MEASUREMENT OF LOW-ENERGY FRUSTRATED VIBRATIONAL-MODES OF CO ON NI(111) VIA INELASTIC ELECTRON-SCATTERING

High-resolution electron energy loss spectroscopy (HREELS) has been used to study the low-energy vibrational modes of bridge-bonded CO on Ni(111) after a well-ordered c(4 x 2) overlayer structure was formed. In this paper we report the spectroscopic observation of two low-energy modes of bridge-bonded CO on Ni(111) which have not been previously reported, as well as the momentum-resolved scattering for one of these modes. At T(s) = 120 K, under impact scattering conditions, a frustrated translation of CO parallel to the surface was observed at an energy loss of 11.8 meV. The dispersion curve measured along the <112BAR> direction of Ni(111) for this mode was dispersionless, indicating that there is no direct lateral interaction between the adjacent CO molecules in this structure. At T(s) = 170 K, using a transient negative-ion SIGMA-shape-resonance to enhance our sensitivity, a frustrated rotation for bridge-bonded CO was also observed at an energy loss of 37.5 meV. The energy of this mode is near the value previously estimated from temperature dependent infrared line shape measurements of the CO stretch peak position and lineshape, and is believed to be responsible for vibrational phase relaxation in the c(4 x 2)-CO-Ni(111) system. The energies of the observed modes are also consistent with those derived from a simple cluster calculation.