Atomic absorption background

For the photons in the spectral region just above the absorption edge, the simple photoeffect is not the only possible channel of reaction with an atom. Accompanying the emission of the photoelectron, additional electrons in the atom can be promoted to more loosely bound orbitals or ejected from the atom. The probability of these events is not a smooth function of photon energy but shows sharp thresholds whenever the coexcitation of a valence or subvalence electron is possible. Depending on the atomic number, these thresholds occur throughout the EXAFS region, superposed onto the structural signal. They are relatively strong and sharp immediately above the edge, and weaker toward the high-energy end. Their contribution is termed the atomic absorption background (AAB): it can ideally be determined on a monatomic sample of an element, with the structural signal completely absent. Very few elements are amenable to such measurement, so that a reverse decomposition of the absorption spectrum into the AAB and the structural signal provides most data. A collection of the AAB for 4p elements (from Ga to Rb) 13 is shown in Fig. 9. In addition to the sharp features, the AAB comprises some smooth saturation contributions: these form a signal of very long periods which can be resolved from the structural signal by the fact that no (real) atomic neighbors could be placed so close to the central atom as to produce such a slow interference oscillation. For materials with a strong structural signal, the long-period approximation to AAB is mostly satisfactory. For disordered materials, however, the structural signal is suppressed and the AAB can prevail. In such a case, the adoption of the exact AAB is mandatory [14].

Fig. 9. Experimentally determined AAB for the 4p elements. [13]

The incidence of major groups of multielectron photoexcitations within the EXAFS spectral region (Fig. 8) shows that ranges of the atomic number with relatively clean EXAFS spectra exist, as e.g. the range of transition metals from Ti to Cu. Some other Z ranges are spoiled by multielectron photoexcitations more seriously, with sharp features in the very midst of the useful EXAFS range (elements in Fig. 9 and early lanthanides) so that the knowledge of exact AAB is necessary in the EXAFS analysis.








Last change: 06-Jun-2006