Near Edge X-Ray Absorption Fine S tructure, NEXAFS, spectroscopy refers to the absorption fine structure close to an absorption edge, about the first 30eV above the actual edge. This region usually shows the largest variations in the x-ray absorption coefficient and is often dominated by intense, narrow resonances. NEXAFS is also called X-Ray Absorption Near Edge Structure, XANES. Today, the term NEXAFS is typically used for soft x-ray absorption spectra and XANES for hard x-ray spectra. A brief overview of the technique is prov ided below.
Opposite to the related x-ray photoemission spectroscopy (XPS or ESCA) technique, where the photon energy is fixed and the electron intensity is measured as a function of electron kinetic energy, in NEXAFS the x-ray energy is scanned and the absorbed x-ray intensity is measured. As illustrated in the Fig. 1 below, NEXAFS spectra can be recorded in different ways. The most common methods are transmission and electron yield measurements. Note that the absorption coeff icient µ is obtained either as the logarithm or the direct ratio of the detected intensities It and Ie and incident intensity Io, as indicated in the Figure.
Fig. 1. Two methods of recording x-ray absorption spectra
The transmission technique requires thin foils while the electron yield technique, often called total electron yield (T EY) detection, can be used for conventional samples. The absorbed x-ray intensity is not measured directly in TEY measurements, but rather the photoelectrons that are created by the absorbed x-rays. X-rays are absorbed through excitations of core electrons to empty states above the vacuum or Fermi level. The created holes are then filled by Auger decay (dominant in the soft x-ray region over x-ray fluorescence). The intensity of the emitted primary Auger electrons is a direct measure of the x- ray absorption process and is used in so called Auger electron yield (AEY) measurements, which are highly surface sensitive, similar to XPS. As they leave the sample, the primary Auger electrons create scattered secondary electrons (see Fig. 1) which dominate the total electron yield (TEY) intensity. The TEY cascade involves several scattering events and originates from an average depth, the electron sampling depth L. Electrons created deeper in the sample lose too much energy to overcome the wo rkfunction of the sample and therefore do not contribute to the TEY. The sampling depth L in TEY measurements is typically a few nanometers, while it is often less than 1 nm for AEY measurements.
NEXAFS is element specific because the x-ray absorption edges of different elements have different energies. This is illustrated in Fig. 2 where XPS and NEXAFS spectra of the same polymer are compared. The XPS spectrum was recorded at a photon energy of 750eV. Both spectra exhibit p ronounced peaks corresponding to C, N, O, and F atoms in the sample. For example, the C K-edge threshold peak in the NEXAFS spectrum lies at a photon energy of about 285eV. This corresponds to a peak in the XPS spectrum at a kinetic energy of 750 eV - 285 eV = 465 eV.
Fig. 2. Comparison of XPS and NEXAFS spectra for a polyimide polymer, whose structural formula is shown in th e white inset.
NEXAFS is also very sensitive to the bonding environment of the absorbing atom. This is already visible in Fig. 2. The NEXAFS spectrum exhibits considerable fine structure above each elemental absorption edge. This fine structure arises from excitations into unoccupied molecular orbitals. In a related picture one can also think of the resonances arising from scattering of the excited low-energy photoelectron by the molecular potential. This structure is considerably larger than the higher energy extended x-ray absorption fine structure (EXAFS), which is extremely weak in polymers. EXAFS is due mostly to single scattering events of a high-energy photoelectron off the atomic cores of the neighbors while NEXAFS is dominated by multiple scattering of a low-energy photoelectron in the valence potential set up by the nearest neighbors. Often one can use a spectral "fingerprint" technique to identify the local bonding environment. Examples are show n in Fig. 3 for carbon atoms with different bonding configurations, often found in polymers. The spectra exhibit chemical shifts within each group similar to XPS spectra but more importantly considerably different fine structure for carbon in different molecular groups. This clearly illustrates the power of NEXAFS to distinguish chemical bonds and local bonding. In many ways it is superior to XPS, which does not provide local structural information.
Fig. 3. Carbon K-edge NEXAFS spectra of different polymers, revealing the sensitivity to molecular functional groups.
Another great asset of NEXAFS spectroscopy is its polarization dependence. Linearly polarized x-rays are best suited for covalent systems like low-Z molecules, macromolecules and polymers, which possess directional bonds. In this case the directional electric field vector of the x-rays can be viewed as a "search light" that can look for the direction of chemical bonds of the atom selected by its absorption edge. An example is shown in Fig. 4 for the benzene molecule. Benzene, C6H6, has unoccupied orbitals of σ and π symmetry which are oriented in and perpendicular to the ring plane, respectively, as shown on the left side. Polarization dependent NEXAFS spectra for benzene chemisorbed on Ag(110) are shown on t he right. When the electric field vector E is aligned along the surface normal, peaks due to the out-of-plane π orbitals are seen and when E is parallel to the surface resonances due to the in-plane σ orbitals are dominant. This shows that benzene lies down on the Ag surface. In fact, benzene is only relatively weakly chemisorbed on Ag. For stronger chemisorption bonds (e.g. C6H6 on Mo or Pt) the π resonances broaden significantly, because the π orbita ls are involved in the bond to the surface.
