Angular distributions for two-photon double ionization of lithium

We present angular distributions for two-photon double ionization of lithium at photon energies of 50 eV (lambda = 25 nm) and 59 eV (lambda = 21 nm). The results are obtained from full-dimensional solution of the two-active-electron time-dependent Schrodinger equation using the time-dependent close-coupling method. We investigate two different double ionization mechanisms. First, we consider direct double ionization of the Li ground state following the absorption of two photons. Secondly, we consider an initial photoexcitation of the 1s2s2p doubly excited state, followed by photoionization of the 2s and 2p electrons. We find significant differences between the angular distributions obtained for these two distinct processes. We also compare the characteristics of the angular distributions for Li with those of other two-electron atoms.