Excitation of sodium clusters in (not so extreme) fs-laser fields

R. Schlipper, R. Kusche, B.v.Issendorff, H.Haberland
Fakultät für Physik, Universität Freiburg, H.Herderstr.3, Freiburg, Germany

Sodium clusters have a strong, collective resonance near 3 eV, which (due to its large oscillator strength) can be multiply excited by a relatively weak fs-laser pulse (10⁹ to 10¹⁰W/cm²) [1]. The kinetic energy distribution of the electrons emitted from such highly excited clusters is purely Boltzmann-like, with a mean energy of about 1 eV. A model which can explain all experimental data is as follows: The laser field couples strongly to the collective motion of the nearly free electron gas in the cluster. The interaction between collective and single particle excitation (time constant about 10 fs) leads to an effective electron temperature of 6000 to 8000K during the laser pulse. The electronic energy flows with a time constant of about one ps into ionic vibrations. Electrons are emitted, as long as the electron gas is hot enough. The cluster cools further by evaporation of atoms, which can finally lead to Coulomb assisted break up.
[1] R.Schlipper, R.Kusche, B.v.Issendorff, H.Haberland, PRL 80, 1194 (1998)