ABSORPTION SPECTROSCOPY AND RELATED TECHNIQUES
A. Kodre, I. Arčon, and J. Padeľnik Gomilšek, Acta Chim.
In the extensive
field of x-ray diagnostic techniques, nowadays mostly implemented on
synchrotron x-ray sources, the x-ray absorption methods offer a relatively
simple tool for structural analysis of materials. The advantage of the
methods is the sensitivity to chemical species and the insensitivity
to the long-range order. An overview of various detection techniques
is given together with the discussion of accuracy of the method with
regard to the spatial resolution and the possible contamination by intra-atomic
contributions, the multielectron photoexcitations.
diagnostic power of x-rays is based on a coincidence of two fortuitous
properties: the wavelength of the x-ray light is of the same order of
magnitude as interatomic distances, and the energy of the x-ray photons
is of the order of binding energies of the most tightly bound electrons.The
first property provides for rich and meaningful interference patterns
of the light in a beam passing a layer of a well-ordered material.From
this the positions of the constituent atoms in the basic unit of a periodic
structure can be deduced, leading to various techniques in XRD ( = X-Ray
Diffraction) developed for either large monocrystals or microcrystalline
powders [1,2]. Even the nano- and mesoscale inhomogeneities
of the materials are discernible in the interference pattern close to
the direct beam (SAXS – Small Angle X-ray Scattering .
second property of x-rays opens a way for a fast and simple elemental
analysis: the binding energies of the core electrons grow monotonically
with atomic number (Moseley law), unaware of chemical periodicity
(Fig. 1). Thus, constituent elemental species of a sample can
be identified simply from the energies of absorption edges in
its x-ray absorption spectrum or from the energies of the characteristic
x-ray lines in its fluorescent radiation. This is the basis of
modern XRF ( = X-Ray Fluorescence) analytic techniques which can
even be exploited for a fast quantitative analysis .
1. The relation between the atomic number Z of an element
and the energy of its K and L x-ray absorption edges (Moseley law).
The simple dependence
of the characteristic energies on the atomic number is also exploited
in X-ray absorption spectroscopy (XAS) [4,5,6].
The basic experiment is very simple: a thin homogeneous sample of the
investigated material is prepared, and the intensities of the incident
and the transmitted x-ray beam are recorded in the stepwise progression
of the incident photon energy (Fig. 2).