THE ENERGIES AND STRUCTURE OF X-RAY K-ABSORPTION EDGES

Improved screening parameters are employed to compute the K threshold energies (E(K)) and photoelectric cross sections (sigma-K) for the abundant atomic species of the interstellar gas. The E(K)-values for neutral C, N, and O differ from the values currently being employed in X-ray astronomy by several percent. The sigma-K are computed in an effective Coulomb field formulation, introducing an "effective Coulomb" threshold, and convenient formulas are given for the extrapolation of the results to lower energies in the neighborhood of the real threshold energy. Within about 30-200 eV above threshold the corrected cross sections display structure associated with direct secondary excitation of outer-shell electrons into the continuum. A unified description of this feature of the cross section is introduced, in terms of the secondary threshold energies and core relaxation factors. Both of these factors are computed with the improved screening parameters associated with the one-electron atomic states of the many-electron systems.