Surface vibrations and the nature of the adsorption state: C2H2 on Pt(111)

Molecular vibrations of C₂H₂ and C₂D₂ adsorbed on Pt(111) at 140 K and ∼300K have been measured by high resolution electron energy loss spectroscopy. The comparison of C₂H₂ and C₂D₂ spectra allows an unambiguous assignment of the observed losses to the excitation of C−H bending, C−H stretching, and C−C stretching modes of nondissociatively adsorbed acetylene. From the relative intensities of losses the hybridisation state is determined to be nearsp ². The C−C stretching frequency indicates a C−C bond order of ∼1.8.     

Spin-polarized photoemission study on the temperature dependence of the exchange splitting of Ni

Using spin-polarized photoemission with high energy- and angle resolution (E=100 meV, =±3°) we have investigated the temperature dependence of the exchange splitting of Ni in the temperature range 0.5T/T _c0.94. At room temperature we find _ex=0.18 eV for the exchange splitting of theS ₄ band at theX point of the Brillouin zone. With increasing temperature the total (spin-averaged) energy distribution shows a narrowing and merges into one peak. The spin-resolved energy distribution curves approach each other and are strongly broadened. A discussion of the data within current theories of itinerant electron magnetism is given. The spectra indicate that neither the pure Stoner model nor the prediction of local band theory, assuming a temperature independent exchange splitting are justified for Ni. We conclude that the exchange splitting decreases with increasing temperature and that transverse as well as longitudinal spin fluctuations are responsible for the broadening of the spin-resoived energy distribution curves.Experimental part of the work performed at Institut für Festkörperforschung of the KFA Jülich and supported by the Deutsche Forschungsgemeinschaft through SFB 125