Electron impact ionization of atomic clusters in ultraintense laser fields

In this paper we report on inner ionization of Xe_n clusters (n = 55- 2171) in ultraintense Gaussian laser fields (peak intensity I = 10¹⁵- 10²⁰ Wcm^-2, pulse width τ= 25 fs, frequency 0.35 fs^-1). The cluster inner ionization process is induced by the barrier suppression ionization (BSI) mechanism and by electron impact ionization (EII), which occurs sequentially with the BSI. We address electron impact ionization of clusters, which pertains to inelastic reactive processes of the high-energy (100 eV–1 keV per electron) nanoplasma. We utilized experimental data for the energy dependence of the electron impact ionization cross-sections of Xe^j+ (j = 1-10) ions, which were fit by an empirical three-parameter Lotz-type equation, to explore EII in clusters by molecular dynamics simulations. Information was obtained on the yields and time-resolved dynamics of the EII levels (i.e., number n_imp of electrons per cluster atom) in the Xe_n clusters and their dependence on the laser intensity and cluster size. The relative long-time (t = 90 fs) yields for EII, n_imp/n_ii (where n_ii is the total inner ionization yield) are rather low and increase with decreasing the laser intensity. In the intensity range I = 10¹⁵-10¹⁶ Wcm^-2, n_imp/n_ii = 0.21 for n = 2171 and n_imp/n_ii = 0.09-0.14 for n = 459, while for I = 10¹⁸-10²⁰ Wcm^-2, n_imp/n_ii = 0.01-0.05. The difference Δn_imp between the EII yield at long time and at the termination of the laser pulse reflects on ionization dynamics by the nanoplasma when the laser pulse is switched off. For Xe₂₁₇₁ in the lower intensity domain, Δ n_imp = 0.9 at I = 10¹⁵ Wcm^-2 and Δn_imp = 0.4 at 10¹⁶ Wcm^-2, reflecting on EII by the persistent nanoplasma under “laser free” conditions, while in the higher intensity domain of I = 10¹⁷ - 10¹⁸ Wcm^-2, Δn_imp is negligibly small due to the depletion of the transient nanoplasma.