Dynamics of multiply charged ions in intense laser fields - art. no. 053402

We numerically investigate the dynamics of multiply charged hydrogenic ions in near-optical linearly polarized laser fields with intensities of order 10(16)-10(17) W/cm(2). The weakly relativistic interaction is appropriately described by the Hamiltonian arising from the expansion of the Dirac equat ion up to the second order in the ratio of the electron velocity v and the speed of light c. Depending on the charge state Z of the ion, the relation of strength between laser field and ionic core changes. We find around Z = 12, typical multiphoton dynamics and for Z = 3 tunneling behavior, however, with clear relativistic signatures. In first order in v/c the magnetic fie ld component of the laser field induces a Z dependent drift in the laser pr opagation direction and a substantial Z dependent angular momentum with rep ect to the ionic core. While spin oscillations occur already in first order in v/c as described by the Pauli equation, spin inducted forces via spin-o rbit coupling only appear in the parameter regime where (v/c)(2) correction s are significant. In this regime for Z = 12 ions, we show strong splitting s of resonant spectral lines due to spin-orbit coupling and substantial cor rections to the conventional Stark shift due to the relativistic mass shift while those to the Darwin term are shown to be small. For smaller charges or higher laser intensities, parts of the electronic wave packet may tunnel through the potential barrier of the ionic core and when recombining, are shown to give rise to keV harmonics in the radiation spectrum. Some parts o f the wave packet do not recombine after ionization and we find very energe tic electrons in the weakly relativistic regime of above threshold ionizati on.