Marijan Šunjić

Surface resonances in vibrational spectroscopy of hydrogen on transition metal surfaces: Pd(100) and Pd(111)

Abstract Measurements of the elastic electron reflectivity of clean and hydrogen-covered Pd(100) and the primary energy dependence of the hydrogen vibrational cross sections are reported, and compared to earlier results for Pd(100) H . In contrast to Pd(111), Pd(100) has a dipole active H vibrational mode and thus the two surfaces serve as models for the H/transition metal system. A model which includes both long-range inelastic and short-range elastic, non-specular scattering is proposed, which accounts for the experimentally observed features of the Pd H system.

Incoherent low-energy atom scattering from adsorbates at low coverages

Abstract We present a theory for the scattering of low-energy atoms from molecules adsorbed at low coverage on a metal surface. The theory is used to obtain the He-CO potential from the measured cross sections when CO is adsorbed on the platinum(111) surface. While the spherical potential can reproduce the energy dependence of the total cross section for a fixed incidence angle, the analysis of the total cross section as a function of incidence angle indicates a strong anisotropy of the long-range van der Waals attraction, arising from the orientation of the dynamical dipole of the adsorbed molecule.

Changing character of electronic transitions in graphene: From single-particle excitations to plasmons

In this paper we clarify the nature of π and π + σ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from

Optical absorption and conductivity in quasi-two-dimensional crystals from first principles: Application to graphene

This paper gives a theoretical formulation of the electromagnetic response of the quasi-two-dimensional crystals suitable for investigation of optical activity and polariton modes. The response to external electromagnetic field is described by current-current response tensor Πμν calculated by solving the Dyson equation in the random phase approximation, where current-current interaction is mediated by the photon propagator Dμν. The irreducible current-current response tensor Π0μν is calculated from the ab initio Kohn-Sham orbitals. The accuracy of Π0μν is tested in the long-wavelength limit where it gives correct Drude dielectric function and conductivity. The theory is applied to the calculation of optical absorption and conductivity in pristine and doped single-layer graphene and successfully compared with previous calculations and measurements.

Theory of core-level spectra in x-ray photoemission of pristine and doped graphene

Spectra of the C1s core hole, created in XPS and screened by electronic excitations in pristine and doped graphene, are calculated and discussed. We find that singular effects in the lineshapes are not possible in the pristine graphene, and their observation should be connected with the doping. However, the structure of the low energy excitation spectrum in the region where the singular behaviour is expected leads to asymmetries in the core hole lineshapes in pristine graphene similar to those in doped graphene. This makes the analysis more complex than in the case of metals and may lead to incorrect or incomplete interpretation of experimental results.

Dynamical effects in electron tunneling

Abstract The problem of dynamical screening for electrons tunneling in the metal-insulator-metal system, based on a generalization of the Johnsons nonlocal theory of exchange-correlation potential, is reviewed. The self-energy is evaluated within the GW approximation for electrons coupled to the long-wavelength charge-density oscillations. Potentials are evaluated analytically, and their local limit is examined. The problem is also solved numerically and the results compared with the analytical (nonselconsistent) results. The method is also extended to the case with an external potential on metal electrodes.

Electron Spectroscopy of Adsorbate Vibrations: The Role of Electronic Surface Resonances

Theoretical description of the electronic vibrational spectroscopy of atoms adsorbed on metal surfaces is developed which takes into account both open and closed (i.e. resonant) scattering channels of incoming electrons. Inelastic scattering cross section for the lowest order vibrational transition is formulated in the two-beam approximation, and the matrix elements are explicitly calculated in the point dipole model. The proper inclusion of the resonant channel scattering leads to the enhancement of the inelastic peaks near resonances, and, assuming the laterally screened induced electron-dipole interaction, explains the partial breakdown of the dipole selection rules and the relative strengths of parallel and normal vibrational excitations, as observed in HREELS of hydrogen adsorbed on transition metal surfaces. The angular dependences of the loss peaks on and off resonances are also discussed.

Quantum ricochets: surface capture, release and energy loss of fast ions hitting a polar surface at grazing incidence

A diffraction mechanism is proposed for the capture, multiple bouncing and final escape of a fast ion (keV) impinging on the surface of a polarizable material at grazing incidence. Capture and escape are effected by elastic quantum diffraction consisting of the exchange of a parallel surface wave vector G=2π/a between the ion parallel momentum and the surface periodic potential of period a. Diffraction-assisted capture becomes possible for glancing angles Φ smaller than a critical value given by Φc ≈2λ/a–|Vim|/E, where E is the kinetic energy of the ion, λ=h/Mv its de Broglie wavelength and Vim its average electronic image potential at the distance from the surface where diffraction takes place. For Φ< Φc, the ion can fall into a selected capture state in the quasi-continuous spectrum of its image potential and execute one or several ricochets before being released by the time reversed diffraction process. The capture, ricochet and escape are accompanied by a large, periodic energy loss of several tens of eV in the forward motion caused by the coherent emission of a giant number of quanta ħω of Fuchs–Kliewer surface phonons characteristic of the polar material. An

Theory of highly charged ion energy gain spectroscopy of molecular collective excitations

This paper discusses the physical mechanism by which a highly charged, energetic ion partly neutralized by electron transfers from a target—a large molecule, a cluster or a solid surface—can create target collective excitations in the process. We develop an analysis for the system of a highly charged ion flying by a fullerene molecule. Our analysis offers a new explanation for the periodic oscillations observed in the high-resolution energy gain spectra of energetic Arq+ ions (q=8, 13, 14, 15) flying by C60 molecules. For the Arq+→Ar(q−s)+ spectra with q=13–15 and s=1 or 2, the observed oscillations of 6 eV periodicity are assigned to energy losses due to multiple, Poissonian excitations of C60 π-plasmons (6 eV quantum). The excitation energy quanta are subtracted from the kinetic energy gained by the ion when one or at most two electrons are transferred to increasingly deep Rydberg states of the ion. The observed 3 eV periodicity for q=8 arises from the specific Rydberg energy levels of ArVIII (Ar7+). The first few shallow levels of this ion are separated by about 3 eV, while some of the pairs of adjacent, deeper levels are also separated by 3 eV. Each deep-level pair produces two interdigitated, Poissonian series of 6 eV π-plasmon excitation peaks resulting in an apparent periodicity of 3 eV throughout the spectra. The broad σ-plasmons (25 eV quantum) are found to contribute a background continuum to the medium- and high-energy regions of the observed spectra. The physical model analyzed here indicates that electronic collective excitations in several other systems could be studied by highly charged ion energy gain spectroscopy at sufficient resolution.

Quantum mechanical response of coupled metallic slabs

The nonlocal dynamical polarizability of two arbitrarily shaped objects coupled by long-range Coulomb forces, when overlap of their electron densities is negligible, is calculated by expressing it in terms of the response functions of separate objects. That enables us to extend the density functional theory approach within the Kohn–Sham scheme to such systems. The method is applied to the system consisting of two metallic slabs within the jellium model. The obtained response function of the total system is used to calculate the correlation energy of a static test charge placed in the system.