Physics and Engineering of Radiation Detection

12.1. Spectroscopy of Photons 687

corresponding spectrum would show peaks at the binding energies of the atom.
Now, since binding energies are specific to the element, the spectrum can be used to
identify the constituents of the sample. This technique is called x-ray photoelectron
spectroscopy or XPS. Note that, as opposed to x-ray absorption spectroscopy, here
one does not need to perform measurements at different x-ray energies. An x-ray
source producing nearly single-wavelength photons is sufficient for photoelectron
spectroscopy.

Eb,2p

Eb,2s

Eb,1s

Eγ

Eγ

Eγ

Eγ Eγ Eγ

Continuum

2p

2s

1s

−Eb,2p −Eb,2s −Eb,1s

Figure 12.1.13: Principle of x-ray photoelectron spec-

troscopy.

The principle of XPS is graphically depicted in Fig.12.1.13. The most tightly
bound electron is the 1s electron, which assumes the lowest energy after absorbing
an x-ray photon. The electrons in the higher energy levels are emitted with higher
energies. Detection of these photons and measurement of their energies then leads
to a spectrum as shown in Fig.12.1.14. The peaks correspond to the transitions
shown in Fig.12.1.13.
Let us now discuss the practical aspects of x-ray photoelectron spectroscopy.
The experimental setup required for XPS is more complicated than for XAFS spec-
troscopy. The reason is that here one is required to not only detect electrons but
also to measure their energies. Since electrons have very short range even in air
therefore the sample and the detector must be kept in a vacuumed chamber (see
Fig.12.1.15). The setup also requires an electron energy analyzer to measure the
photoelectron energy.
Different types of energy analyzers exist but probably the most common is the
so called hemispherical deflection analyzer or HDA. HDA is also known as con-
centric hemispherical analyzer or CHA since it mainly consists of two concentric
hemispheres. The purpose of these hemispheres is to set a passband of electron
energies. That is, to block all electrons that are outside a defined energy range. A
sketch of the HDA is shown in Fig.12.1.16. The first part of the apparatus consists