Comments on the photoionization of helium

Abstract
The continuous oscillator strength df/d epsilon for photoionization of atomic helium is evaluated in the dipole length and velocity formulations using a 50-parameter correlated ground-state wave function together with both Hartree-Fock and polarized-orbitals free-state representations. The results are compared with those obtained from 6-parameter and 20- parameter correlated ground-state wave functions. It is found that if we take a particular free-state representation and calculate df/d epsilon using increasingly more accurate ground-state wave functions then the dipole velocity results tend much more rapidly to a constant value than do those obtained from the dipole length formula. However, if one chooses a particular ground-state wave function and compare the two continuous oscillator strengths calculated with the Hartree-Fock and polarized-orbital free-state representations, then, at low photon energies, there is much better agreement between the two dipole length values than there is between the dipole velocity results.

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