Biomaterials Tutorial

NEXAFS

Heather Canavan
National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/BIO)

Near edge X-ray absorption fine structure (NEXAFS), or also called X-ray absorption near-edge structure (XANES), is capable of revealing the oxidation state and binding environment of the individual atoms within molecules.  In addition, this technique can determine the orientation of molecules absorbed onto surfaces.   NEXAFS is based on the excitation of an electron from a core orbital to an unoccupied molecular orbital by an X-ray beam.

XAS Theory

As with X-ray Photoelectron Spectroscopy (XPS), http://www.nb.engr.washington.edu/moreinfo/tutorials/techniques/esca/ X-ray absorption spectroscopy (XAS) relies on the photoelectric effect, whereby photoelectrons are created with kinetic energy determined by the energy of an incident photon and the binding energy of the bound electron.  The primary difference between the two techniques is that in XPS, the core electrons are ejected into the continuum. Whereas in XAS, they are promoted into unoccupied molecular orbitals or the continuum.

Due to the wave/particle duality of light, the ejected photoelectron can also be regarded as a spherical wave.  This wave expands concentrically, and may be scattered by encounters with neighboring atoms.  Therefore, the final photoelectron wave consists of two terms: the outgoing wave and the backscattered wave.  The interference of the two waves creates a unique wave pattern that depends on the positions and type of neighboring atoms. 

XAS Spectrum

Experimental results from XAS analysis are given in terms of a spectrum where the absolute energy of the incoming X-rays is given on the x axis, while the normalized absorption coefficient is given on the y axis.  In general, the resulting spectra are composed of three regions: 1) a pre-edge region, which usually shows a smooth exponential decay but may include some features; 2) the near edge X-ray absorption fine structure (NEXAFS) region, extending from the absorption edge to ~+50 eV due to multiple scattering events; and 3) the extended X-ray absorption fine structure (EXAFS) region, which is primarily due to single scattering. (See Figure 1)

Figure 1.  Sulfur K-edge X-ray Absorption Spectroscopy (XAS) spectrum indicating the absorption edge (E0) at 2472 eV, the near-edge NEXAFS region (up to 50 eV > E0), and the extended XAS or EXAFS region (> 50 eV above E0).

XAS Interpretation

The NEXAFS region—also called X-ray absorption near edge spectroscopy (XANES) region—may be used as a “fingerprinting” technique; by comparing the spectrum of the sample of interest to that of accurate reference molecules or materials, the oxidation state and binding environment of the absorber atom may be determined.  To obtain information on the electronic structure of the absorber atom, as well as the geometric arrangement of its neighbors, complex mathematical computer analysis programs are used to analyze the spectra. One example of such a program is the FEFF8 code http://leonardo.phys.washington.edu/feff/ developed at the University of Washington (Seattle, WA) [1].

XAS Instrumentation

NEXAFS (and all XAS detection) relies on the use of synchrotron radiation to illuminate the sample. The investment in equipment and the size of the facility generally dictates that synchrotron light sources are therefore nationally owned resources, such as the National Synchrotron Light Source (NSLS) http://www.nsls.bnl.gov/default.asp at Brookhaven National Laboratory  http://www.bnl.gov/world/.  Synchrotron radiation is produced by the acceleration of high-energy particles inside of an evacuated hollow tube.  The path of the particles is bent by pulsed magnetic fields to form a circular path.  As they are accelerated around the curved path, the electrons emit a continuous spectrum of light, called “white light.”  This white (or non-monochromatic) energy beam is then directed away from the storage ring down a tangential path called the “beamline.”  In the beamline, a series of mirrors, lenses, filters, and monochromators are used to produce a collimated beam of light at the desired wavelength.  Using a double monochromator, the wavelength may be tuned while the position of the beam remains static.

XAS Detection

XAS can be detected in transmission, fluorescence, or electron yield mode.  Each of these methods is suited to different sample types.  Transmission mode is traditionally used for the analysis of thin layers of solid samples, or solvated analyte in solution.  For thicker samples, fluorescent photons are measured from the relaxation of the excited atom after photoionization.  To analyze surface species, detection of the Auger electron is especially useful. Lytle presents an excellent review of each of these techniques, as well as the theory of XAS [2].

XAS Samples

XAS experiments do not always require the use of high vacuum, and therefore, allow the determination of the local structure and electronic properties of almost any atom, regardless of its phase.  It is therefore possible to obtain NEXAFS spectra from samples in the solid, liquid, or gas phase, as well as solids adsorbed onto surfaces.  NEXAFS can be used as either a measure of the bulk properties of a surface (using fluorescence and transmission mode) or as a surface-sensitive technique (using a glancing angle in fluorescence mode, or in electron yield mode). The information-rich quality of the NEXAFS method, combined with its ability to analyze absorber atoms in almost any phase makes this technique extremely valuable to the study of biomaterials.

References:

1. Ankudinov AL, Ravel B, Rehr JJ, Conradson SD. Real-space multiple-scattering calculation and interpretation of X-ray-Absorption Near-Edge Structure. Physical Review B 1998; 58(12): 7565-7576.

2. Lytle FW. Experimental X-ray absorption spectroscopy. 1988; Beijing. p 1-89.