Dynamics of clusters and molecules in contact with an environment

Abstract

We present recent theoretical investigations on the dynamics of metal clusters in contact with an environment, deposited or embedded. This concerns soft deposition as well as irradiation of the deposited/embedded clusters by intense laser pulses. The description of these complex and demanding compounds employs a hierarchical model in an extension of a Quantum-Mechanical/Molecular-Mechanical (QM/MM) approach where the cluster electrons are described by Time-Dependent Density-Functional Theory (TDDFT) and the constituents of the more inert environment by classical equations of motion. Key ingredients are the polarization potentials where, in particular, our QM/MM implementation takes care to include the full dynamical polarizability of the substrate. This is crucial for an appropriate modeling of dynamical scenarios. We discuss the observables accessible in that model, from quantum-mechanical cluster electrons, from classical cluster ions and from the degrees of freedom of the environment (positions, dipole polarizabilities). We discuss examples of applications for two typical test cases, Na clusters deposited on a MgO(001) surface and Na clusters in/on an Ar substrate. Both environments are insulators with sizeable polarizability. They differ in their geometrical and mechanical properties. We first survey the low-energy properties of these compounds, structure and optical response. We work out the impact of surface corrugation and of polarizability. We analyze the difference between deposited and embedded clusters. The second part discusses the dynamics of soft deposition processes, for Na clusters impinging on Ar(001) or MgO(001) surfaces. We analyze charge and size effects, and details of energy transfer to the environment. We show how the deposition process can create “hot spots” in the surface where sizeable amounts of energy are stored in internal excitations of the substrate atoms. Finally, we consider laser irradiation of embedded/deposited Na clusters. These systems serve as generic test cases for chromophore effects. We discuss a broad range of scenarios, from “gentle” to “strong” irradiation processes. The key effect is ionization through the laser pulse. We analyze the effect of the substrate on the angular distribution of emitted electrons and the effect of ionization on the substrate and the interface interaction. The case of strong excitations shows a dramatic change of cluster dynamics due to the environment, in particular hindered (or delayed) Coulomb explosion.