Two-electron Excitation and Ionization in an Intense Laser Field

Abstract We present a simple model for two-electron excitation and ionization of an atom in the presence of an intense laser field. We show, in particular, how the Coulomb interaction, via configuration interaction, prevents the excitation and ionization from being collective in all but the most intense fields. By collective we mean that each electron has the same spatial wavefunction: this implies that the electron pair may be described by a restricted time-dependent Hartree-Fock wavefunction. The configuration interaction prevents collective behaviour not only through the process of auto-ionization but also through the mixing of doubly excited independent electron states into the ground state. We are able to give criteria for establishing true collective excitation and ionization of our model two-electron system in an intense field.