Spin-orbit effects in aluminum photoionization

The eigenchannel R-matrix approach, in conjunction with the multichannel quantum-defect theory and the LS{→}jj recoupling frame transformation, is used to calculate the photoionization spectrum of Al i below the 3s3p ¹{P}ᵒ$ ionization threshold (photon energies in the range 0.44{≤}{}{ω}{≤}0.98 Ry). Relativistic channel mixing is incorporated in the calculations by a recoupling frame transformation and by the inclusion of experimental fine-structure threshold energies. This mixing enables autoionization of resonances whose decay would otherwise be forbidden in the pure LS-coupling scheme. The calculated J-dependent energies and widths agree well with those of experimental resonances. The complicated relativistic spectrum, with up to 11 interacting channels, provides an experimentally realizable testing ground for studies of statistical properties of resonances. The spectrum below the 3s3p ³{P}ᵒ$ ionization threshold exhibits the Wigner and Porter-Thomas distributions of positions and widths of resonances, respectively. While portions of the aluminum spectrum appear to be random, according to these measures, there remains much underlying regularity in the level spacing and width distributions.