Electron impact ionization of atomic clusters in ultraintense laser fields

摘要

In this paper we report on inner ionization of Xe-n clusters ( n = 55-2171) in ultraintense Gaussian laser fields ( peak intensity I = 10(15)- 10(20) W cm(-2), pulse width tau = 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 Xej+ ( 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 X(e)n clusters and their dependence on the laser intensity and cluster size. The relative long-time ( t = 90 fs) yields for EII, n(imp)(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 = 10(15)- 10(16) Wcm(-2), n(imp)(ii) = 0.21 for n = 2171 and n(imp)(ii) = 0.09- 0.14 for n = 459, while for I = 10(18)- 10(20) Wcm(-2), n(imp)(ii) = 0.01-0.05. The difference Delta n(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 Xe-2171 in the lower intensity domain, Delta n(imp) = 0.9 at I = 10(15) Wcm(-2) and Delta n(imp) = 0.4 at 10(16) Wcm(-2), reflecting on EII by the persistent nanoplasma under "laser free" conditions, while in the higher intensity domain of I = 10(17)- 10(18) Wcm(-2), Delta n(imp) is negligibly small due to the depletion of the transient nanoplasma.