52 Experimental Methods
Oxide pO2 mbar T K MnOx rate Å/s Post-anneal T K
Mn3O4 2.7 104 473 0.8 na.
Mn2O3 7 104 473 0.8 873 (1 hour)
MnO2 103 473 0.1 773 (1 hour)
Table 3.2: Deposition parameters resulting in crystalline Mn oxides.
evaluated and c) high stability in energy and beam position. These requirements
are all met in synchrotron sources that can deliver more than ve orders
of magnitude higher ux compared to laboratory sources. The energy of the
photons delivered from the synchrotron is selected with monochromators.
The method is based on exciting the core electrons of atoms in a sample by
absorption of photons. The photon energy required to excite a given electron
depends on the energy level of the electron and the unoccupied state that can
be lled. In XAS, the absorption of photons as a function of energy is used for
analysis, which is in contrast to X-ray Photoelectron Spectroscopy, where the
energies of photoelectrons are analysed. The absorption of photons is typically
measured for a specic range corresponding to a certain "edge" indicating the
core electron that is excited by the photons. The K edge corresponds to exciting
the 1s, L1-3 edges correspond to 2s and 2p and M1-5 edges correspond to 3s, 3p
and 3d. At the same time the K edge requires the highest photon energy while
the M5 requires the lowest. X-rays can penetrate deep into samples so when
transmitted X-rays or uorescence radiation is used for analysis, XAS is a bulk
sensitive technique. Transmission mode is the simplest detection method, where
ion chambers before and after the sample are used to evaluate how much of the
beam is absorbed. Fluorescence radiation originates from a decay of the excited
state, where electrons from higher energy levels ll the empty core states and
emit new photons with characteristic energies. This is shown in gure 3.6a. It
is also possible to measure electron yield originating from the Auger process, in
which case the technique becomes surface sensitive. This process is shown in
gure 3.6b. In the Auger process, an electron from a higher energy level in an
excited atom lls an empty core level and the energy dierence is transferred to
another electron which is released into vacuum as a photoelectron.