Inner-orbital binding energies

The observed decrease in the inner orbital binding energies, measured for M(CeH5)4 (M = C to Pb) and some other compounds, with increasing atomic size has been found to be similar to that obtained theoretically from atomic SCF calculations [41]. 

In electron correlation treatments, it is a common procedure to divide the orbital space into various subspaces orbitals with large binding energy (core), occupied orbitals with low-binding energy (valence), and unoccupied orbitals (virtual). One of the reasons for this subdivision is the possibility to freeze the core (i.e., to restrict excitations to the valence and virtual spaces). Consequently, all determinants in a configuration interaction (Cl) expansion share a set of frozen-core orbitals. For this approximation to be valid, one has to assume that excitation energies are not affected by correlation contributions of the inner shells. It is then sufficient to describe the interaction between core and valence electrons by some kind of mean-field expression. 

In Fig. 2 we compare the 6s orbitals obtained for the two different couplings of the ion core. The difference in the calculations for these two orbitals is that the A a-coefficient for the exchange-correlation term in the Hartree-Slater Hamiltonian is varied to shift the calculated orbital energy to agree with the respective binding energy. The Hartree-Slater orbital for the 6s [ Fi/2 core] is also shown in Fig. 2. The inner nodes in this orbital are removed to obtain the 6s pseudoorbital. 

The main peaks in X-ray Photoelectron Spectroscopy (XPS) for molecules appear because of the photoionization of core electrons. In addition, satellite peaks on the high binding energy side of the main peak have often been observed. These peaks are generally referred to as shakeup satellite peaks. In the sudden approximation, the shakeup process which accompanies photoionization can be considered as a two-step process. First, a core electron is emitted as a photoelectron, creating an inner shell vacancy. In the next step, electron(s) in the same molecule transfer from valence orbital(s) to unoccupied orbital(s) with relaxation of orbital energies. It is important to study these satellites in order to understand the valence and excited states of molecules (1). 

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