The orientation and bonding of CO on Mo(100) using anglehyphen;resolved photoelectron spectroscopy and nearhyphen;edge xhyphen;ray absorption fine structure

摘要

The nature of the species formed by CO chemisorption of Mo(100) has been investigated using anglehyphen;resolved ultraviolet photoelectron spectroscopy (ARUPS) and nearhyphen;edge xhyphen;ray absorption spectroscopy. Highhyphen;resolution electron energy loss spectroscopy (HREELS) indicates the formation of two distinct types of CO. At coverages greater than 50percnt; of saturation, chemisorbed CO exhibits a CO stretching frequency of sim;2100 cmminus;1corresponding 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 coverageslessthan 50percnt; of saturation, CO exhibits an extraordinarily low stretching frequency of sim;1200 cmminus;1. Both NEXAFS and ARUPS measurements unequivocally indicate that at low coverages CO is tilted at sim;40deg; to the surface normal. Since CO is tilted with respect to the surfaceatlowcoverages, this effect cannot be ascribed to adatomndash;adatom interactions, and a careful measurement of the positions of the photoelectron peaks of the tilted molecule indicates that both the 1pgr; and the 5sgr; 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.