Association

2.4. Hard X-ray photoemission spectroscopy 23
losses is introduced by using an imaginary part of the momentum, k ⊥ = k (real)
⊥
+ ik (im)
⊥ . Since
we are interested only in binding energies and chemical shifts of deep core-levels (negligible
k dependence), a basic picture according to the three-step model is sufficient for this thesis.
2.4.2. Spectral features and their interpretation
Figure 2.14.: Schematics of photoemission spectroscopy. After Saiht (2009). 221
In photoemission spectroscopy, a solid sample is irradiated by monochromatic photons which
excite electrons from occupied states in the solid. If the photoelectrons have a large enaugh
kinetic energy, then they can be detected by an electron energy analyzer (Fig. 2.14). A photoemission
spectrum usually comprises peaks which correspond to the kinetic energy of the
photoelectrons, E kin = hν − E bin − φ 0 , where hν is the energy of the light, φ 0 denotes a work
function which the photoelectrons have to overcome in order to reach a free electron state in
the vacuum, and E bin is the binding energy of the electron’s initial state in the solid.
While the description of photoemission spectra by the three-step model can be very successful,
the one-electron approximation neglects many body interactions in the final state, as
well as surface effects or intra-atomic exchange interactions. We are leaving the one-electron
picture here, and subsequently introduce selected spectral features due to multiple electron
interactions in the final states, which are relevant to EuO core-level spectra.
Intrinsic satellites of core-levels. After emission of the photoelectron from orbital k, the
remaining N − 1 electrons will reorder to minimize their energy. This includes final state
configurations in which the (N − 1) system of the core-level interacts with electron near the
Fermi level. The transition matrix element must then be calculated by summing over all possible
final states. In a photoemission spectrum, the core-level line appears then asymmetric,