Rmatrix with pseudostates calculation for single and double ionization of helium by photon impact

Abstract

We have applied the $R$ matrix with pseudostates (RMPS) method to calculate single and double ionization of helium by photon impact. Using the velocity gauge of the dipole operator, excellent agreement with high-precision experimental data is obtained for photon energies between threshold and 200 eV. While some discrepancies remain between results from different gauges, we demonstrate that the RMPS method can produce satisfactory results if either the length, velocity, or acceleration form of the dipole operator is used.