DOUBLE MULTIPHOTON IONIZATION OF A MODEL ATOM

A direct solution of the time-dependent Schrodinger equation (TDSE) fo r a two-electron model atom in a strong electromagnetic field is used to extract probabilities for both single and double electron ionizatio n. For three-photon ionization at a moderately strong intensity, the T DSE calculations find single ionization at 62%, single ionization with excitation at 10%, and double ionization at 3%. The sequential proces s of escape from an already once ionized atom is found to make a subst antial 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 mo del atom. Although in qualitative agreement with the TDSE results, the TDUHF results show significantly less single ionization with excitati on and have a different angular distribution for double ionization.