Photoionization cross section of neutral calcium including double electron resonances

Many-body perturbation theory has been used to calculate the photoionization cross section of neutral calcium from threshold at 6.11 to 70 eV. Near-threshold resonance structure due to double electron excitation has been included and is compared with the results of several experiments. At higher energy, resonance structure due to $3p$ excitations to bound states with $l=0\mathrm{} \mathrm{and} 2$ has also been calculated and compared with a measurement by Mans-field and Newsom. The $3p^{5}4s^{2}3d{}_{}{}^1{}_{}{}^{}P$ resonance is found to be particularly prominent, in agreement with experiment. Our results are also compared with a recent calculation using the relativistic random-phase approximation.