Adsorption sites and long range order — vibrational spectra for CO on Ni(111)

Vibrational excitations of CO chemisorbed on Ni(111) have been studied using high resolution electron energy loss spectroscopy. The measurements have been performed at 140 K as function of CO exposure and revealed characteristic loss peaks at 400 cm −1, 1810 cm −1 (shifting to 1910 cm −1 with increasing coverage), and at 2050 cm −1. The losses are assigned to the C Ni vibration and to the C O stretching modes for bridge and on-top adsorption sites, respectively. The normalized loss intensities for the C O stretching vibrations show a peculiar dependence on coverage and can be correlated with the occurrence of the c(4 × 2) and (√ × √7)R19.2° LEED structures. At low coverage and for the well ordered c(4 × 2) structure all CO molecules occupy bridge bonded adsorption sites. From the interpretation of the loss intensity dependence on exposure it is concluded that the dynamical dipole moment for bridge bonded CO is by a factor of three larger than for CO in on-top positions.