Convergent close coupling calculations of two-photon double ionization of He

We review recent applications of the convergent close-coupling (CCC) method to studies of two-photon double ionization (TPDI) of He. In a weak-field regime, the electron-photon interaction can be treated within the lowest order perturbation theory (LOPT) whereas the electron-electron interaction is included in full. The intermediate states of the target can either be represented by a discrete set of B-splines in a box or summed over with an average weight (closure approximation). In a non-perturbative regime, we solve the time-dependent Schrödinger equation on a square-integrable basis and project this solution on a set of CCC final states. In both regimes, we are able to calculate reliably the total intgrated and full differential TPDI cross-sections. Besides purely numerical results, we introduce a convenient analytical parametrization of the TPDI amplitude in the manner similar to single photon double ionization. Aided with this parametrization, we observe two distinctly different modes of correlated motion of the photoelectron pair. We also derive the angular anisotropy parameters and the recoil ion momentum distribution for TPDI of He. The latter can be compared with recent experimental observations.