X-ray absorption spectroscopy
- The X-ray near-edge absorption spectroscopy (often abbreviated as XANES or NEXAFS) provides information about unoccupied electron states in the atomic orbitals of the chemical element being examined and thus about the chemical composition of a sample to be examined. One subgroup is X-ray near-edge absorption spectroscopy with circularly polarized X-rays. This technique uses X-ray dichroism (XMCD) and is used to study the magnetization of a sample.
- The EXAFS (from English extended x-ray absorption fine structure , EXAFS) provides information on the bond lengths in a sample. Measurements with this technology on surfaces are also referred to as SEXAFS ("s" stands for English surface ).
In all of these methods, the absorption of the X-rays is measured in the area of an absorption edge . If an X-ray quantum has enough energy, it can knock an electron out of an orbital close to the nucleus; at this energy, therefore, the absorption of the X-rays increases sharply.
Measurement of the X-ray absorption coefficient
All X-ray absorption spectroscopy techniques have in common that a source of X-rays with variable energy is required. Nowadays synchrotron radiation from an electron storage ring (e.g. BESSY II in Berlin) is normally used, which is monochromatized in order to obtain radiation of a certain energy from the continuous spectrum. The monochromatic X-ray radiation enters the sample and is (partially or completely) absorbed there.
The ratio of the X-ray intensity before and after the interaction with the sample is measured and the absorption is thus determined. This is the simplest method, but it requires that the sample is thin enough that X-rays can still pass through. With low X-ray energies (absorption edges of carbon, nitrogen or oxygen), the samples would have to be extremely thin (in the range of a micrometer). In the transmission measurement, both the inside of the sample and its surface contribute to the absorption; however, the contribution of the surface is much weaker than that of the volume and therefore cannot be determined separately.
Ionization counters are used to determine the X-ray intensity .
Auger and sample flow measurement
The excitation of an electron leaves an unoccupied state , which is filled up again after an extremely short time (a few femtoseconds or less) by an electron of higher energy. The gain in potential energy is typically associated with the excitation of another electron ( Auger process ). The number of Auger electrons, i.e. the Auger electron yield , is therefore proportional to the number of electrons originally excited by the X-ray radiation, i.e. the total electron yield and thus a measure of the absorption coefficient.
The relatively high-energy Auger electrons, however, are inelastically scattered after a short distance (a few tenths of a nanometer to a few nanometers) and therefore quickly lose their energy, which instead absorbs other electrons. When the electrons have reached a relatively low energy (below approx. 10 eV) they have a greater mean free path and can therefore leave the sample more numerous than the Auger electrons.
If the Auger electrons are measured, information is therefore only obtained about the X-ray absorption in the immediate area of the surface. When measuring all emitted electrons with a secondary electron multiplier , e.g. B. a Channeltron , slightly deeper areas of the sample are recorded (a few nanometers). With powerful X-ray sources ( synchrotrons ), the entire electron current is in the range of nanoampere and can also be measured as a sample current, which is caused by the "loss" of the emerging electrons. The latter requires that the primary intensity is measured using the same method and that the sample flow is normalized (divided) with it. This measurement of the so-called I 0 intensity is often carried out on a gold mesh, since gold is inert and has hardly any absorption structure in the soft X-ray range.
Another possible process for “filling up” the unoccupied state is the emission of X-rays , ie a fluorescence photon. This process is far less likely in the soft X-ray range (typically in the per mil range) than the Auger process. However, the X-rays can penetrate a thicker layer than the electrons, so a larger part of the sample is captured and the disadvantage of the lower intensity is partially compensated for. The proportion of the direct surface in the measurement signal is therefore lower, which is advantageous if one is interested in the spectroscopy of the interior of the sample and impurities on the surface are a problem.
When measuring directly at the absorption edge, the so-called self-absorption or saturation effect occurs, i.e. with the X-ray energies at which the absorption is higher, the primary beam penetrates less into the sample and thus only a smaller sample volume is recorded. This effect is irrelevant when measuring the electron yield because electrons no longer come from greater depths where the primary beam is weakened. However, fluorescence measurements still reach this depth because the exit depth of the secondary radiation is comparable to the penetration depth of the primary beam. The fluorescence radiation is therefore no longer exactly proportional to the absorption. The ratio of fluorescence events to the number of absorbed X-ray quanta is called fluorescence yield (Engl. Fluorescence yield ).
- Douglas A. Skoog, James L. Leary: X-ray Spectroscopy (XRS). In: Douglas A. Skoog, James L. Leary (Eds.): Instrumental Analytics . Springer, Heidelberg 1996, ISBN 978-3-662-07916-4 , pp. 387-414.
- Grant Bunker: Introduction to XAFS: A Practical Guide to X-ray Absorption Fine Structure Spectroscopy. Cambridge University Press, Cambridge 2010, ISBN 978-0-521-76775-0 .