P. M. Echenique

Theory of surface plasmons and surface-plasmon polaritons

Collective electronic excitations at metal surfaces are well known to play a key role in a wide spectrum of science, ranging from physics and materials science to biology. Here we focus on a theoretical description of the many-body dynamical electronic response of solids, which underlines the existence of various collective electronic excitations at metal surfaces, such as the conventional surface plasmon, multipole plasmons and the recently predicted acoustic surface plasmon. We also review existing calculations, experimental measurements and applications.

Attosecond spectroscopy in condensed matter

Comprehensive knowledge of the dynamic behaviour of electrons in condensed-matter systems is pertinent to the development of many modern technologies, such as semiconductor and molecular electronics, optoelectronics, information processing and photovoltaics. Yet it remains challenging to probe electronic processes, many of which take place in the attosecond (1 as = 10-18 s) regime. In contrast, atomic motion occurs on the femtosecond (1 fs = 10-15 s) timescale and has been mapped in solids in real time using femtosecond X-ray sources. Here we extend the attosecond techniques previously used to study isolated atoms in the gas phase to observe electron motion in condensed-matter systems and on surfaces in real time. We demonstrate our ability to obtain direct time-domain access to charge dynamics with attosecond resolution by probing photoelectron emission from single-crystal tungsten. Our data reveal a delay of approximately 100 attoseconds between the emission of photoelectrons that originate from localized core states of the metal, and those that are freed from delocalized conduction-band states. These results illustrate that attosecond metrology constitutes a powerful tool for exploring not only gas-phase systems, but also fundamental electronic processes occurring on the attosecond timescale in condensed-matter systems and on surfaces.

The existence and detection of Rydberg states at surfaces

It is shown that the surface barrier potential can confine electrons in surface states, which because of the coulombic tail on the potential form a Rydberg series. This previously known result is made more rigorous by a discussion of lifetime broadening of the states. Observability of the states via LEED experiments is also investigated. The Rydberg series should give rise to structure in LEED curves which can in principle be resolved for all members of the series as n to infinity .

Density functional calculation of stopping power of an electron gas for slow ions

Abstract We describe the first calculation of the stopping power of an electron gas for slow ions using the density-functional formalism. We evaluate the nonlinear self-consistent potential around the ion and from scattering theory determine the energy loss directly. Comparison with the results of linear theory is made.

Image potential states on metal surfaces: binding energies and wave functions

We present self-consistent pseudopotential calculations of both surface and image potential states on simple metal surfaces: Li(110), Na(110), Be(0001), Mg(0001), Al(100), and Al(111). The local density approximation (LDA) is used to describe the one-electron potential inside the film and in the surface region. In the vacuum space (at z>z im ) the LDA potential is replaced by the image potential. A one-dimensional potential proposed recently is constructed for 14 simple and noble metal surfaces. By using this model potential we study wave functions and binding energies of the image states and also image plane position trends for these metal surfaces.

Interaction of slow multicharged ions with solid surfaces

Abstract The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on “Interaction of Slow Highly Charged Ions with Solid Surfaces”, which has been carried out during the last three years. After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of “hollow atoms” formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understanding of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface.

Theory of inelastic lifetimes of low-energy electrons in metals

Abstract Electron dynamics in the bulk and at the surface of solid materials are well known to play a key role in a variety of physical and chemical phenomena. In this article we describe the main aspects of the interaction of low-energy electrons with solids, and report extensive calculations of inelastic lifetimes of both low-energy electrons in bulk materials and image-potential states at metal surfaces. New calculations of inelastic lifetimes in a homogeneous electron gas are presented, by using various well-known representations of the electronic response of the medium. Band-structure calculations, which have been recently carried out by the authors and collaborators, are reviewed, and future work is addressed.

Direct observation of electron dynamics in the attosecond domain

Dynamical processes are commonly investigated using laser pump–probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10-18 s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10-15 s). Here we monitor the dynamics of ultrafast electron transfer—a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices—on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.

Dynamic Screening of Ions in Condensed Matter

Publisher Summary The chapter presents a review on dynamic screening of ions in condensed matter. The chapter discusses the physical aspects of the wake, related to energy loss and dynamic screening of the ion. Processes characteristic of various speed regimes are analyzed. Near-adiabatic interactions in the regime υ ≪ υ 0 involve mainly electrons at the top of the valence electron sea. The density functional method allows accurate evaluation of the energy loss rate and the effective charge of the ion. When υ>υ, electrons respond to the impulse of the swiftly moving ion as if they were nearly free. Perturbation theory is then valid for the evaluation of energy loss or effective charge, but corrections must be made for ions with large atomic number or for many-body effects appearing as υ → υ 0 . Empirical theory has been developed to describe phenomena occurring in this regime. This chapter presents a many-body self-energy approach to obtain a priori probabilities for energy loss and for capture and loss. Results that have been obtained for low atomic number ions penetrating several elemental solids are described.

Role of Bulk and Surface Phonons in the Decay of Metal Surface States

We present a comprehensive theoretical investigation of the electron-phonon contribution to the lifetime broadening of the surface states on Cu(111) and Ag(111), in comparison with high-resolution photoemission results. The calculations, including electron and phonon states of the bulk and the surface, resolve the relative importance of the Rayleigh mode, being dominant for the lifetime at small hole binding energies. Including the electron-electron interaction, the theoretical results are in excellent agreement with the measured binding energy and temperature dependent lifetime broadening.