Field ionization effects on ion acceleration in laser-irradiated clusters

Field ionization is included in our two-dimensional particle-in-cell simulations of femtosecond laser pulse interactions with submicron water clusters. Though laser field does not penetrate into the interior of submicron clusters, field ionization is shown to be efficient there due to the electric fields induced by quasineutrality violation due to expelled electrons by laser wave together with hot electron bunches which cross the cluster in the next half-period of the laser. Thus, oxygen atoms are ionized up to Z≥6Z≥6 in the whole submicron cluster by 40 fs laser pulses of amplitude a0≥0.5a0≥0.5. The impact of oxygen ion charge state on the spectrum of accelerated protons is shown and comparison of the spectrum with simulations with fixed ion charge states is presented.