + u ) de-excitation in thermal collisions on metal surfaces by Stracke et al. [Surf. Sci. 396 (1998) 212] who first proposed this de-excitation mechanism. We find this resonance mediated process to be very efficient.

Theoretical results on the F − ion formation in scattering from Al and Ag surfaces are presented. Negative ion fractions are studied over a wide range of incident energies and scattering angles. Our description of the ion–surface charge transfer takes into account the multistate effects originating from the 2p 6 closed shell structure of the F − ion. Based on the parameter-free study of the dynamics of resonant charge transfer we find that the parallel velocity effects are quite important down to projectile energies of 0.5 keV. This is further confirmed by comparison between the present results and available experimental data [S. Ustaze et al., Surf. Sci. 414 (1998) L938; M. Maazouz et al., Surf. Sci. 409 (1998) 189; S. Ustaze, L. Guillemot, V.A. Esaulov, personal communications].

The Li and Cs alkali-adsorbate-induced states on a model Cu(111) surface are studied theoretically using the coupled angular mode and the wave packet propagation methods. Their energy position and their decay rate via one-electron transfer into the metal are calculated. An unoccupied alkali-induced state is found with a rather long lifetime, much longer than those found in the case of free-electron metal surfaces. This is attributed to the combined effect of the hybridisation of the alkali levels and of the Cu(111) projected band gap along the normal to the surface that blocks the electron transfer between the adsorbate and the metal. This qualitatively explains the experimental finding of a long-lived state by Bauer et al. [Phys. Rev. B 55 (1997) 10 040] and Ogawa et al. [Phys. Rev. Lett. 82 (1999) 1931].

A wave-packet propagation method is applied to the treatment of the resonant charge transfer (RCT) process in the interaction between an ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion and a Cu(111) surface. Using a model description of the Cu(111) electronic structure, it is shown that the RCT efficiency is deeply influenced by the presence of the Cu(111) projected band gap, that partially blocks the electron transfer in the direction normal to the surface. The differences between the RCT process on a free electron metal surface and on a Cu(111) surface are discussed. The two cases are associated with very different pictures of the electron transfer. In particular, the importance of the Cu(111) surface state for the decay of the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion is demonstrated. The effect of the band gap is also shown to strongly depend on the interaction time. For short interaction times (large collision velocities), the electron wave packet does not have enough time to probe the metal band structure and the RCT on a Cu(111) surface is very similar to that on a free electron surface. For long interaction times (low collision velocities), the RCT efficiency is drastically reduced by the presence of the band gap. The wave-packet propagation method is also used to discuss the validity of the rate equation approach in the case of a free electron metal target.