Ab initio studies of the X-ray absorption edge in copper complexes. I. Atomic Cu^(2+) and Cu(II)Cl_2

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, Ss, and Sp transitions and the corresponding shakeup processes for Cu atom and for a Cu(II) model system, CuCl_2. For CuCl_2 the lowest strong transitions have the character 1s → 4p (f=0.00133). 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., 1s^(2)3d^(9) → 1s^(1)3d^(10)). 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.)