Charging of metal clusters in helium droplets exposed to intense femtosecond laser pulses

We review the strong field (10¹³–10¹⁶ W cm⁻²) laser excitation of metal clusters (Cd_N, Ag_N and Pb_N) embedded in He nanodroplets. Plasmon enhanced ionization obtained by stretching the laser pulses to several hundreds of femtoseconds or by using dual pulses with a suitable optical delay leads to a Coulomb explosion of highly charged atomic ions. The charging dynamics can be well described by corresponding semiclassical Vlasov simulations. The influence of the He environment on the ionization process and on the final charge distribution is discussed. Evidence is found that He²⁺ is generated in collisions with highly charged metal ions. In contrast, singly and doubly charged ions with low recoil energies induce the formation of He snowballs with a distinct shell structure around the ion. Laser intensity thresholds for snowball formation and for the ionization of clusters are investigated by applying intensity selective scanning.