In X-ray Absorption spectroscopy, a core electron is excite d into an empty valence state above the Fermi level by the absorption of a x-ray photon with energy tuned to the ionization energy of the electron. This event occurs when the excitation energy of the photoelectron is not high enough to reach the ionization continuum. The XAS spectrum records the absorption intensity as a function of the incoming photon energy.

XAS is divided into two regimes; Near Edge X-ray Absorption Fine Structure (NEXAFS) for bound states and low energy resonances in the continuum, and Extended X-ray Absorption Fine Structure (EXAFS) when the outgoing electron is well above the ionization continuum. In the NEXAFS regime the spectroscopy provides information about the empty electronic states above the Fermi level. The dipole selection rules (Dl = +/- 1, Ds = 0, Dj = 0, +/- 1) which dictate the absorption process and the p olarization of the synchrotron radiation allow us to probe the population and the symmetry character of different orbitals and obtain information about the orientation of the molecules adsorbed on the surface. The latter case is falls within NEXAFS and is illustrated in the figure above center.

In a one-electron picture th e XA spectrum reflects the unoccupied density of states where the position of the Fermi level is porvided by the XPS core level binding energy (see XPS above). In this way it provides element-specific information about the density of states, local atomic structur e, lattice parameters, molecular orientation, the nature, orientation, and length of chemical bonds as well as the chemical state of the sample. Molecular orientation is manifest in the XAS spectra as shown for N2 adsorbed on Ni(100) shown at left. The total intensity of the XA spectrum is given by the number of unoccupied states in the inital state, while the XA spectral shape reflects the density of states for the core hole state. An example of this can be found above right, which is a enlarged view of the pi-bonding network ob tained using XAS.