On efficiency of double ionization of a three-electron atom in moderate laser field intensities

We study double ionization in a one-dimensional model of a three-electron atom exposed to strong short laser fields. The corresponding ab initio simulations reveal a characteristic ``knee'' in the ionization yield at lower field intensities than in comparable two-electron systems. To understand the origin of this shift, a quantitative semi-analytic model for double ionization of a three-electron atom has been created. In this model, ionization is treated as a combination of different scenarios and channels. Each channel is simulated with either Quantitative Rescattering Theory (QRS) or two-electron ab initio model. Analysis of the semi-analytical results indicates that the shifted part of the ``knee'' is dominated by a recollision-excitation channel in which the rescattering electron initially escapes the parent atom from an inner orbital. This counterintuitive phenomenon is then discussed and explained.