Tricyclohexylphosphine adsorbed on rhodium

Tricyclohexylphosphine (TCHP) adsorbates on rhodium, prepared both from sol ution and by sublimation in UHV are studied by X-ray photoelectron spectros copy (XPS), infrared reflection-absorption spectroscopy (IRAS), and tempera ture-programmed desorption (TPD). XPS and IRAS are used to investigate the molecular orientation of the adsorbates and the coordination to the surface . TPD is used to study the stability of the adsorbates. Molecular surface i nteractions causing chemical shifts in the core level spectra of the adsorb ates on rhodium are investigated using multilayer films and adsorbates on g old as references. In the solution deposition procedure, freshly evaporated rhodium samples are shortly exposed to air, resulting in oxygen covered su rfaces. The P(2p) core level XPS spectrum shows only one type of phosphorus with a P(2(p3/2)) binding energy about 2.7 eV higher than when adsorbed on gold. This is in agreement with molecular adsorption through oxygen to the surface. To study the interaction of TCHP with clean rhodium we also prepa red a TCHP adsorbate by an in situ sublimation process in UHV. One dominati ng state of phosphorus is observed with a P(2p3/2)binding energy about 2.6 eV lower than when adsorbed from solution, but close to what was found when adsorbed on gold. This indicates formation of a soft donor/acceptor bond d ue to a direct coordination of the molecule through the phosphorus lone pai r orbital to the rhodium surface. The soft donor/acceptor and oxygen bridgi ng to rhodium lead to a large electronic reorganization of the phosphorus a tom, which manifests itself as substantial infrared peak shifts of the cycl ohexyl modes, especially the CH2 stretches. Combined IRAS and TPD measureme nts show molecular desorption at 465 K for the UHV prepared monolayer, whic h is consistent with strong chemisorption to the rhodium surface.