K-SHELL PHOTOIONIZATION OF ATOMIC BORON

The direct and conjugate shake-up cross sections for the 2p --> 3p, 3s and 2s --> 2p, 3s transitions are calculated for atomic boron in a cl ose-coupling approximation using the R-matrix method. In the photon en ergy interval 17-45 Ryd we find that the total cross section for these secondary processes represents about 30% of the single inner-shell ph otoionization cross section. The 2s --> 3s excitation gives the domina nt contribution (80% of the total), while the 2p --> 3p and 2s --> 2p excitation contributions are broadly comparable with each other in thi s energy range. The contribution from the 2p --> 3s transition is negl igible. It is then probable that the 2s shake-off excitation dominates the 2p one, giving rise mostly to terms of the 1s2s2p configuration. Our results suggest that the ionization equilibrium of multiply charge d B, C, Na, Mg, Al and Si in a low-density soft-x-ray photoionized pla sma will be perturbed strongly by such processes.