Interpretation of Shakeup Mechanisms in Copper L‑Shell Photoelectron Spectra

We report on an original full ab initio quantum
molecular approach designed to simulate Cu 2p X-ray photoelectron
spectra. The description includes electronic relaxation/correlation
and spin–orbit coupling effects and is implemented within nonorthogonal
sets of molecular orbitals for the initial and final states. The underlying
mechanism structuring the Cu 2p photoelectron spectra is clarified
thanks to a correlation diagram applied to the CuO4C6H6 paradigm. This diagram illustrates how the energy
drop of the Cu 3d levels following the creation of the Cu 2p core
hole switches the nature of the highest singly occupied molecular
orbital (H-SOMO) from dominant metal to dominant ligand character.
It also reveals how the repositioning of the Cu 3d levels induces
the formation of new bonding and antibonding orbitals from which shakeup
mechanisms toward the relaxed H-SOMO operate. The specific nature,
ligand → ligand and metal → ligand, of these excitations
building the satellite lines is exposed. Our approach finally applied
to the real Cu­(acac)2 system clearly demonstrates how a
definite interpretation of the XPS spectra can be obtained when a
correct evaluation of binding energies, intensities, and relative
widths of the spectral lines is achieved.