CHEMISORPTION OF CO ON NBC(111) AT 80-K - ANGLE-RESOLVED PHOTOEMISSION-STUDY

Angle-resolved photoemission spectroscopy with synchrotron radiation has been used to study the adsorption of CO on a NbC(111) surface at 80 K. CO is adsorbed molecularly on the NbC(111) surface. Binding energies of the CO valence levels are in good agreement with previously published results for metal-CO systems. The adsorbed CO forms a (1 X 1) overlayer. The two-dimensional band dispersions of CO valence levels in the (1 X 1) phase are measured in the GAMMA-MBARBAR direction. The considerable dispersions (approximately 0.35 eV for both 4-sigma and 5-sigma + 1-pi levels) are observed as in the case for the periodic CO layer on the metal surfaces. A shape resonance of the 4-sigma level of the adsorbed CO is observed at approximately 32 eV, which is nearly the same energy as that for the gas phase CO. The 4-sigma cross section at the resonance energy is measured as a function of incidence angles and the result is consistent with a molecular orientation normal to the surface.