Surface-extended X-ray absorption fine structure (SEXAFS) is the surface-sensitive equivalent of the EXAFS technique.
This technique involves the illumination of the sample by high-intensity X-ray beams from a synchrotron and monitoring their photoabsorption by detecting in the intensity of Auger electrons as a function of the incident photon energy.
Surface sensitivity is achieved by the interpretation of data depending on the intensity of the Auger electrons (which have an escape depth of ~1–2 nm) instead of looking at the relative absorption of the X-rays as in the parent method, EXAFS.
The photoabsorption can therefore be monitored by direct detection of these Auger electrons to the total photoelectron yield.
The absorption coefficient versus incident photon energy contains oscillations which are due to the interference of the backscattered Auger electrons with the outward propagating waves.
In SEXAFS, an electron detector and a high-vacuum chamber is required to calculate the Auger yields instead of the intensity of the transmitted X-ray waves.
The energy analyzer gives rise to better resolution while the electron multiplier has larger solid angle acceptance.
The oscillatory component of the photoabsorption originates from the coupling of this reflected wave to the initial state via the dipole operator Mfs as in (1).
The Fourier transform of the oscillations gives the information about the spacing of the neighboring atoms and their chemical environment.
Thus, with a proper choice of the absorption edge and characteristic Auger transition, measurement of the variation of the intensity in a particular Auger line as a function of incident photon energy would be a measure of the photoabsorption cross section.
The intensity ratio between the Auger electron and X-ray emissions depends on the atomic number Z.
The cross section of photoabsorption is given by Fermi's golden rule, which, in the dipole approximation, is given as where the initial state, i with energy Ei, consists of the atomic core and the Fermi sea, and the incident radiation field, the final state, ƒ with energy Eƒ (larger than the Fermi level), consists of a core hole and an excited electron.
ε is the polarization vector of the electric field, e the electron charge, and ħω the x-ray photon energy.
Assuming single-scattering and small-atom approximation for kRj >> 1, where Rj is the distance from the central excited atom to the jth shell of neighbors and k is the photoelectrons wave vector, where ħωT is the absorption edge energy and Vo is the inner potential of the solid associated with exchange and correlation, the following expression for the oscillatory component of the photoabsorption cross section (for K-shell excitation) is obtained: where the atomic scattering factor in a partial wave expansion with partial wave phase-shifts δl is given by Pl(x) is the lth Legendre polynomial, γ is an attenuation coefficient, exp(−2σi2k2) is a Debye–Waller factor and weight Wj is given in terms of the number of atoms in the jth shell and their distance as The above equation for the χ(k) forms the basis of a direct, Fourier transform, method of analysis which has been successfully applied to the analysis of the EXAFS data.
The number of electrons arriving at the detector with an energy of the characteristic WαXY Auger line (where Wα is the absorption edge core-level of element α, to which the incident x-ray line has been tuned) can be written as where NB(ħω) is the background signal and
is the probability that an excited atom will decay via WαXY Auger transition, ρα(z) is the atomic concentration of the element α at depth z, λ(WαXY) is the mean free path for an WαXY Auger electron, θ is the angle that the escaping Auger electron makes with the surface normal and κ is the photon emission probability which is dictated the atomic number.