Enhanced ionization of the two-electron molecule H-2 in intense laser fields: Mechanism of the creation of doorway states

We investigate the mechanism of enhanced ionization in the two electron mol ecule Hz subjected to an ultrashort, intense laser pulse by solving exactly the time-dependent Schrodinger equation for a one-dimensional model. Resul ts of the simulation are analyzed by using the "field following" three adia batic states that are adiabatically connected with the lowest three essenti al electronic states of Hz as a field changes. Ionization is enhanced when the excited ionic state H-H+ is most efficiently created from the covalent ground state HH in the level dynamics prior to ionization. An analytic expr ession for the ionic and covalent crossing condition is also obtained in te rms of the three essential states and agrees well with the numerical result s. As the internuclear distance R decreases, the population of the H-H+ cre ated increases whereas the ionization rate from a pure H-H+ state decreases owing to the stronger attraction by the distant nucleus. As a result, the ionization is most enhanced at intermediate internuclear R (approximate to6 ) range where covalent and ionic configurations cross.