Ricardo Díez Muiño

Time-dependent electron phenomena at surfaces

Femtosecond and subfemtosecond time scales typically rule electron dynamics at metal surfaces. Recent advance in experimental techniques permits now remarkable precision in the description of these processes. In particular, shorter time scales, smaller system sizes, and spin-dependent effects are current targets of interest. In this article, we use state-of-the-art theoretical methods to analyze these refined features of electron dynamics. We show that the screening of localized charges at metal surfaces is created locally in the attosecond time scale, while collective excitations transfer the perturbation to larger distances in longer time scales. We predict that the elastic width of the resonance in excited alkali adsorbates on ferromagnetic surfaces can depend on spin orientation in a counterintuitive way. Finally, we quantitatively evaluate the electron–electron and electron–phonon contributions to the electronic excited states widths in ultrathin metal layers. We conclude that confinement and spin effects are key factors in the behavior of electron dynamics at metal surfaces.

Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions

Photoemission with a twist Attosecond time-resolved spectroscopy provides the ability to probe the fastest electronic processes in atoms and solids. Yet the photoemission process from solids is not fully understood. Siek et al. studied photoemission from the layered van der Waals material WSe2 and found that electron emission occurs as a sequence of events that are apparently time-ordered with respect to rising angular momentum of the involved initial states (see the Perspective by Yakovlev and Karpowicz). This result will help provide a more detailed picture of the photoemission process. Science, this issue p. 1274; see also p. 1239 Attosecond time-resolved spectroscopy reveals angular momentum–induced delays in solid-state photoemission. Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe2) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe2, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.

Modeling surface motion effects in N2 dissociation on W(110): Ab initio molecular dynamics calculations and generalized Langevin oscillator model

Accurately modeling surface temperature and surface motion effects is necessary to study molecule-surface reactions in which the energy dissipation to surface phonons can largely affect the observables of interest. We present here a critical comparison of two methods that allow to model such effects, namely, the ab initio molecular dynamics (AIMD) method and the generalized Langevin oscillator (GLO) model, using the dissociation of N2 on W(110) as a benchmark. AIMD is highly accurate as the surface atoms are explicitly part of the dynamics, but this advantage comes with a large computational cost. The GLO model is much more computationally convenient, but accounts for lattice motion effects in a very approximate way. Results show that, despite its simplicity, the GLO model is able to capture the physics of the system to a large extent, returning dissociation probabilities which are in better agreement with AIMD than static-surface results. Furthermore, the GLO model and the AIMD method predict very similar energy transfer to the lattice degrees of freedom in the non-reactive events, and similar dissociation dynamics.

Dynamic screening and energy loss of antiprotons colliding with excited Al clusters

Abstract We use time-dependent density functional theory to calculate the energy loss of an antiproton colliding with a small Al cluster previously excited. The velocity of the antiproton is such that non-linear effects in the electronic response of the Al cluster are relevant. We obtain that an antiproton penetrating an excited cluster transfers less energy to the cluster than an antiproton penetrating a ground state cluster. We quantify this difference and analyze it in terms of the cluster excitation spectrum.

Energy Dissipation Channels in Reactive and Non-reactive Scattering at Surfaces

One of the main challenges in theoretical gas-surface studies is to incorporate into the dynamics energy exchange to both lattice vibrations and electronic excitations, keeping the accuracy of a multidimensional ab-initio potential energy surface for describing the gas/metal interaction. In this chapter, we review some recent advances in the subject and will present a theoretical framework recently developed that allows to evaluate within a full dimensional dynamics the combined contribution of both excitation mechanisms. This objective has been accomplished by combining the Generalized Langevin Oscillator model for phonon excitations and the Local Density Friction Approximation for electronic excitations. The inclusion of both effects allows one to address such fundamental questions as which is the relative importance of phonon and electron-hole pair excitations as energy dissipation channels and to what extent the adiabatic calculation can capture the basic physics of the dynamics and provide accurate results. Results on several systems and on different elementary gas-surface processes (dissociation, scattering, and molecular adsorption) are used to enrich the discussion. We show that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.

Quantum Monte Carlo programming : for atoms, molecules, clusters, and solids

A first monte-carlo example Variational Quantum-Monte-Carlo for a One-Electron System Two electrons with two adiabatically decoupled nuclei: Hydrogen molecule Three electrons: Lithium Atom Many- electron confined systems Many- electron atomic aggregates: Lithium cluster Infinite number of electrons: Lithium solid Diffusion quantum Monte- Carlo (DQMC)

Dynamic screening of a localized hole during photoemission from a metal cluster

Recent advances in attosecond spectroscopy techniques have fueled the interest in the theoretical description of electronic processes taking place in the subfemtosecond time scale. Here we study the coupled dynamic screening of a localized hole and a photoelectron emitted from a metal cluster using a semi-classical model. Electron density dynamics in the cluster is calculated with time-dependent density functional theory, and the motion of the photoemitted electron is described classically. We show that the dynamic screening of the hole by the cluster electrons affects the motion of the photoemitted electron. At the very beginning of its trajectory, the photoemitted electron interacts with the cluster electrons that pile up to screen the hole. Within our model, this gives rise to a significant reduction of the energy lost by the photoelectron. Thus, this is a velocity-dependent effect that should be accounted for when calculating the average losses suffered by photoemitted electrons in metals.