Double-photoionization calculations of the helium metastable 2^1,3S states

Theoretical results are presented for double photoionization and ionization with excitation of the metastable 2^1,3S states of helium. We employ the momentum space formulation of the close-coupling theory to describe nonperturbatively the electron-electron interaction in the final state. A large Laguerre basis is used to obtain convergent close-coupling (CCC) results. In addition, we employ a B-spline basis for the target states. The presented results cover continuously the photon energy range from the double-photoionization threshold to the asymptotic limit of infinite photon energy. Near the threshold our data generally support the calculations of van der Hart et al. [Phys. Rev. A 57, 3641 (1998)]. At large photon energies our results merge continuously with the asymptotic values derived from the metastable state 2^1,3S wave functions. Proportionality between the double-to-single photoionization cross-sections ratio and the cross section of electron-impact ionization of the ion is tested to examine the relative contributions of different mechanisms of the two-electron photoionization in the ground and metastable states of the helium atom.