Extreme ionization of Xe clusters driven by ultraintense laser fields

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xen clusters (n=2-2171, initial cluster radius R0 =2.16-31.0 Å) driven by ultraintense infrared Gaussian laser fields (peak intensity IM = 1015 - 1020 W cm-2, temporal pulse length τ=10-100 fs, and frequency =0.35 fs-1). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of { Xeq+ }n with q=2-36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (IM = 1018 - 1020 W cm-2) inner ionization is dominated by BSI. At lower intensities (IM = 1015 - 1016 W cm-2), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the { Xeq+ }n ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe2171 at τ=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at IM = 1015 - 1016 W cm-2 establishes an ultraintense laser pulse length control mechanism of extreme ionization products.