Quantum stability of ultrathin metal overlayers on semiconductor substrates

The adsorption of one monolayer of 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) on the Ag(1 1 1)-surface was studied using the normal incidence X-ray standing waves (XSW) technique. Results regarding two key-issues are presented: Most prominent, the precise adsorbate–substrate bonding distance could be evaluated to 3.02 ± 0.02 Å (for the “relaxed monolayer”-structure). This value is significantly smaller than a van der Waals bonding distance and clearly indicates the chemisorptive bonding character. Concordant results were obtained from both O 1s photo- and O KLL Auger electron emission. This was enabled by the development of a data analysis procedure––the second issue addressed––which takes into account non-dipolar contributions to the photoemission as well as electron-stimulated Auger excitations. The latter effect adds a fraction to the total Auger yield being as high as 50% and hence may be important for any XSW study using Auger signals.