Two-electron excitations in atomic calcium. I

The pattern of electron correlations is examined for odd-parity L=1 states of calcium over a wide range of energies from far below the 4s threshold up to the 4p threshold of ${\mathrm{Ca}}^{+}$. By combining quantum-defect theory with a small-scale eigenchannel R-matrix calculation, good agreement with the observed photoabsorption spectrum is obtained. The two-electron probability densities and channel-interaction parameters are analyzed as functions of the total energy. A novel interference between direct (4sɛp-3dɛp) and indirect (4sɛp-4pɛs-3dɛp) channel-interaction amplitudes greatly reduces the width of the 3d6p autoionizing resonance compared to adjacent series members.