Dynamical resonant electron capture in atom surface collisions: H- formation in H-Al(111) collisions

Negative ion formation by neutral hydrogen atom grazing scattering from a LiF(100) surface

H⁻ conversion during H⁰ grazing scattering on a LiF(100) surface, includes H⁻ affinity electron loss to a surface image state.

Nonadiabatic dynamics in energetic negative fluorine ions scattering from a Si(100) surface

The dependence of the negative-ion fractions on incident energy and angle is reported for 8.5-22.5 keV F(-) ions scattered from a Si(100) surface at a fixed scattering angle of 38°. The negative-ion fraction increases monotonically with incident velocity for specular scattering. In particular, the variation of the fraction with incident angle is bell shaped for a given incident energy. We interpret this variation using the incident-velocity effect at short distances where the yield of negative ions depends on the number of initial neutrals. It strongly indicates that at short distances, a dynamical equilibrium population is never achieved. This nonadiabatic feature is supported by simple calculations using modified rate equations.

The effects of electron transfer on the energy loss of slow He²⁺, C²⁺, and C⁴⁺ ions penetrating a graphene fragment

Electronic energy loss in the collision processes of slow ions with a graphene fragment is investigated by combining ab initio time-dependent density functional theory calculations for electrons with molecular dynamics simulations for ions in real time and real space. We study the electronic energy loss of slow He²⁺, C²⁺, and C⁴⁺ ions penetrating the graphene fragment as a function of the ion velocity, and establish the velocity-proportional energy loss for low-charged ions down to 0.1 a.u. One mechanism clarified in the simulations for electron transfer is polarization capture, which is effective for bare ions at low velocities. The other one is resonance capture, by which the incident ion can capture electrons from the graphene fragment to its electron affinity levels, which have the same, or nearly the same, energy as those of the electron donor levels. The results demonstrate that the nonlinear behavior of energy loss of C⁴⁺ is attributed to the large number of electrons captured by this multi-charged ion during the collision.

Multi-orbital non-crossing approximation from maximally localized Wannier functions: the Kondo signature of copper phthalocyanine on Ag(100)

We have developed a multi-orbital approach to compute the electronic structure of a quantum impurity using the non-crossing approximation. The calculation starts with a mean-field evaluation of the system's electronic structure using a standard quantum chemistry code; here we use density functional theory (DFT). We transformed the one-electron structure into an impurity Hamiltonian by using maximally localized Wannier functions. Hence, we have developed a method to study the Kondo effect in systems based on an initial one-electron calculation. We have applied our methodology to a copper phthalocyanine molecule chemisorbed on Ag(100), and we have described its spectral function for three different cases where the molecule presents a single spin or two spins with ferro- and anti-ferromagnetic exchange couplings. We find that the use of broken-symmetry mean-field theories such as Kohn-Sham DFT cannot deal with the complexity of the spin of open-shell molecules on metal surfaces and extra modeling is needed.

Localized description of surface energy gap effects in the resonant charge exchange between atoms and surfaces

The resonant charge exchange between atoms and surfaces is described by considering a localized atomistic view of the solid within the Anderson model. The presence of a surface energy gap is treated within a simplified tight-binding model of the solid, and a proper calculation of the Hamiltonian terms based on a LCAO expansion of the solid eigenstates is performed. It is found that interference terms jointly with a surface projected gap maximum at the Γ point and the Fermi level inside it, lead to hybridization widths negligible around the Fermi level. This result can explain experimental observations related to long-lived adsorbate states and anomalous neutral fractions of low energy ions in alkali/Cu(111) systems.

Grazing incidence diffraction of keV helium atoms on a Ag(110) surface

Diffraction of fast atoms at grazing incidence has been recently demonstrated on the surface of alkali halides and wide band gap semiconductors, opening applications for the online monitoring of surface processes such as growth of ultrathin layers. This Letter reports energy resolved diffraction of helium on Ag(110) metal surface showing that a band gap is not mandatory to restrict the decoherence due to electron-hole pair excitations by the keV projectile. Measurement of the energy loss, which is in the eV range, sheds light on the scattering process.

Calculation of transition probability for neutralization process of +1 ion based on the Noziéres and Dominicis theory

We investigate the neutralization probability of a +1 ion moving parallel to a metal surface, mainly using the Noziéres and Dominicis theory (ND theory), which was applied to the edge singularities in x-ray absorption and emission spectra in metals. In particular, we focus on the effects due to the sudden disappearance of the image potential in the neutralization process. From the analysis, we found that the neutralization probability is given by [Formula: see text], where D and Γ are the band width and the gamma function. The factor α = 2δ/π-(δ/π)(2) is given by the phase shift (δ: phase shift on Fermi surface), and the energy difference between the initial and final states, Δ, includes the kinetic energy of the ion and various other phenomenological factors. The singularity caused from the sudden disappearance of the image potential enhances or reduces the neutralization probability depending on the symmetry of atomic orbital of the ion. When the ion velocity is too fast to form an image potential, the neutralization process would proceed without the creation of an image potential, accompanying an excitation of a surface plasmon; as a result of plasmon excitation the ion velocity becomes slower than the plasma frequency because of screening effects. In this paper, we apply the ND theory to the case of slow ion motion and discuss the dynamics.

Surface miller index dependence of Auger neutralization of ions on surfaces

Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories is performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.

Role of the 2D surface state continuum and projected band gap in charge transfer in front of a Cu(111) surface

Electron capture by Li+ and H projectiles in grazing scattering from Cu(111) and Cu(110) surfaces is studied experimentally and theoretically. Whereas data for Cu(110) can be described by established theoretical methods treating resonant charge transfer with a free-electron metal, data for Cu(111) show pronounced deviations from this approach. We interpret our observations by the effect of the projected L-band gap of the Cu(111) surface. In particular, the quantum states of reduced dimension (2D surface state continuum) play a dominant role in electron transfer.