Core Level Spectroscopy
Creation and Decay of Core Holes
Spectroscopic Techniques

X-ray Photoelectron (XPS) X-ray Absorption (XAS) X-ray Emission (XES) Auger Electron (AES)

Core holes are created by the ionization of a core electron in XPS and by excitation in XAS .

The XPS and XAS final states are highly unstable
and the core hole decays by non-radiant Auger relaxation
(AES) or by radiant x-ray emission
processes (

XPS and AES probe the unoccupied electronic stru cture, while XAS projects the unoccupied
valence states of the system onto a particular

A brief description of the each of the different spectroscopies illustrated by schematic pictures
of the creation and decay
with data measured for
N2 adsorbed on Ni(100) can be found by scrolling
dow n or following the links above.

For a more detailed description, please refer to "Applications of core level spectroscopy to adsorbates,"
Anders Nilsson
. Journal of Electron Spectroscopy and Related Phenomena 126 (2002) 3-42.


X-ray Photoelectron Spectroscopy
< /font> (XPS)

XPS is based on the creation of a core hole via ionization and provides a method to study the geometric, electronic and chemical properties of a sample.

In XPS, photons with sufficient energy hn are absorbed by a system causing core electrons to be ejected from the samp le. If the energy of the photons,
hn, is larger than the binding energy of the electron (Eb), the excess energy is converted to kinetic energy of the emitted photoelectron (Ek).
Knowledge of the incoming photon (hn) energy and the work function of the spectrometer (f) and measurement of the kinetic energy via an electron analyzer makes it possible to calculate the binding energy according to: Eb = hn + Ek + f.

Since binding energies of core electrons are characteristic for elements in a certain chemical environment, XPS allows for a determination of the atomic compositions of a sample or the chemical state of a certain element, as well as electronic structure and band structure. In many cases chemical shifts can be used to draw direct conclusions on the local coordination in a system and the electronic change upon adsorption.

This information can be used to distinguish different adsorption sites of molecules adsorbed on a surface as shown above right in the XPS spectrum for N2 perpendicularly adsorbed on a Ni(100) surface. Here two well-separated N 1s peaks are observed with a chemical shift of 1.3 eV. The peak with the lowest binding energy, 399.4 eV, corresponds to ionization o f the outer N atom, whereas the high binding energy peak at 400.7 eV is due to ionization of the inner N atom. No such clear splitting is observed in the XAS below.

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X-ray Absorption Spectroscopy (XAS

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 ). 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.

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X-ray Emission Spectroscopy (XES)

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.

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Auger Electron Spectroscopy (AES)

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After a core electron is ejected, the ionized atom is in a high excited state. In the Auger process, one electron f alls from a higher level to fill an initial core hole in the K-shell and the energy liberated in this process is simultaneously transferred to a second electron. A fraction of this energy is required to overcome the binding energy of this second electron and the remainder is retained by this emitted Auger electron as kinetic energy. Unlike the one hole final state of XES, Auger decay leads to a two-hole state under emission of an Auger electron.

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.

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Core Spectroscopies Introduction
Creation and Decay of Core Holes