Enhancement of strong-field two-electron ionization via doubly excited states

We have performed nonperturbative time-dependent calculations of single and double ionization of atomic calcium by short and intense laser pulses using a two-active electron model (TAE). It is shown that the significant enhancement of the Ca²⁺ yield observed in a recent experiment [1] using feedback control techniques originates from the time asymmetry of the pulse shape. The initial part of the pulse prepares a coherent superposition of doubly excited states which is transferred into the double electron continuum at later times. An asymmetric shape, with a slowly decreasing tail, therefore favors the production of Ca²⁺.