PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

Experimental Techniques 47
Figure 2.15: The x-ray loses its intensity via interactions with material. The absorption
coefficient decreases smoothly with higher energy, except for special photon
energies. When the photon energy reaches a critical value for a core electron transition,
the absorption coefficient increases abruptly.
2.4.2 Detection techniques
The normal way to measure the x-ray absorption is in the transmission mode [21]. The
intensity of the x-ray is measured before and after the sample and the percentage of
transmitted x-rays is determined. Such transmission mode of experiment is standard
for hard x-rays. But for soft x-rays they are difficult to perform because of the strong
interaction of soft x-rays with matter and requires a very thin sample of 0.1 µm to
obtain a detectable signal. An alternative to the tansmission mode experiment is the
”Yield mode”, where the decay product of the core hole (created in x-ray absorption)
will be measured. The decay of the core hole gives rise to an avalanche of electrons,
photons and ions escaping from the sample surface. There are four types of detection
methods [21] in yield mode XAS measurement such as Auger electron yield (or partial
yield), fluorescence yield, ion yield and total electron yield.
In Auger electron yield method, the electrons of a certain energy range (i.e electrons
of a specific Auger decay channel of the core hole) are detected. It is found
that the mean free path of a 500 eV Auger electron is of the order of 20 Å. Hence