X-Ray Analysis

Determining the structure of a crystalline substance on the atomic level is referred to as X-ray analysis or X-ray structural analysis. This is accomplished by observing the diffraction of suitable irradiations at the crystal lattice. Suitable irradiation may be mono-chromatic X-ray radiation, which can be generated by an X-ray tube. Molybdenum K_α radiation is frequently used. Its wavelength of 73.01 pm is ideally suited for diffraction at C-C bonds. Diffraction can be carried out at a monocrystal or at a crystalline powder. Monocrystal diffraction provides more precise results than diffraction at a crystalline powder. The observed diffraction pattern of the X-ray radiation allows to infer the crystal structure by means of a special computer software. From this diffraction pattern the geometry of the unit cell can be computed, which allows to deduce the array of the atoms within this unit cell. Note that with X-ray structural analysis, the radiation is diffracted at the electrons, the so-called core and valence electrons. This complicates the localisation of elements like hydrogen. Localising these elements is possible only by neutron diffraction analysis, where neutrons interact with the atomic nucleus.

X-rays

X-rays are electromagnetic waves of a photonic energy between 100 eV and several MeV, and of a wavelength between 1 nm and 10^-3 nm. Consequently, X-ray radiation is located in the electromagnetic spectrum between UV and gamma radiation. It was discovered by Wilhelm Conrad Röntgen, who is honoured by the German denomination of x-rays, röntgenstrahlung. The energy ranges of gamma and X-ray radiation overlap in a broad area. Their common characteristic is the fact that both are electromagnetic radiation of equal energy. They are thus physically equivalent. The difference between them is their origin. In contrast to gamma rays, X-rays originate not from processes in the atomic nucleus but from high-energy electron processes.

Generation of X-ray radiation

X-ray radiation is generated in the X-ray tube. The position of the maximum is dependent on the level of the operating voltage. The X-rays in the X-ray tube are generated by strong acceleration of charged particles and by high-energy transitions in the electron shells of atoms or molecules. Both effects are used in the X-ray tube where electrons are accelerated. Afterwards, these electrons hit a block of metal, by which they are profoundly slowed, knocking other electrons out of the shells of the metallic atoms. The generated electron holes are refilled with other electrons, resulting in X-ray radiation of an energy level that is specific for the used element, the so-called characteristic X-ray radiation.

Literature

• http://de.wikipedia.org/w/index.php?title=R%C3%B6ntgenstrahlung&oldid=86983003 (called: 28.03.11).

• http://de.wikipedia.org/w/index.php?title=R%C3%B6ntgenabsorptionsspektroskopie&oldid=82631152 (called: 28.03.11).

• http://www1.tu-darmstadt.de/fb/ms/fg/sf/uebung/V_210108_d_sw.pdf (called: 28.03.11).