Structure determination of ammonia on Cu(111)

The local structure of ammonia adsorbed on Cu(lll) has been determined usin g N Is scanned-energy-mode photoelectron diffraction. The molecule is found to occupy an atop bonding site with a Cu-N bond length of 2.09 +/- 0.03 An gstrom, but with large amplitude vibrations of the molecule parallel to the surface, consistent with a systematic trend found in our studies of specie s adsorbed in atop sites. We find no evidence for a significant static offs et of the molecule from the atop site (which was found on the lower-symmetr y Cu(110) surface), but a small offset of this kind cannot be distinguished from the effects of the large dynamic displacement. As a first test for th is photoelectron diffraction technique we have also investigated the effect of including the weakly scattering H atoms in the analysis. This does lead to preferred sites for the H atoms consistent with expectations, but their precision of location is too poor to provide information of chemical signi ficance.