Intense-laser-field-enhanced ionization of two-electron molecules: Role of ionic states as doorway states

We investigate the mechanism of enhanced ionization in two-electron molecules by solving exactly the time-dependent Schrodinger equation for a one-dimensional H-2 in an ultrashort, intense (I greater than or equal to 10(14) W/cm(2)) laser pulse (lambda = 1064 nm). Enhanced ionization in two-electron systems differs from that in one-electron systems in that the excited ionic state H-H+ regarded as the dominant doorway state to ionization crosses the covalent ground state HH in field-following time-dependent adiabatic energy. An analytic expression for the crossing condition obtained in terms of the lowest three states agrees with the numerical results. The gap at the avoided crossing decreases the initial covalent component and promotes electron transfer to H-H+. As the internuclear distance R decreases, the population of the H-H+ created increases, whereas the ionization rate from a H-H+ decreases owing to the stronger attraction by the distant nucleus. As a result, the rate has a peak at R approximate to 6 a.u., where most adiabatic states avoid each other with considerable gaps.