Ab initio studies of the x-ray absorption edge in copper complexes. I. Atomic Cu2+ and Cu(ii)Cl2

As a first step in the study of the structure at x-ray absorption edges, we have examined the discrete transitions corresponding to the atomic 1s 3d, 4s, 4p, 5s, and 5p transitions and the corresponding shakeup processes for Cu atom and for a Cu(ii) model system, CuCl2. For CuCl2 the lowest strong transitions have the character 1s 4p (f=0.001 33). About 7.5 eV lower is a group of transitions involving 1s 4p simultaneous with ligand-to-metal shakedown. About 18.7 eV below the main peak is a weak (65 times weaker) quadrupole-allowed transition corresponding to 1s 3d (i.e., 1s23d9 1s13d10). These results are in agreement with typical assignments of x-ray absorption spectra of Cu(ii) systems except that the middle transition is usually assigned as 1s 4s, whereas we find this transition to be 1s 4p plus shakedown. (Transitions of the character 1s 4s are calculated but have intensities far too low to have been observed.)