Quantum defect theory IV. The absorption of radiation by calcium atoms

Photoionization cross sections and oscillator strenghts for the absroption of radiation by neutral atomic calcium in its ground state are calculated with allowance for configuration interaction effects, using radial wave functions with asymptotic forms obtained on application of many-channel quantum defect theory. Calculations are first carried out assuming a two-channel system, an attempt subsequently being made to improve the results obtained by including a third channel. In the case of the two-channel approximation, for which the fitting to experimental data is fairly accurate, it would appear that where there is discrepancy between theory and experiment this is due to neglect of the third channel. For the three-channel case, the discrepancies are probably due to inaccuracies in the fitting procedure. A further source of inaccuracy occurs when there is cancellation between positive and negative contributions to the total line strength matrix element.