Electrons as probes of dynamics in molecules and clusters: A contribution from Time Dependent Density Functional Theory

Abstract

a b s t r a c tThere are various ways to analyze the dynamical response of clusters and molecules to electromagnetic perturbations. Particularly rich information can be obtained from measuring the properties of electrons emitted in the course of the excitation dynamics. Such an analysis of electron signals covers observables such as total ionization, PhotoElectron Spectra (PES), Photoelectron Angular Distributions (PAD), and ideally combined PES/PAD. It has a long history in molecular physics and was increasingly used in cluster physics as well. Recent progress in the design of new light sources (high intensity, high frequency, ultra short pulses) opens new possibilities for measurements and thus has renewed the interest on these observables, especially for the analysis of various dynamical scenarios, well beyond a simple access to electronic density of states. This, in turn, has motivated many theoretical investigations of the dynamics of electronic emission for molecules and clusters up to such a complex and interesting system as C 60 . A theoretical tool of choice is here Time-Dependent Density Functional Theory (TDDFT) propagated in real time and on a spatial grid, and augmented by a Self-Interaction Correction (SIC). This provides a pertinent, robust, and efficient description of electronic emission including the detailed pattern of PES and PAD. A direct comparison between experiments and well founded elaborate microscopic theories is thus readily possible, at variance with more demanding observables such as for example fragmentation or dissociation cross sections.The purpose of this paper is to describe the theoretical tools developed on the basis of real-time and real-space TDDFT and to address in a realistic manner the analysis of electronic emission following irradiation of clusters and molecules by various laser pulses. After a general introduction, we shall present in a second part the available experimental results motivating such studies, starting from the simplest total ionization signals to the more elaborate PES and PAD, possibly combining them and/or resolving them in time. This experimental discussion will be complemented in a third part by a presentation of available theoretical tools focusing on TDDFT and detailing the methods used to address ionization observables. We shall also discuss the shortcomings of standard versions of TDDFT, especially what concerns the SIC problem, and show how to improve formally and practically the theory on that aspect. A long fourth part will be devoted to representative * P. Wopperer et al. / Physics Reports ( ) -results. We shall illustrate the use of total ionization in pump and probe scenarios with fs lasers for tracking ionic dynamics in clusters. More challenging from the experimental point of view is pump and probe setups using attosecond pulses. The effort there is more on the capability to define proper signals to be measured/computed at such a short time scale. TDDFT analysis provides here a valuable tool in the search for the most efficient observab...

Studies of spuriously shifting resonances in time-dependent density functional theory

“…It is a promising candidate for modeling non-equilibrium electron dynamics since it can capture the correlation effects with a relatively cheap computational cost. Although the performance of available functionals for non-equilibrium dynamics has been much less explored, promising results have been reported [7][8][9][10][11]. On the other hand, work on small systems where numerically-exact or high-level wavefunction methods are applicable, has shown that the approximate TDDFT functionals can yield significant errors in the simulated dynamics [12][13][14][15][16][17][18][19][20][21][22][23][24][25].…”

Similarities and Differences Between Nuclei and Metal Clusters

“…73,74 The setup has been shown to provide a versatile tool for many studies involving a detailed analysis of various decay channels as, e.g., electron emission following in particular laser irradiation scenarios. 75,76 The TDLDA part is handled as described in sections 2 and 3. Ionization (the doorway to fission) is computed via absorbing boundary conditions.…”

On the analysis of photo-electron spectra

“…The latter term is a functional of the local density (r, t) = i |ϕ i (r, t)| 2 and we work here within the time-dependent version of the Local Density Approximation (TDLDA) with the parametrization of Perdew and Wang [39]. The TDLDA is complemented by an efficient self-interaction correction term [40] which allows us to properly describe the ionization threshold [41,42].…”