Autoionization in the uv Photoabsorption of Atomic Calcium

Detailed measurements of the absolute photoabsorption cross section of atomic calcium between 2028 and 1753 Å and 1589 and 1424 Å are presented. Structure is displayed in the absorption cross section below 1589 Å, confirming previous theoretical predictions of autoionization in this region. However, the observed peak magnitudes are an order of magnitude smaller than suggested by theory. A theoretical analysis has succeeded in achieving better agreement between experiment and the two-channel quantum-defect theory for the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}P_1^0{}_{}{}^{}$ transitions in the 2028-1753-Å region, both in the magnitude and in the general shape of the absorption profile.