Association

24 2. Theoretical background
or even comprises distinguishable additional satellite lines. 80 Intrinsic photoemission satellites
may occur due to the reduced Coulomb interaction from the (N − 1) core-level, so that
the valence level can accept one more electron. This “apparent valence change” due to the
photoexcitaiton is included in the observable final state, and referred to as shake-up satellite
at higher binding energy. A discussion on the intrinsic satellites of Eu 3d photoemission in
given in Ch. 5.1.
Plasmons One class of extrinsic loss events is the excitation of a free electron gas in the
sample by photoelectrons. In this case, a photoelectron dissipates a fraction of its kinetic
energy to collective excitations with the plasma frequency ω 0 in metallic layers. These are
of characteristic energy and referred to as plasmons. The plasmon excitation probability is
dependent on the photoelectron’s kinetic energy as calculated in Inglesfield (1983): for high
kinetic energy (hard X-ray excitation), plasmon losses have a reduced probability. 89 In this
thesis, fast photoelectrons from Eu core-levels excite plasmons in Al and Si layers which are
well known in literature, 90,91 and thus can easily be included into a quantitative evaluation
of the core-level spectra (Ch. 5.1).
Chemical shifts In photoemission, we are interested in the chemical states of the thin film
material. This information is provided by the chemical energy shift, first explained by Fadley
et al. (1967) – interestingly on the example of native europium oxide. 92 By measuring this
energy shift (generally 0.2–10 eV) one can investigate the initial state valency in ionic crystals.
The chemical shift ΔE bin of core-level binding energies between two chemical phases A and
B in a solid is given by
ΔE chem (A,B)=K ·(Q A − Q B )+(V A − V B ). (2.22)
Here, K is the coupling constant from the Coulomb interaction between valence and core
electrons, and Q denotes the charge transfer on the particular ion (e. g. +2 for an oxidized
cation). Thus, the first term in eq. (2.22) describes the difference in the electron–electron
interaction between the core orbital and the valence charges in a certain ionic bounding. The
second term (V A − V B ) represents the interaction of the atoms A and B with the remaining
crystal. For oxides, this is a Madelung-type energy. 80 As a rule of thumb, ions with higher
coordination number (e. g. octahedral vs. tetrahedral) or with larger charge transfer (e. g.
Eu 2+ vs. Eu 3+ ) exhibit also a larger chemical shifts to higher binding energy.
Surface core-level shifts. Energetic shifts can also arise from an altered electronic configuration
in the surface layers of a solid. For low-energetic photoelectrons, the escape depth is
in the range of the surface layers, and surface shifts are likely to be observed. This is approximately
the case for the deeply bound Eu 3d photoelectrons analyzed in this thesis. However,
surface shifts are usually small ( 3 eV) and less pronounced in HAXPES experiments due to
the predominant bulk character of the photoelectrons.
Spin–orbit coupling. During photoionization of a core-level, a core hole is created with
spin s ∗ = 1/2. A dominant spin–orbit coupling assumed, a doublet peak is eminent due to the
final states coupling j = l ± s ∗ in orbitals with an angular momentum l 0 (i. e. the p, d, f ,