Normal Incidence X-ray standing wavefield (NIXSW)

Normal Incidence X-ray Standing Wavefield (NIXSW) is a surface measurement technique to locate adsorption sites with the added advantage that it is technically also feasible to determine the site distribution for simple surfaces.

In this technique, a Bragg-reflection is established from the bulk substrate planes and when the incidence and reflected X-ray wavefields interfere, a standing wave perpendicular to the scattering planes is produced. The absorption at the adsorbate atom site is measured as a function of the photon energy about the Bragg reflection energy, allowing the determination of the adsorbate-substrate layer spacing. By repeating the measurement from different planes, the adsorbate site is determined by triangulation. The measurements also yield another structural parameter, the coherent fraction, which for simple single site structures is a measure of local disorder in the structure. In more complicated cases where an adsorbate occupies more than one type of symmetrical site on the surface, the distribution of the occupied sites can be determined.

The technique is suitable for a range of metal, oxide and semiconductor single crystal surfaces and Bragg energies for many of the low index surfaces of these materials is covered in the photon energy range of 1800 – 4000 eV. Adsorbate-substrate layer spacings can generally be determined to an accuracy of 0.02 Å (or 0.002 nm). A number of structures have been studied on SRS Beamline 4.2, including endofullerenes on Silicon (111) and Ag (111) substrates, halogens on Ag and Cu (111), and metals (such as Sn and Bi) on Cu (100) to name but a few.

station 4.2