On the mechanism of strong-field double photoionization in the helium atom

We perform time-dependent quantum calculations on the ionization dynamics o f helium in intense ultrashort laser fields. A linear model of the atom is used to extract single- and double-ionization yields for a 780 nm excitatio n. For an intensity of 10(15) W cm(-2) clear evidence is found for a direct double-ionization process which is caused by rescattering of an electron f rom the core in the presence of the field. A momentum-space analysis of the wavefunctions shows that in this case both electrons are preferentially em itted in the same direction and with different momenta.