X-ray absorption spectroscopy

While direct photoemission spectroscopy offers an experimental approach to to the occupied electronic bands of a solid state, XANES (x-ray absorption near-edge spectroscopy) or NEXAFS (near-edge x-ray absorption fine structure) is a technique to characterize surfaces by evaluation of unoccupied electronic states. In contrast to inverse photoemission spectroscopy the experimental setup simply requires a monochromatically tunable light source and an electron energy analyzer so that XANES measurements can be performed at each synchrotron radiation source of sufficient energy. Due to the sharp transitions in molecular systems near-edge absorption spectroscopy is one of the preferred experimental techniques to study organic thin films.

The XANES principle is based on the determination of the x-ray absorption coefficient m depending on the photon energy hn at a fixed angle of illumination q. As the optical excitation of a core level electron requires the binding energy EB as a minimum photon energy, the transgression of this energy will coincide with an increased absorption coefficient. This leads to the formation of absorption edges, which may be indexed by their atomic subshells (K,L,M...). Beyond the absorption edge the intensity of a monochromatic x-ray passing through a medium of thickness d will follow the absorption law

whereby m depends the atomic number Z of the medium and decreases with increasing photon energy hn [1]. However, the fine structure of this element-specific edge of the absorption coefficient is influenced by the energy of unoccupied electronic levels, as it is depicted in Fig. 1(a). Only a sufficient photon energy enables the photoexcitation of a core level electron beyond the vacuum level Evac. After 10×10-14 sec [2] the ionized atom may relax by occupation of the core hole with an electron from the valence band (VB), while the generated energy will normally not be used for the emission of a flourescence photon (probability 1 %), but will be absorbed for the vaccum emission of an Auger electron (probability 99%) from the valence band. In case of a non-sufficient energy for the emission of the primary electron, it may be excited into a conduction band (CB) level, so that a similar relaxation process becomes possible. This spectator process then results in the emission of only one Auger electron.

Figure 1: Following the excitation of a core level electron several ways of relaxation are possible (a). Thereby secondary electrons are generated which undergo multiple scattering processes before they leave the crystal structure as low-energy photoelelecrons (b) (picture from S. Woedtke, Ph.D. thesis).

 Alternatively the core hole may be reoccupied by the core level electron itself, so that the excitation energy is finally used for the emission of a valence electron. As the final state of this participator process is comparable to a direct photoemission process and as both mechanisms may happen concurently, the participator excitation is also called resonant photoemission.
The number of generated secondary electrons is thereby directly proportional to the x-ay absortion cross section [3]. On their way to the crystal surface these electrons undergo multiple scattering processes with other electrons (Fig. 1(b)), so that their number is multiplied while their averaged energy is reduced. Consequently, from the atomic layers near the surface up to 50 Å depth low-energy photolectrons are emitted. For the determination of the absorption coefficient m depending on the photon energy two techniques are possible. The integrated detection of all emitted electrons (total electron yield) as well as the selective detection of electrons of fixed energy (partial electron yield) as a function of hn will lead to equivalent structures in the spectra [4].

The typical XANES experiment measures the photoelectron intensity for photon energies beginning from the absorption edge til 50 eV beyond the edge energy [5]. Although photon energies below the ionization threshold allow electronic transitions into unoccupied elctronic bands or molecular orbitals, the spectral features are not directly related to the unoccupied density of states (UDOS) [6], which can be probed with inverse photoemission spectroscopy. In particular, p-conjugated molecular systems [7] exhibit a strong excitonic interaction between core electron and hole, so that the transitions show shifted energetic positions in the absorption spectra [8]. In case of an inhomogeneous intramolecular charge distribution the same electronic transition may even be found as slightly shifted double structures [9]. Nevertheless the well-structured molecular absorption spectra are a powerfull tool for the identifaction of chemical components and redox-induced changes.

References

[1]
D.C. Koningsberger, XAFS spectroscopy - Physical Principles, Data-Analysis and Applications (Lecture Notes, 2nd International School and Symposium on Synchrotron Radiation in Natural Science, Jaszowiec, 1996).
[2]
C. Wagner, W. Riggs, L. Davis, and J. Moulder, in Handbook of x-ray photoelectron spectroscopy, edited by G.E. Muilenberg (Perkin Elmer Corporation, Eden Prairie, Minnesota, 1979).
[3]
C. Wöll and M. Wühn, Spektroskopie von elektronischer Struktur und molekularer Orientierung mittels NEXAFS (Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, Bochum, 1999).
[4]
J. Stoehr, NEXAFS Spectroscopy, Vol. 25 of Springer Series in Surface Science (Springer Verlag, Berlin, 1996).
[5]
H. Rumpf, Diplomarbeit, Physikalisches Institut (Rheinische Friedrich-Wilhelms-Universität Bonn) (1998).
[6]
R. Schwedhelm, L. Kipp, A. Dallmeyer, M. Skibowski, Phys. Rev. B 58, 13176 (1998).
[7]
R. Mitsumoto et al., Physica B 208-209, 543 (1995).
[8]
M. Nyberg et al., Phys. Rev. B 60, 7956 (1999).
[9]
J. Taborski et al., Journ. of Electr. Spectr. and rel. Phenomena 75, 129 (1995).