Double multiphoton ionization of a model atom

A direct solution of the time-dependent Schrödinger equation (TDSE) for a two-electron model atom in a strong electromagnetic field is used to extract probabilities for both single and double electron ionization. For three-photon ionization at a moderately strong intensity, the TDSE calculations find single ionization at 62%, single ionization with excitation at 10%, and double ionization at 3%. The sequential process of escape from an already once ionized atom is found to make a substantial contribution to the double-ionization probability. Multiphoton-ionization probabilities are also obtained from a solution of the time-dependent unrestricted Hartree-Fock (TDUHF) equations for the same model atom. Although in qualitative agreement with the TDSE results, the TDUHF results show significantly less single ionization with excitation and have a different angular distribution for double ionization.