Effects of The Nanoplasma Electrons on Coulomb Explosion of Xenon Clusters

Abstract

The traditional description of Coulomb explosion (CE) of multicharged clusters driven by ultraintense, near-infrared, femtosecond laser fields (peak intensities of 1015−4 × 1016) W cm−2 at pulse lengths of 25−230 fs) under the initial conditions of complete and vertical outer ionization is transcended in the presence of a nanoplasma within the cluster. Unique features of the energetics of CE of many-electron elemental Xen (n = 55−6099) clusters manifest the interrelations between electron dynamics of outer ionization and nuclear CE dynamics. Molecular dynamics simulations of high-energy electrons and ions focused on the cluster size and laser parameter domains, where the contribution of electron impact inner ionization is important, the population of the persistent nanoplasma is considerable, and the effects of the nanoplasma on CE nuclear dynamics and energetics are significant. Relations were established for the cluster size and the laser parameter dependence of CE energetic attributes, that is, the average and maximal kinetic energies and the energy distribution, which mark the breakdown of the initial conditions of the complete cluster vertical ionization and of the simple electrostatic models for this situation. Incomplete vertical outer ionization, which retains a persistent nanoplasma, is exhibited for short pulses and low intensities. The CE energetics is then described in terms of the lychee model, where the persistent nanoplasma is initially confined within a neutral sphere in the center of the cluster while ions from an exterior shell undergo CE. This model accounts for the cluster size dependence of the average and maximal CE energies, for their initial site dependence, and for the narrow kinetic energy distribution at low energies. Complete or incomplete nonvertical ionization, when the time scales for electron outer ionization dynamics and nuclear CE dynamics are inseparable, is exhibited for long pulses. For complete nonvertical ionization, we relied on an empirical analysis of the cluster size scaling of the average and maximal CE energies, which can be reasonably well described in terms of the simple electrostatic model for complete cluster vertical outer ionization. For incomplete nonvertical outer ionization, CE kinetic energy distribution for long pulses is broad with a pronounced structure, which is due to a distribution of {Xeq+} ions with different charges. The gross features of the initial site dependence of CE energies and the broad distribution of the kinetic energies for incomplete nonvertical outer ionization can be rationalized in terms of the lychee model, which is strictly applicable for incomplete vertical ionization.