X-ray Standing Waves

The standing wave field is produced by Bragg reflection of monochromatic X-rays from a single crystal substrate. When scanning through a Bragg condition the phase of the X-ray interference field changes by π and, consequently, the nodal planes shift by half a lattice constant. (Move mouse pointer over the picture on the left.)

Due to photoexcitation processes within the adlayer electrons and fluorescence light are emitted. The variation of the photoelectron (fluorescence) yield when scanning through the Bragg condition - by changing the photon energy or incidence angle - depends on the adsorbate position, see this simulation.

• The monolayer films are prepared in situ by organic molecular beam deposition.
• Synchrotron light with tunable energy is used to match the Bragg condition in backreflection geometry.
• The photoelectrons created by the XSW field are measured with a hemispherical electron analyzer. For each element a series of spectra is obtained which can be used to extract the relative photoelectron yield.

In a project carried out at beamline ID32 of the ESRF we studied the adsorption geometry and orientational ordering of chlorogallium phthalocyanine molecules (GaClPc) on Cu(111) by using the x-ray standing wave technique, photoelectron spectroscopy, and quantum mechanical calculations. We find that for submonolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a Cl-down configuration by forming a covalent bond to the substrate. For bilayer coverages the x-ray standing wave data indicate a coexistence of the Cl-down and Cl-up configurations on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.