Breakdown of sequential tunnel ionization in ultrashort electromagnetic pulses

Abstract

We consider double ionization of one- and two-dimensional atomic systems modeling neutral helium, xenon and the negative bromine ion in intense low-frequency electromagnetic fields. By solving numerically the two-electron time-dependent Schr{ö}dinger equation under conditions which exclude recollisions, we demonstrate that while for pulses of a few tens of femtoseconds duration and longer the sequential single-electron approximation perfectly describes the ionization dynamics, for pulses as short as few femtoseconds this picture breaks down entirely, and the electron-electron interaction suppresses the rate of ionization by roughly one order of magnitude.