Core-ionization spectra

When orbitals are contracted in the core, spin-orbit effects are much greater and many p, d, and f core ionization spectra show multiplet structure. 

The SAC-CI method is useful to study both the main and satellite peaks in the core-ionization spectra as well as valence-ionization spectra. In this section, we explain the SAC-CI applications to the CEBE [114], inner-shell satellite spectmm [114,115] and vibrational spectrum of inner-shell ionization [116]. 

Given the complicated and variable nature of CO binding to palladium surfaces, it is not surprising that, until fairly recently, theoretical studies have struggled to adequately model palladium surface chemistry. The molecules PdCO and Pd2CO (bridging GO coordination) have been evaluated as models for GO on a palladium surface in a number of studies. The core ionization spectra of these complexes have been calculated and compared to experimental data for... 

We have tacitly assumed that the photoemission event occurs sufficiently slowly to ensure that the escaping electron feels the relaxation of the core-ionized atom. This is what we call the adiabatic limit. All relaxation effects on the energetic ground state of the core-ionized atom are accounted for in the kinetic energy of the photoelectron (but not the decay via Auger or fluorescence processes to a ground state ion, which occurs on a slower time scale). At the other extreme, the sudden limit , the photoelectron is emitted immediately after the absorption of the photon before the core-ionized atom relaxes. This is often accompanied by shake-up, shake-off and plasmon loss processes, which give additional peaks in the spectrum. 

The X-ray photoelectron spectrum of the core ionization of an atom in a molecule consists of peaks and bands corresponding to transitions to various excited states. None of these transitions corresponds to the formation of the Koopmans theorem frozen-orbital ionic state, which is a completely hypothetical state. However, the center of gravity of the various peaks and bands lies at the energy corresponding... 

We should finally briefly discuss the calculation of spectra for the surface adsorbates which we will use to verify the theoretical models and to assign peaks in the spectra. The calculation of XES spectra has been discussed extensively previously [3]. Briefly we have shown that the ground state orbitals provide a balanced description of initial and final state and calculate the spectrum as the dipole transition between the valence orbitals and the selected Is core level [21]. The success of this approach relies on similar charge transfer screening in the core-ionized initial (or intermediate) state as for the valence-ionized levels. XES thus reflects the ground state molecular orbitals. 

A semi-quantitative description of the core level spectrum and the charge-transfer process can be obtained from a simple two-level MOLCAO-model based on the sudden approximation 155 157,160). Here, we follow the formulation of Larsson157) and consider the influence of a core hole on a single electron in an MO formed by linear combination of AO s Ul and uM centred on the ligands (L) and the central metal ion (M). In the ground state, before ionization, the electron is in a bonding orbital... 

The PS spectrum has a strong peak centered at 285 eV associated with the direct photoionization of the core levels and a low-energy satellite peak at 291.6 eV arising from shake-up transitions ( tt it ) accompanying core ionization. These... 

As an example of a spectrum obtained with a 10 m instrument (grating radius 10m) a recording of the carbon K emission line from the CO2 molecule is shown in Fig.5.7. As can be seen, the high resolution reveals a clear structure due to molecular vibration. Through careful analysis of a spectrum of this kind it is possible to evaluate the C-O bond length very accurately in the core-ionized molecule. It turns out that the bond length is shortened by about 2% when the Is core vacancy has been formed in the carbon atom. From the linewidth it is also possible to evaluate the natural lifetime of the C Is state (Sect.9.4.5). The lifetime is of the order 10" s. Atomic structure research using X-ray emission spectroscopy has been discussed in [5.9-13]. 

Figure 8.19 X-ray photoelectron spectrum, showing core and valence electron ionization energies, of Cu, Pd, and a 60% Cu and 40% Pd alloy (face-centred cubic lattice). The binding energy is the ionization energy relative to the Fermi energy, isp, of Cu. (Reproduced, with permission, from Siegbahn, K., J. Electron Spectrosc., 5, 3, 1974)...

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