Charging dynamics of metal clusters in intense laser fields

Clusters of heavy metal atoms in strong femtosecond laser-light fields unde rgo multi-ionization with the loss of hundreds of electrons. The cross sect ion largely exceeds that of corresponding isolated atoms, which leads in th e case of Pb-N to a complete ionization of the 4f shell with a light intens ity of 1.2 x 10(15) W/cm(2). Experimental investigations on Pb and Pt clust ers with variable pulse widths and, for the first time, with the pump&probe technique give insight into the dynamics of the coupling of electromagneti c radiation into the clusters. Both approaches support the picture accordin g to which, after an initial charging, the clusters expand due to Coulomb f orces. This expansion is accompanied by a reduction of the electron density and at the same time by an increase of the optical sensitivity. Once the p lasmon energy of the diluted nanoplasma approaches the photon energy, the c harging efficiency increases significantly. The experimental observations a re confirmed by random-phase approximation (RPA) calculations of the optica l response, including molecular-dynamics simulations of the expanding syste ms.