Effects of the nanoplasma on the energetics of Coulomb explosion of molecular clusters in ultraintense laser fields

We report on theoretical and computational studies of electron and nuclear energies in the Coulomb explosion of (D2) n/2 clusters (n=250-33 000, cluster radius R0 =11 -55) coupled to ultraintense Gaussian laser fields (laser peak intensities IM = 1015 - 1018 W cm-2, pulse widths τ=25-50 fs, and frequency ν=0.35 fs-1). Molecular dynamics simulations were fit by semiempirical relations for the average Eav and maximal EM ion energies and for their dependence on the cluster radius (R0) and on the laser parameters. This revealed two kinds of Coulomb explosion domains separated by the border radius R0 (I), which marks complete cluster outer ionization and which depends on IM and τ, (i) the cluster vertical ionization (CVI) domain (R0 < R0 (I)), where Eav, EM R02 is independent of the laser parameters, (ii) the non-CVI domain (R0 > R0 (I)), which prevails at lower laser intensities over a broad region of cluster sizes (i.e., at IM = 1015 W cm-2, R0 > R0 (I) =6.2 for τ=25 fs, and R0 > R0 (I) =9.5 for τ=50 fs). The effects of the persistent nanoplasma on Coulomb explosion in the non-CVI domain are manifested by a distinct cluster size dependence, i.e., Eav R0η (η=0) and EM R0η (η=1) for R0 > (2.0-2.5) R0 (I), and by a bimodal distribution of the ion kinetic energies. The energetics of Coulomb explosion in the non-CVI domain and its dependence (or independence) on the cluster size and laser parameters was semiquantitatively described by a cold nanoplasma model, which is based on a lychee configuration of the cluster charge, and which induces Coulomb explosion in the presence of the persistent nanoplasma. The results of our analyses are general for the cluster size dependence of the energetics of Coulomb explosion of multicharged elemental and molecular Xen, (C H4) n, and (DI)n clusters in the non-CVI domain.