Diploma - Max Planck Institute for Solid State Research

18 3 Experimental foundations
mean free inelastic scattering path of electrons in solids follows a general curve
(cf. fig. 2.3) yields for the used energy range [45. . .140] eV approximately [4. . .8] Å,
thus PE is rather surface sensitive. Hence, to obtain clean surfaces with few adsorbed
atoms / molecules, we prepare them in-situ by cleaving (a stick which
has been glued previously onto the sample is used as lever to split the sample,
see fig. 3.1). The difference between surface and bulk states can experimentally
be distinguished by choosing different surfaces or by quenching of surface emission
involving the deposition of adlayers. In ab-initio calculations, one can use
the contribution of the first layer to the band structure of a supercell (slab) as
an estimate, where the translational invariance is broken parallel to the surface
normal. Nevertheless, also the top layer contributes to bulk states, and therefore
one should generally use the states, the charge density of which is localized
at the surface. As the DFT calculations deal solely with itinerant electrons, the
PE of rather localized 4f electrons has to be incooperated in another way. The
insignificant overlap in all three spatial dimensions of the respective states allows
to use calculated atomic PE spectra as a first approximation. The 4f emission
of Eu adlayers is shifted towards higher binding energies due to the asymmetry
in the potential at the surface. Since the lower coordination number effects
the distribution of valence electrons, the binding energy of europiums’ 4f orbitals
changes and therefore the shift of the surface emission is comparable to that of a
surface core level shift (predictable by a Born-Haber process [33]).
• As a coarse approximation atomic cross sections for photoemission will be used
to analyze the corresponding spectra. Basically it has been shown, that they are
different in solids (e.g. revealing several Fano resonances [34]) caused by deviations
in the shape of the wave function, particularly due to different boundary conditions
in the solid and the free atom. However, the calculation is non-trivial and in zeroth
order the atomic cross sections are a suitable estimate.
3.2 General setup
The photoemission spectra were taken at two experimental setups located at different
synchrotron sources each having its unique beamline layout and endstation characteristic.
Their specifics will be discussed below.
Generally, a synchroton radiation source consists of a booster unit composed of diverting
coils and accelerating parts, which take charged particles to high energies (typically
several GeV). The variation of the magnetic field according to the particles’ speed keeps
them on a closed orbit. Afterwards they are induced into a storage ring where the energy
loss (proportional to the amount of synchrotron radiation) is compensated by linear accelerators
mounted into the ring so the particles remain at an orbit with constant speed.
Since the radiation at high energies peaks strongly in the forward direction in contrast