12.1. Spectroscopy of Photons 689
an electric field in the volume subtended by them. All the electrons coming from
the focusing lenses experience this electric force. The higher energy electrons get
deflected toward the outer hemisphere while the lower energy ones go toward the in-
ner hemisphere. Hence only electrons having energy within a narrow band will pass
through the hemispheres and get detected by the electron detector (see Fig.12.1.16).
The energy and intensity measurements are done by the lens parameters and the
electron detector counts.
Focusing LensesElectron DetectorLow−EnergyHigh−Energy
ElectronBand−PassElectronDeflection AssemblyHemisphericalSlitSlitElectrons
SlitElectronsFigure 12.1.16: Conceptual design of a hemispherical deflection
analyzer.C.3 X-rayDiffractionSpectroscopy(XDS)
The XAFS spectroscopy and the XPS we discussed earlier do not give us insight into
the internal physical structure of the material. The reason is that these techniques
exploit the processes leading to the absorption of x-ray photons and then detection
of particles emitted as a result. Since there is no correlation between the emitted
particles and the bonding structure of the material, therefore one can not deduce
the physical structure from the measurements.
X-ray diffraction spectroscopy or XDS is a technique that can actually be used to
determine the internal structure of crystalline materials. As the name suggests, this
technique is based on the process of diffraction of x-ray photons. To understand the
process, the reader is referred to Fig.12.1.17, which depicts a simplified picture of
three atomic planes of a crystalline lattice. Parallel beams of x-rays are shown to be