Within the framework of multichannel quantum-defect theory, Seaton’s scrR matrix is transformed to a modified reactance matrix with null submatrices for the open-open and degenerate closed-closed channel couplings. Using this coupling matrix, parametric expressions for the photoionization cross section in the autoionization region analogous to Fano’s resonance formula are derived for four cases: (i) many open channels and one closed channel, (ii) many open channels and many closed channels which do not interact either directly or through open channels, (iii) one open channel and many degenerate closed channels, and (iv) one open channel and two nondegenerate closed channels. The utility of the derived expressions in fitting experimental data is demonstrated using absorption spectra of Xe, Ge, Ca, and Sr.

• Received 18 September 1986

DOI:https://doi.org/10.1103/PhysRevA.35.2484