Multichannel continuum states by a least-squares approach in a spline basis: application to He and H- photoionization

A recently proposed algorithm for the determination of the continuum wavefunction employing a B-spline basis set expansion and a least-squares approach is extended to a full multichannel formulation employing a general close coupling expansion of the continuum wavefunction. Photoionization of the He atom up to the n=4 threshold and H - up to the n=3 threshold are described with a pure close coupling wavefunction (16 and 9 channels respectively), and an accurate full CI expansion for the ground state. Excellent agreement with previous accurate calculations is obtained for total and partial cross sections, resonance parameters and angular distributions. For He a nine-channel wavefunction allows photoionization to be described up to 2 keV. Modification of the boundary conditions to describe long Rydberg series is successfully tested. The proposed algorithm appears to be a general and flexible approach for the description of multichannel continuum wavefunctions by basis set expansion.