Quantum control of double ionization of calcium

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. It is shown that the significant enhancement of the Ca2+ yield observed in a recent experiment [E. Papastathopulous, M. Strehle, and G. Gerber (unpublished)] using feedback control techniques originates from the time asymmetry of the pulse shape. Numerical simulations have been performed for various asymmetrical pulses. The initial part of the pulse prepares a coherent superposition of 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+. Single-active-electron calculations have also been performed to demonstrate the significant role played by electron correlations and by doubly excited states