Photoionization - excitation of helium using an R-matrix with pseudostates method

Abstract

We have developed an alternative R-matrix with pseudostates (RMPS) method for incorporating a two-electron continuum description into the wavefunction of electron - ion collision calculations. This method is similar in spirit to various recent treatments of the pseudostate expansion, most notably the R-matrix approaches of Meyer and Greene, and Bartschat et al. Our approach differs in that we: (1) utilize a direct sum of several bases: the physical target orbitals, additional MCHF pseudo-orbitals for the He ground state, R-matrix continuum orbitals, and, on occasion, Laguerre orbitals, and (2) rely on a different approach for the creation of an orthonormal basis. Since we use the Belfast codes that are based on a Wigner - Eisenbud R-matrix treatment, we also need to introduce a modified Buttle correction. The method is first tested on the s-wave (Temkin - Poet) model for electron-impact excitation and ionization of . Then it is applied to the calculation of cross sections and angular distributions for the photoionization - excitation process (n = 2,3). We find that these results are greatly improved by the inclusion of the two-electron continuum description, which is more important for the ground state, but plays a role in the final photoionized states as well. A highly correlated multiconfiguration Hartree - Fock expansion is used to represent the ground state, and from the configuration interaction coefficients, asymptotic ratios are determined and compared with other theoretical results. Results are also compared with recent high-energy measurements for the n = 2,3 cross section ratios and angular distribution parameters. This pseudostate expansion allows us to predict photo - double-ionization cross sections, . Of particular significance, we find good agreement between our length and velocity gauge results, indicating that our ground-state correlation is sufficiently converged for the present system.