Vibrational excitations and structure of CO adsorbed on Ni(100)

Abstract

Vibrational excitations and structure of CO chemisorbed on the clean Ni(100) surface have been investigated by high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). The Ni(100) c(2 × 2) CO structure produces two vibrational losses at 59.5 and 256.5 meV interpreted as due to excitations of the Ni-C and C-O stretching vibrations of CO linearly bonded to the Ni atoms. At higher coverages a quasihexagonal CO structure develops which produces vibrational losses that are substantially shifted and spread in energy because of the incoherency of this CO structure with respect to the Ni(100) net. At low CO coverage, where no ordered structure forms at the temperatures investigated (173, 293 K), the vibrational loss spectrum reveals a temperature dependent mixing of two bond configurations, one of which is the linear. The other, which is dominating at 173 K produces vibrational losses at 44.5, 81.5 and 239.5 meV which are suggested to be due to excitations of the symmetric and asymmetric Ni-C and the C-O stretching vibrations of CO bridge bonded to two nearest neighbour Ni atoms.