In X-ray Absorption spectroscopy, a core electron is excited into unocupied atomic/molecular orbitals above the Fermi level. XAS is divided into two regimes: Near Edge X-ray Absorption Fine Structure (NEXAFS) for bound states and low energy resonances in the con tinuum, and Extended X-ray Absorption Fine Structure (EXAFS) when the outgoing electron is well above the ionization continuum. (An excellent reference for NEXAFS by Jo Stohr can be found at www-ssrl.slac.stanford.edu/stohr/ ). These transitions are caused by the absorption of an x-ray photon with energy tuned by means of synchrotron radiation to the ionization energy of the electron and lead to a pronoun ced fine structure in the XAS spectrum.The XA spectra records the absorption intensity as a function of the incoming photon energy.

The figure at the center above is a enlarged view of the p-bonding network obtained with XAS. The total intensity of the spectrum is given by the number of unoccupied states in the inital state, while the spectral shape reflects the density of states for the core hole state. In this way XAS provides element-specific information about the density of states, local atomic structure, lattice parameters, molecular orientation, the nature, orientation, and length of chemical bonds as well as the chemical state of the sample.

Due the localization of the core hole created at a certain atom, the unoccupied states are projected on this atom. In the soft x-ray regime (K-edges of C, N, O), NEXAFS transitions are governed by dipole selection rules and consequently the absorption cross-sections show a polarization dependent angular anisotropy. By means of polarization dependent NEXAFS measurements it is therefore possible to determine the orientation of molecular adsorbates. Molecular orientation for N2 adsorbed on Ni (100) obtained using NEXAFS is illustrated above right.

In systems with inequivalent atoms of the same element, the XA spectrum can become complicated due to overlapping spectral features. The energy separations between these features are usually small and the shape and intensity of the spectral components may vary significantly, precluding a straightforward separation. However, with the help of Auger Electron Spectroscopy (AES), an XA spectrum can be decompos ed into its individual components.

X-ray emission spectroscopy (XES) is a classical technique to study the electronic structure of bulk samples, for which it is ideally suited due to the large information depth (roughly 0.1 m for soft X-rays). In XES the core hole created by an XA process is filled by the decay of a valence electron into the lower lying core ho le and energy is conserved by the emission of an x-ray photon of matching energy. This technique can be made surface sensitive through a selective, resonant excitation of a core-hole state at the atomic center of an adsorbate. Due to the localization of the core-hole, XES provides a detailed element-specific picture of the local electronic structure around a given atomic site with no contribution from the much larger number of atoms in the substrate. The emitted radiation is dominated by the decay of valence electrons from the same atomi c center. XES therefore probes the occupied valence states in an atom-specific projection.

In the case of highly oriented systems, e.g. the case of N2 standing up on Ni(100) shown above, angular- dependent XES enables the separation of states of different symmetry of the involved orbitals. An important consequence is that one can study states of symmetry which result solely from the chemical bonding. The maximum x-ray emission is generally found in the direction perpendicular to the spatial orientation of the involved atomic p-orbitals. By switching the direction of detection from normal to grazing emission, orbitals of different spatial orientation are probed. In normal emission geometry, only valence states of p-symmetry contribute to the x-ray emission signal, whereas in grazing emission geometry both p- and s-orbitals are probed. A simple subtraction procedure reveals s states only.

In general, since the initial ionization is non-selective and the init ial hole may therefore be in various shells, there will be many possible Auger transitions for a given element - some weak and some strong in intensity. Auger Spectroscopy is based upon the measurement of the kinetic energies of the emitted electrons which are independent of the mechanism of initial core hole formation. Each element in a sample will give rise to a characteristic spectrum of peaks at various kinetic energies. An example can be found above in the case of N2 adsorbed on Ni (100), where we see separation of the N 1s XAS 2p resonance for N2/Ni(100) into absorpti on peaks from the inner and outer atoms, respectively.

In this way, AES allows not only for a quantitative compositional analysis of the surface of interest, but provides a tool to separate the two XA features for the two inequivalent N2 atoms in more detail. Since the shapes and intensities of these subspectra are not a priori known this information can not be obtained from a direc t analysis of the absorption spectra.