X-ray near-edge absorption spectroscopy
The near edge x-ray absorption fine structure , English near-edge x-ray absorption fine structure or X-ray absorption near edge structure spectroscopy , short NEXAFS or XANES spectroscopy is the X-ray absorption spectroscopy belonging spectroscopy methods for the study of solid surfaces . It records the unoccupied electron - states and the frequency and the spatial position of atoms or may molecules investigate on the surface.
Basics
With NEXAFS , strongly bound electrons close to the nucleus are lifted into an unoccupied state in the valence band or into an unoccupied atomic or molecular orbital using X-rays .
The basis of this technique is the appearance of X-ray absorption edges: an X-ray quantum can only knock an electron out of a strongly bound state (i.e. orbital close to the nucleus) if the energy is sufficient for the electron to enter an unoccupied state (or the continuum ) reached. If the energy of the X-ray radiation is increased from a value at which this is not (yet) possible to a value at which such a process is possible, the X-ray absorption of a material increases suddenly. As soon as the energy is high enough for the electron to reach the continuum, the absorption remains at a high value, which only slowly decreases as the energy increases. The sudden increase in absorption is called the absorption edge; and in the area of the absorption edge, the absorption depends on how many unoccupied states of a certain energy are available for the electron.
The adjacent figures show the formation of the X-ray fine structure when an incident plane wave is absorbed by an atom (left figure). After absorption, the absorbing atom in turn emits a spherical photoelectron wave (middle figure), which is then scattered by surrounding atoms (right figure). On the one hand (blue) there are single scattering processes that are the cause of the EXAFS structures, on the other hand (orange) there are also multiple scattering processes that are the cause of the NEXAFS / XANES structures in the detected spectrum.
In both cases there is interference (arrows), which provide the basis for quantitative investigations in NEXAFS / XANES or EXAFS measurements.
In order to avoid misunderstandings, however, it must be made clear that the XANES area cannot be described exclusively by the multiple scattering described above. While the EXAFS area relatively far from the absorption edge is essentially dominated by the interference of the emitted photoelectrons, the area around ± 100 eV around the absorption edge can in part be very strongly determined by intra-atomic absorption bands. (Excitation of electrons close to the nucleus into unoccupied states which are energetically just below the photoionization energy.) The part of the multiple scattering of the photoelectrons in the XANES range described above can therefore take a back seat. In particular, the so-called “pre-peak” is often assigned to a quadrupole transition; the direct emission of a photoelectron is not yet possible in this energy range in the traditional sense.
Experimental technique
To measure a NEXAFS spectrum, the X-ray energy must be varied, for this purpose monochromatic X-ray radiation is filtered out from synchrotron radiation (large wavelength range) . The wavelength is varied with the monochromator in a small range (a few eV ) around the absorption edge.
The absorption is usually not determined directly via the attenuation of the X-ray beam, but rather the "filling in" of the orbital that has become free is observed. The Auger electrons are released, these and / or the secondary electrons generated by them can be detected with a secondary electron multiplier , for example a Channeltron , and are a measure of the X-ray absorption. This detection method is known in English as Auger electron yield or total electron yield . Since the electrons can only travel short distances in the solid (Auger electrons approx. 1 nm until they lose energy; low-energy electrons a few nanometers), this method is only sensitive to the area near the surface.
Another method of determining the absorption is to measure the X-ray radiation that can arise when the orbital orbital that has become free is filled. It is thus measured X-ray fluorescence ( fluorescence yield , engl. Fluorescence yield ). Since the light elements, which are mostly examined with NEXAFS, only emit X-ray quanta with a very low probability (a few per thousand) when filling the orbital near the core and an Auger process usually takes place instead, the X-ray yield is lower than the electron yield, but the soft X-ray radiation can penetrate thicker layers of material than Electrons. With the detection of the X-ray radiation, deeper layers can be recorded.
Applications
- NEXAFS is often used to investigate reactions of molecules on surfaces, for example for basic investigations of catalyst technology . The frequency of different types of molecules can be tracked using the NEXAFS spectrum. The molecules must have different vacant states in order to distinguish them.
- Since synchrotron radiation is normally linearly polarized, in certain cases the orientation of the molecular orbitals recorded with NEXAFS can be determined by varying the angle between the incident synchrotron radiation and the sample. This makes it possible, for example, to determine whether a certain type of molecule is lying flat on a surface or whether the molecules are "upright" on the surface.
NEXAFS usually measures the absorption edges of carbon (X-ray energy = 285 eV), nitrogen (400 eV) or oxygen (530 eV).
Related and Complementary Techniques
- Another technique for determining the unoccupied electron states is inverse photoemission . While inverse photoemission records all unoccupied states, NEXAFS only measures the unoccupied states at the location of an atomic type (the atomic type to which the measured absorption edge belongs, i.e. only oxygen atoms, for example).
- The X-ray absorption in a larger area (a few hundred electron volts) above the absorption edge is recorded with (surface) extended x-ray absorption fine structure ( EXAFS or SEXAFS). With (S) EXAFS bond lengths and the number of neighboring atoms can be determined.
See also
literature
- Jochen Haase: SEXAFS and NEXAFS. X ‐ ray absorption spectroscopy on adsorbate ‐ covered surfaces . In: Chemistry in Our Time . tape 26 , no. 5 , 1992, pp. 219-231 , doi : 10.1002 / ciuz.19920260506 .
- Joachim Stöhr: NEXAFS Spectroscopy . Springer, 1992, ISBN 3-540-54422-4 .