Fig. 4. Polarization dependent NEXAFS spectra of benzene chemisorbed on Ag(110), illustrating the capability to determine molecular orientations.
Modern NEXAFS spectroscopy uses linear as well as circularly polarized light to probe the anisotropy of charge and spin. In the following we will discuss in more detail the power of x-ray polarization in conjunction with NEXAFS. In particular, we will discuss different polarization effects called X-ray Linear Dichroism (XLD), X-ray Magnetic Linear Dichroism (XMLD) and X-ray Magnetic Circular Dichroism (XMCD).
The remarkable capabilities of NEXAFS for the study of charge and spin phenomena in materials derive from the direct coupling of the x-ray electric field vector to the charge and the coupling of the x-ray angular momentum to the angular (orbital) momentum of the charge. The absorption processes are governed by electric dipole transitions that obey strict selection rules on angular momentum conservation and couple different core shells to specific valence shells. In x-ray absorption, sensitivity to the spin arises indirectly from the coupling of the orbital moment to the spin moment in the valence shell through the spin-orbit coupling.
Determination of charge orde r - XLD: THE XLD effect is that discussed in conjunction with Fig. 4. In a more general formulation, the x-ray absorption intensity of linear polarized x-rays directly probes the quadrupole moment of the local charge around the absorbing atom through a search-light-like effect. A quadrupole moment exists in all cases where the local charge has lower than cubic symmetry. The absorption intensity is at a maximum if the x-ray electric field vector is aligned along the direction of maximum charg e (hole) density in the atomic volume surrounding the absorbing atom, e.g. along an empty molecular orbital. The XLD effect provides the basis for the determination of the orientation of chemisorbed molecules on surfaces and the orientational order in polymers or liquid crystals. Here one determines the average orientation of a selected molecular orbital (e.g. a π orbital). The measured angular dependence determines the quadrupole moment or order parameter of the charge distribution of th e selected orbital (J. Electr. Spectr. Rel. Phenom. 98-99, 189 (1999), (J.Magn. Magn. Mater. 200, 470 (1999)).
Determination of spin order through charge order - XMLD: In the
absence of spin order, linear dichroi
sm NEXAFS spectroscopy can only determine
charge order in systems where the absorbing atom has lower than cubic symmetry.
However, in the presence of spin order the spin-orbit coupling leads to
preferential charge order relative to the spin direction even in cubic systems.
This effect is the basis for the determination of the spin axis in ferromagnetic
and especially antiferromagnetic systems by means of x-ray magnetic linear
spectroscopy. Since the electric field vector oscillates in time along an axis
and the radiation may be absorbed at any time, linearly polarized x-rays are
only sensitive to axial not directional properties. Hence one can determine the
orientation of the antiferromagnetic or ferromagnetic axis, but the spin
direction itself cannot be determined. The fact that the XMLD intensity depends
on the relative orientation of the electric field vector and the magnetic axis
can be used for
Determinatio n of spin and angular momentum order - XMCD: Right and left handed circularly polarized x-rays possess opposite angular momenta which are transferred in the x-ray absorption process to the photoelectron excited from a core shell. The photoelectron then possesses a well defined angular momentum and in a one-electron picture one may view the empty valence shell as a detector of this momentum. The x-ray magnetic circular dich roism (XMCD) absorption intensity, defined as the intensity difference measured with left and right circularly polarized light, is linked through sum rules to the size of the orbital and spin momenta of the empty valence states. Angle dependent measurements in magnetic fields can determine the anisotropies of the orbital moment and of the spin density in the unit cell (also called the intra-atomic magnetic dipole moment). Since han ded circularly polarized x-rays have directionality they can also detect the direction of the spin and orbital moments, a fact utilized in XMCD microscopy of ferromagnets.
The basic polarization effects in x-ray absorption spectroscopy are summarized in Fig. 5 below. Discussion of the fourth effect, X-ray Natural Linear Dichroism, is beyond the scope of this brief overview.
Fig. 5. Overview of polarization dependent effects in NEXAFS. The references to the original observation of the effects are given. The shown data do not in all cases correspond to these references but are chosen to illustrate the size of the various effects.
For a more detailed discussion the reader is referred to: