Breakdown of sequential tunnel ionization in ultrashort electromagnetic pulses

Abstract: We consider double ionization of negative bromine ion ${\rm Br}^-$ in intense linearly polarized low-frequency laser fields. By solving numerically the two-electron time-dependent Schr\"odinger equation we demonstrate that while for electromagnetic 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 a few femtoseconds this picture breaks down entirely, and the electron-electron interaction suppresses the rate of ionization by approximately one order of magnitude. We also show weak but clear signatures of the collective tunneling effect in the photoelectron density distribution.