Highly Charged Rydberg Ions from the Coulomb Explosion of Clusters.

Ion emission from a nanoplasma produced in the interaction of intense optical laser pulses with argon clusters is studied resolving simultaneously charge states and recoil energies. By applying appropriate static electric fields we observe that a significant fraction of the ions Ar-q (q = 1-7) has electrons with binding energies lower than 150 meV; i.e., n(Ryd) >= 15 levels are populated. Charge state changes observed on a mu s time scale can be attributed to electron emission due to autoionizing Rydberg states, indicating that high-l Rydberg levels are populated as well. The experiments support theoretical predictions that a significant fraction of delocalized electrons, which are bound with hundreds of eV to the nanoplasma after the laser exposure, fill up meV bound ion states in the adiabatic expansion. We expect the process to be relevant for the long-term evolution of expanding laser-induced dense plasmas in general.