Philipp Wopperer

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

Various ways to analyze the dynamical response of clusters and molecules to electromagnetic perturbations exist. 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 total ionization, Photo-Electron Spectra, Photoelectron Angular Distributions, and ideally combined PES/PAD, with a long history in molecular physics, also increasingly used in cluster physics. Recent progress in the design of new light sources (high intensity and/or frequency, ultra short pulses) opens new possibilities for measurements and thus has renewed the interest on the analysis of dynamical scenarios through these observables, well beyond a simple access to a density of states. This, in turn, has motivated many theoretical investigations of the dynamics of electronic emission for molecules and clusters. 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. 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 aim of this paper is to review the available experimental results motivating such studies, describe the theoretical tools developed on the basis of real-time and real-space TDDFT to address in a realistic manner the analysis of electronic emission following irradiation of clusters and molecules by various laser pulses, discuss representative results, and finally give some future directions of investigations.

On the dynamics of photo-electrons in C60

We explore photo-electron spectra (PES) and photo-electron angular distributions (PAD) of C60 with time-dependent density functional theory (TDDFT) in real time. To simulate experiments in gas phase, we consider isotropic ensembles of cluster orientations and perform orientation averaging of the TDDFT calculations. First, we investigate ionization properties of C60 by one-photon processes in the range of VUV energies. The PES map the energies of the occupied single-particle states, while the weights of the peaks in PES are given by the depletion of the corresponding level. The different influences can be disentangled by looking at PES from slightly different photon frequencies. PAD in the one-photon regime can be characterized by one parameter, the anisotropy. This single parameter unfolds worthwhile information when investigating the frequency and state dependences. We also discuss the case of multi-photon ionization induced by strong infrared laser pulses in C60. In agreement with measurements, we find that the PES show a regular comb of peaks separated by the photon energy. Our calculations reveal that this happens because only very few occupied states of C60 near the ionization threshold contribute to emission and that these few states happen to cooperate filling the same peaks. The PAD show a steady increase of anisotropy with increasing photon order.

On the role of resonances in photoionization of metal clusters

We analyze electron emission from irradiated clusters by means of time-dependent density-functional theory (TDDFT) in real time. We focus on photo-electron spectra (PES) which deliver an invaluable tool to explore static and dynamical properties of irradiated species. We discuss, in particular, the role of resonances in the PES once the laser frequency is below the emission threshold which implies multiphoton processes. We show that the resonances in the electronic spectrum lead to the occurrence of several peaks in the PES and also strongly affect the standard scaling relations between ionization and the number of required photons for electronic emission.

Dynamics of irradiation: from molecules to nano-objects and from material science to biology

We give a brief review of theoretical and computational tools developed for the description and analysis of irradiation dynamics of cluster and molecules. We illustrate the capabilities of the method on the demanding example of C60 irradiated by various laser fields.

Laser excitation of clusters: observables from electron emission

We give a brief review of the theoretical description of photo-electron spectra (PES) and photo-angular distributions (PAD) and discuss a few selected, typical results. The description is based on time-dependent density-functional theory at the level of the local-density approximation augmented by a self-interaction correction which is crucial for a quantitative assessment of emission processes. Coordinate-space grids are used together with absorbing boundary conditions. We discuss the basic features and trends of PES and PAD for two typical test cases, the clusters Na8 and C60.