Near Edge X-ray Absorption Fine Structure

NEXAFS (Near Edge X-ray Absorption Fine Structure) is an element-specific electron spectroscopic technique which is highly sensitive to bond angles, bond lengths and the presence of adsorbates. It is widely used in surface science and has also been used to study polymers and magnetic materials. NEXAFS is synonymous with XANES (X-ray Absorption Near Edge Structure) but NEXAFS by convention is usually reserved for soft X-ray spectroscopy (photon energy less than 1000 electron volts). NEXAFS is distinguished from the closely related EXAFS method in that NEXAFS concentrates on fine structure within about 30 eV of the absorption edge while EXAFS considers the extended spectrum out to much higher electron kinetic energies.

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Theory

The fundamental phenomenon underlying NEXAFS is the absorption of an x-ray photon by a core level of an atom in a solid and the consequent emission of a photoelectron (see the first Figure). The resulting core hole is filled either via an Auger process or by capture of an electron from another shell followed by emission of a fluorescent photon. The difference between NEXAFS and traditional photoemission experiments is that in photoemission, the initial photoelectron itself is measured, while in NEXAFS the fluorescent photon or Auger electron or an inelastically scattered photoelectron may also be measured. The distinction sounds trivial but is actually significant: in photoemission the final state of the emitted electron captured in the detector must be an extended, free-electron state. By contrast in NEXAFS the final state of the photoelectron may be a bound state such as an exciton since the photoelectron itself need not be detected. The effect of measuring fluorescent photons, Auger electrons, and directly emitted electrons is to sum over all possible final states of the photoelectrons, meaning that what NEXAFS measures is the total joint density of states of the initial core level with all final states, consistent with conservation rules. The distinction is critical because in spectroscopy final states are more susceptible to many-body effects than initial states, meaning that NEXAFS spectra are more easily calculable than photoemission spectra. Due to the summation over final states, various sum rules are helpful in the interpretation of NEXAFS spectra. When the x-ray photon energy resonantly connects a core level with a narrow final state in a solid, such as an exciton, readily identifiable characteristic peaks will appear in the spectrum. These narrow characteristic spectral peaks give the NEXAFS technique a lot of its analytical power as illustrated by the B 1s π* exciton shown in the second Figure.

Synchrotron radiation has a natural polarization that can be utilized to great advantage in NEXAFS studies. The commonly studied molecular adsorbates have sigma and pi bonds that may have a particular orientation on a surface. The angle dependence of the x-ray absorption tracks the orientation of resonant bonds due to dipole selection rules.

Experimental considerations

NEXAFS spectra are usually measured either through the fluorescent yield, in which emitted photons are monitored, or total electron yield, in which the sample is connected to ground through an ammeter and the neutralization current is monitored. Because NEXAFS measurements require an intense tunable source of soft x-rays, they are performed at synchrotrons like the CLS, ALS, NSLS, ELETTRA, BESSY, ANKA, the Daresbury SRS, SSRL, ESRF, Spring 8 or SOLEIL. Because NEXAFS soft x-rays are absorbed by air, the synchrotron radiation travels from the ring in an evacuated beam-line to the end-station where the specimen to be studied is mounted. Specialized beam-lines intended for NEXAFS studies often have additional capabilities such as heating a sample or exposing it to a dose of reactive gas.

Significance

The great power of NEXAFS derives from its elemental specificity. Because the various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. Buried layers are very important in engineering applications, such as magnetic recording media buried beneath a surface lubricant or dopants below an electrode in an integrated circuit. Because NEXAFS can also determine the chemical state of elements which are present in bulk in minute quantities, it has found widespread use in environmental chemistry and geochemistry. The ability of NEXAFS to study buried atoms is due to its integration over all final states including inelastically scattered electrons, as opposed to photoemission and Auger spectroscopy, which study atoms only with a layer or two of the surface.

References

NEXAFS Spectroscopy by J. Stöhr, Springer 1992, ISBN 3-540-54422-4.