X-ray standing waves as probes of surface structure: Incident beam energy effects

Abstract

The sensitivity to x-ray beam energy of structure measurements using x-ray standing waves (XSW) generated under conditions of total external reflection has been determined. To this end, the optical properties of the system were examined in a theoretical analysis to identify possible energy-dependent components such as surface roughness. The analysis shows that, provided surface roughness is small (Debye–Waller factor less than 10 Å) and the energy range covered in the XSW measurements lies within several keV, its contribution can be accounted for satisfactorily by a simple Debye–Waller factor. In addition, a series of XSW measurements were made on Langmuir–Blodgett films of manganese arachidate (C20:0) on a gold mirror surface at three incident x-ray beam energies in the 7–11.2 keV range. The XSW data were analyzed to account for the Debye–Waller factor. No obvious dependence on incident x-ray energy was found. These results demonstrate that the contribution of surface roughness to the x-ray fluorescence yield profile is minimal under these conditions. Thus, mirrors of the type and quality used in these experiments are useful in XSW measurements where multiple element types are incorporated as structural labels in organic thin films and at surfaces. We also demonstrate that the resolving power of the XSW method is sufficient to distinguish and to locate two separate probe atom layers in a single Langmuir–Blodgett film.