Our Research Activities
The core of our activities concerns the dynamics of clusters. One can sort the various explored paths along three major directions of research. In the first place, we focus on intrinsic dynamical properties of clusters as revealed by moderate external excitations. The second research axis deals with the response of clusters when subjected to a possibly intense external field which, to a large extent, shapes the response of the system. The third aspect covers the numerous theoretical developments motivated by the description of cluster dynamics in the various situations and domains of excitations explored in the two previous items.
Time and energy resolved analysis (intrinsic cluster dynamics)
At moderate perturbations, the cluster response dominantly reflects its own (structure and dynamical) properties. This first item covers such situations (which for the simplest ones can also be addressed in purely static pictures). The optical response in metal clusters provides a typical example. But we also pursue detailed investigations of photoelectron spectroscopy (energy and/or angle resolved) and of pump and probe scenarios at moderate excitations.
Free clusters in external fields
This general title covers several aspects of our activities sharing the
common denominator that the observed dynamics is a result of the
cluster in interaction with an external (static or time dependent)
field and not only of the cluster itself. The related phenomena lie in
the adiabatic regime (plasmon, harmonic generation) as well as
strongly non adiabatic situations. Extensive studies have thus been led
on the various scenarios encountered by clusters irradiated by intense
laser beams or hit by energetic highly charged projectiles.
Molecules and clusters in contact with a polarizable environment
Clusters can be more
easily handled experimentally when they are produced in contact with an environment
(deposited on a surface or embedded in a matrix). This concerns various experiments and a large
amount of experimental data. We have thus developed a generalized
Quantum Mechanics / Molecular Mechanics (QM/MM) method in the sense
that electronic degrees of freedom of the environment can be
explicitely treated dynamically. This hierarchical approach allows us
to explore various dynamical scenarios, as optical response of
deposited clusters, deposition processes, irradiation of embedded
clusters by an intense laser field, etc.
Understanding of cluster dynamics represents a complex task which requires elaborate theoretical tools. Density Functional Theory (DFT) represents here a robust starting point which allows to address various situations. We use DFT at various levels of sophistications (Local Density Approximation, Self Interaction Correction) in our time dependent approach. The basic tool is Time Dependent LDA in the quantal Kohn Sham picture, but we have also developed semi-classical schemes, in terms of the Vlasov-LDA approximation, possibly complemented by dynamical correlations. Exploratory investigations are also led to account for fluctuations by means of stochastic extensions of time dependent mean field theories.