Photoionization of Atomic Sodium for Astrophysics

Near-threshold photoionization of atomic Na has been of interest both for basic physics and for astronomical applications for more than 70 years, beginning with the pioneering calculations of Seaton in 1951 and the experimental work of Hudson and Carter in 1967. The existence of a Cooper minimum in the threshold region makes accurate computation of the near-threshold cross section and recombination rates problematic since the details of the very small photoionization cross section in the region of the minimum is highly sensitive to the details of the calculation, specifically, the multiconfiguration/multichannel many-body wave functions of the initial and final states. Here we theoretically quantify this Cooper minimum in detail and obtain reliable near-zero cross sections and rate coefficients in the photon energy range just above threshold.