Photoionization of atomic sodium near threshold

R-matrix with pseudostates (RMPS) calculations have been carried out for photoionization of atomic sodium near threshold. The large RMPS atomic orbital and configuration basis allows for very accurate computations of low-energy photoionization cross sections up to $\approx 30$ eV, the energy range for which the RMPS calculations were optimized. Consistency checks for accuracy include, first, the excellent agreement found between length- and velocity-gauge theoretical results, a necessary but not sufficient requirement for having a converged wave function. A second accuracy quantification is the excellent prediction of the position of the Cooper minimum compared to experimental results. Particular attention is paid to the Cooper minimum occurring just above threshold, and the spin-orbit splitting of minima, resulting in a nonzero total cross section. Our RMPS results away from the minimum are found to be lower than the experimental data, and we make the case that the experimental magnitudes are an overestimate. A third important affirmation of the present accuracy is the continuity found between the $3s\to np$ bound-bound discrete oscillator strength density below threshold—see Wiese et al. [W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Probabilities, Vol. 2: Sodium Through Calcium; A Critical Data Compilation (US Government Printing Office, Washington, DC, 1969)]—and the $3s\to \epsilon p$ bound-continuum RMPS oscillator strength density above threshold. These three somewhat independent tests of the accuracy of the computed cross sections add confidence to our recommending the present RMPS results as the most reliable extant data for low-energy Na photoionization (and the earlier Wiese et al. results for the discrete states).