Ionization dynamics of Na-93(+): dependence on laser pulse length

We present a numerical study of the electron dynamics of Na-93(+) in laser pulses of fixed total energy and pulse lengths varying between 20 and 400 f s. The frequency of the laser is 3.1 eV, i.e. slightly above the plasmon fr equency of Na-93(+). It turns out that there is a clear-cut transition betw een a short-pulse and a long-pulse regime for pulse lengths around 200 fs. For short pulses, resonance with the collective plasmon mode leads to stron g ionization of more than 10 electrons on the average. On the other hand, i onization of long pulses is much less drastic and proceeds mainly via, a st eady heating of the cluster valence electron cloud. These observations and the proposed ionization mechanism are in general accordance with recent exp erimental findings.