Low-frequency vibrations of alkali atoms on Cu(001)

Phonons on Cu(001) surface covered by submonolayer alkali metals

We present a theoretical investigation of the structural and vibrational properties of ordered 2D phases formed by the Li, Na and K atoms on the Cu[Formula: see text] surface. The lattice relaxation, phonon dispersions and polarization of vibrational modes as well as the local density of states are calculated using the embedded-atom method. The obtained structural parameters and vibrational frequencies are in close agreement with available experimental results.

Inducing regioselective chemical reactivity in graphene with alkali metal intercalation

First principles calculations demonstrate that alkali metal atoms, intercalated between metal substrates and adsorbed graphene monolayers, induce localised regions of increased reactivity. The extent of this localisation is proportional to the size of the alkali atom and the strength of the graphene-substrate interaction. Thus, larger alkali atoms are more effective (e.g. K > Na > Li), as are stronger-interacting substrates (e.g. Ni > Cu). Despite the electropositivity of these alkali metal adsorbates, analysis of charge transfer between the alkali metal, the substrate and the adsorbed graphene layer indicates that charge transfer does not give rise to the observed regioselective reactivity. Instead, the increased reactivity induced in the graphene structure is shown to arise from the geometrical distortion of the graphene layer imposed by the intercalated adsorbed atom. We show that this strategy can be used with arbitrary adsorbates and substrate defects, provided such structures are stable, towards controlling the mesoscale patterning and chemical functionalisation of graphene structures.

Theory of surface phonons at metal surfaces: recent advances

Recent studies of the surface dynamics of Al(001) and Cu(111) based on density functional perturbation theory have substantiated the existence of subsurface optical phonon resonances of all three polarizations, thus confirming early predictions of the embedded-atom method. The hybridization of the shear-vertical optical resonance with the longitudinal acoustic phonon branch accounts for the ubiquitous anomalous acoustic resonance as an intrinsic feature of metal surfaces. The DFPT calculation of the phonon-induced surface charge density oscillations shows that helium atom scattering spectroscopy (HAS) can indeed probe subsurface resonances. This opens new perspectives to HAS for the measurement of subsurface phonon dispersion curves in thin films, as proved by recent HAS studies on Pb and Fe ultrathin films on copper. After discussing these recent advances, this paper briefly reviews other important trends of surface dynamics expressed in recent years.

Dynamics of electron distributions probed by helium scattering

Helium atom scattering (HAS) is the most important tool for surface science investigations. The analysis of helium scattering off a solid surface allows for a detailed analysis of its structural and dynamical properties. In this work we show how the dynamics of electron distributions at a metal surface can be investigated by HAS in the adiabatic approximation. First we examine the anticorrugating effect, namely the property of the He-surface potential of those metal systems in which the classical turning points of He beams are farther away from the surface layer at the bridge than at top sites. Anticorrugation for the system He/Cu(111) is examined in detail by a density functional theory (DFT) calculation and compared with the corrugating behaviour of He/Al(111). To explain such an effect the charge polarization of the system is crucial. Second we consider theoretically a surprising restricted diffusion result in the normal direction for Na adatoms on Cu(001) at coverages larger than 0.04 ML, obtained by measurements with spin polarized (3)He beams. From DFT calculations for this system a model for the description of the He-surface interaction based on the effective medium theory, which accounts for the observed phenomenon, is discussed. We show that the surface charge distribution probed by HAS is altered by the local concentration of the diffusing adatoms which is fluctuating with time and producing variations in the apparent height of the adatom measured by HAS. Our calculations demonstrate that such electronic dynamical rearrangements can be probed by the (3)He spin echo technique, which could be extended to other studies of surface electronic properties.

Decay of hybridized electronic states of a Na cluster on Cu(001)

The electronic properties of a Na9+ cluster are markedly changed when deposited on a Cu(001) surface. Particularly, the lifetime of the (hybridized) single-particle electronic states are drastically enhanced indicating a change in electronic correlations upon absorption. To capture this effect, we developed a Green's function approach based on the configuration interaction technique. The calculated lifetimes (16.5 to 33 fs) of excited electronic states are in line with experimental observations. Our new method demonstrates the feasibility of accounting accurately for electronic correlation in large, nonperiodic systems.

Adsorbate-induced enhancement of electrostatic noncontact friction

We study the noncontact friction between an atomic force microscope tip and a metal substrate in the presence of bias voltage. The friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the tip surface by the bias voltage. We show that the friction can be enhanced by many orders of magnitude if the adsorbate layer can support acoustic vibrations. The theory predicts the magnitude and the distance dependence of friction in good agreement with recent puzzling noncontact friction experiment [B. C. Stipe, H. J. Mamin, T. D. Stowe, T. W. Kenny, and D. Rugar, Phys. Rev. Lett. 87, 096801 (2001).]. We demonstrate that even an isolated adsorbate can produce high enough friction to be measured experimentally.

Resonant photon tunneling enhancement of the van der Waals friction

We study the van der Waals friction between two flat metal surfaces in relative motion. For good conductors, we find that normal relative motion gives a much larger friction than for parallel relative motion. The friction may increase by many orders of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case, the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes.

Surface femtochemistry: observation and quantum control of frustrated desorption of alkali atoms from noble metals

This review presents a case study of the direct, real-time observation of a surface photochemical reaction, namely the frustrated photodesorption of alkali atoms from noble metal surfaces. Charge transfer excitation of an electron from the metal substrate into an unoccupied resonance of the alkali atom instantaneously turns on the repulsive Coulomb force inducing the nuclear motion of both the adsorbate and substrate atoms. The incipient nuclear wave packet dynamics are documented for the case of Cs/Cu(111) through the accompanying change in the surface electronic structure. The intimate view of atoms attempting to escape the surface bond highlights the unique role of the substrate in the electronic and nuclear dynamics that ultimately determine the product yields. Moreover, slow dephasing of the coherent polarization is exploited to demonstrate the control of nuclear wave packets through the phase of the excitation light.