Ionization dynamics of Na93+: dependence on laser pulse length
We present a numerical study of the electron dynamics of Na 93 + in laser pulses of fixed total en-ergy and pulse lengths varying between 20 and 400 fs. The frequency of the laser is 3.1 eV, i.e. slightly above the plasmon frequency of Na 93 + . It turns out that there is a clear-cut transition between 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 strong ionization of more than 10 electrons on the average. On the other hand, ionization of long pulses is much less drastic and proceeds mainly via a steady heating of the cluster valence electron cloud. These observations and the proposed ionization mechanism are in general accordance with recent experimental findings.
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