The orientation and bonding of CO on Mo(100) using angle-resolved photoelectron spectroscopy and near-edge x-ray absorption fine structure

Abstract

The nature of the species formed by CO chemisorption of Mo(100) has been investigated using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and near-edge x-ray absorption spectroscopy. High-resolution electron energy loss spectroscopy (HREELS) indicates the formation of two distinct types of CO. At coverages greater than 50% of saturation, chemisorbed CO exhibits a CO stretching frequency of ∼2100 cm−1 corresponding to a CO molecule chemisorbed in an atop site. ARUPS indicates that at these coverages CO chemisorbs with its axis perpendicular to the surface in an analogous manner to that commonly observed on transition metal surfaces. At coverages less than 50% of saturation, CO exhibits an extraordinarily low stretching frequency of ∼1200 cm−1. Both NEXAFS and ARUPS measurements unequivocally indicate that at low coverages CO is tilted at ∼40° to the surface normal. Since CO is tilted with respect to the surface at low coverages, this effect cannot be ascribed to adatom–adatom interactions, and a careful measurement of the positions of the photoelectron peaks of the tilted molecule indicates that both the 1π and the 5σ orbitals participate in surface bonding. A bonding model is proposed that is in accord with these observations in which the CO molecule chemisorbs into a fourfold hollow site.