Charging dynamics of metal clusters in intense laser fields

Clusters of heavy metal atoms in strong femtosecond laser-light fields undergo multi-ionization with the loss of hundreds of electrons. The cross section largely exceeds that of corresponding isolated atoms, which leads in the case of Pb_N to a complete ionization of the 4f shell with a light intensity of 1.2×10¹⁵ W/cm². Experimental investigations on Pb and Pt clusters with variable pulse widths and, for the first time, with the pump&probe technique give insight into the dynamics of the coupling of electromagnetic radiation into the clusters. Both approaches support the picture according to which, after an initial charging, the clusters expand due to Coulomb forces. 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 plasmon energy of the diluted nanoplasma approaches the photon energy, the charging efficiency increases significantly. The experimental observations are confirmed by random-phase approximation (RPA) calculations of the optical response, including molecular-dynamics simulations of the expanding systems. Received: 11 November 1999 / Published online: 13 July 2000