A close look at Ar photoion spectra around the K edge: non-diagram transitions and double photoionization

Relative abundances of the various ionic charges resulting from photoionization of argon by monochromatized synchrotron radiation at energies between 3150 and 4900 eV have been measured with low statistical error. Despite the fact that already, right at the onset of K excitation (3203.5 eV), almost the full 1s binding energy is deposited in the atom, the relative abundances change smoothly over the region of np excitation up to 6 eV above the limit. While this behaviour, caused by Rydberg shake-off (about 0.5 probability at the 1s to 4p resonance) and by recapture of the photoelectron via post-collision interaction, appears to be understood there is no explanation for a gradual change of relative abundances extending over tens of eV below the lowest K excitation. K+M and K+L single photon double excitations both show up as a smooth rise of the average ionic charge q from the respective threshold to a plateau at considerably higher energy, the overall increase of q being 0.206+or-0.02 and 0.13+or-0.01.