Embedded metal cluster in strong laser fields

Abstract

We discuss microscopic mechanisms of the violent dynamics following strong laser excitation of a metal cluster embedded in a rare gas matrix, taking as test case Na8@Ar434. This covers at least two aspects: first, it represents the typical experimental situation of metal clusters handled in raregas matrices or a finite drop of surrounding raregas material, and second, it serves as a generic test case for highly excited chromophores in inert surroundings addressing questions of energy transport and perturbation of the medium. We simulate the process up to 10 ps using a mixed quantum mechanical (for the electrons) and classical (ions and atoms) approach and analyze the emerging dynamics with respect to all basic constituents: cluster electrons, cluster ions, and matrix atoms. We find several stages of relaxation taking place with time scales from a few fs to over a few ps and much slower processes remaining for long after the simulation. A particularly interesting aspect is that the surrounding raregas material stabilizes a highly charged metal cluster which would otherwise explode without delay.

Laser-driven nonlinear cluster dynamics

“…Theoretical analysis has yet to cope with the great variety of material combinations. For an example using the generic test system of metal cluster in a rare gas matrix, see (Fehrer et al, 2007a(Fehrer et al, , 2008. A thorough study of surface-deposited cluster subject to strong laser pulses still is a matter of future studies.…”

Dipole excitations of Ar substrate in contact with Na clusters

“…In order to exemplify the point, we have also plotted in Fig. 8 the actual position of the most external Na ions and this makes the pattern clear, with an effect only along radial coordinate due to the oblate shape of the irradiated cluster [33,34]. Apart from that detail, we find again a dominantly static charge effect explaining the dipole polarizations of Ar atoms, much similar to the deposition case with localization prevailing again and little own dynamics.…”

“…The laser pulse is kept short to concentrate the ionization to a fairly well defined initial time and the intensity is tuned such that the irradiation leaves the cluster with a net 3+ charge. A Coulomb explosion of the cluster is hindered by the matrix which stabilizes that high charge state, but allows a sizable oblate expansion of the cluster [33,34]. The point is illustrated in the upper panel of Fig.…”

“…Such methods are often called quantum-mechanicalmolecular-mechanical (QM/MM) models and have been applied for instance to chromophores in biomolecules [17,18], surface physics [19,20], materials physics [21,22,23,24], embedded molecules [25] and ion channels of cell membranes [26]. We have developed such a QM/MM modeling, primarily in the case of Na in contact with Ar [27,28,29] and successfully applied this method to deposition dynamics on finite Ar clusters [30] or on Ar surfaces [31], as well as to irradiation scenarios in the case of embedded clusters both in the linear (optical response [32]) and non linear (hindered explosion [33,34]) domains. More recently, we have extended the modeling to a MgO substrate [35].…”