Tricyclohexylphosphine Adsorbed on Rhodium

Tricyclohexylphosphine (TCHP) adsorbates on rhodium, prepared both from solution and by sublimation in UHV are studied by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRAS), and temperature-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 interactions causing chemical shifts in the core level spectra of the adsorbates on rhodium are investigated using multilayer films and adsorbates on gold as references. In the solution deposition procedure, freshly evaporated rhodium samples are shortly exposed to air, resulting in oxygen covered surfaces. The P(2p) core level XPS spectrum shows only one type of phosphorus with a P(2p3/2) 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 prepared a TCHP adsorbate by an in situ sublimation process in UHV. One dominating 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 due to a direct coordination of the molecule through the phosphorus lone pair orbital to the rhodium surface. The soft donor/acceptor and oxygen bridging to rhodium lead to a large electronic reorganization of the phosphorus atom, which manifests itself as substantial infrared peak shifts of the cyclohexyl modes, especially the CH2 stretches. Combined IRAS and TPD measurements show molecular desorption at 465 K for the UHV prepared monolayer, which is consistent with strong chemisorption to the rhodium surface.