AVS 45th International Symposium
    Surface Science Division Monday Sessions
       Session SS2-MoM

Paper SS2-MoM2
An Atom Specific Probe of the Surface Chemical Bond Using X-ray Emission Spectroscopy

Monday, November 2, 1998, 8:40 am, Room 309

Session: Molecular Adsorbates on Metals
Presenter: J. Hasselstrom, Uppsala University, Sweden
Authors: J. Hasselstrom, Uppsala University, Sweden
A. Fohlisch, Uppsala University, Sweden
O. Karis, Uppsala University, Sweden
A. Nilsson, Uppsala University, Sweden
M. Nyberg, Stockholm University, Sweden
L.G.M. Pettersson, Stockholm University, Sweden
J. Stohr, IBM Almaden Research Division
Correspondent: Click to Email
When a molecule is adsorbed on a metal surface by chemical bonding new electronic states are formed. The direct observation and identification of these states is still an experimental challenge. In this contribution we will show how X-ray Emission Spectroscopy (XES), in spite of its inherent bulk sensitivity, can be used to investigate adsorbed molecules. We have applied XES to CO and NH@sub 3@ adsorbed on single crystal Cu surfaces. These molecules have lone pair orbitals in its @sigma@-systems and adsorb with these towards the substrate. Due to the localization of the core excited intermediate state, XE spectroscopy allows an atom specific probe of the valence electrons, i.e. the molecular contributions can be separated from those of the substrate. In combination with ab initio calculation, new details of the chemical bond formed at the surface can be revealed. It is, for CO adsorbed on Cu(100), found that the surface chemical bond formation can be related to the @pi@-system interacting with the metal d band. The overall interaction of the @sigma@-system, i.e. the 5@sigma@ lone pair, is found to be repulsive. In contrast, the @sigma@-system, involving the 3a@sub 1@ lone pair, of ammonia is found to comprise the main covalent contribution to the adsorption energy of the NH@sub 3@/Cu(110) system; the degenerated 1e orbitals, often referred to as the N-H @pi@-system preserve much of its molecular character. Our results indicate that the different bonding mechanisms can be related to the energy position of the lone pair valence orbitals as compared to the metal bands, and furthermore to the corresponding symmetries of the lowest unoccupied molecular orbitals.