2 affinity level of adsorbed CO: Bonding and dispersion

The inverse photoemission spectrum of CO adsorbed on the CU(100) surface reveals a split CO 2derived feature. For the ordered overlayers at high coverage, band dispersion of both components is clearly observed. We present evidence from cluster calculations that the splitting is caused by the occurrence of two classes of negative-ion final states. In the first (lower-energy) class, the added electron is in an orbital which is bonding with respect to Cu and CO and in the second class in an orbital which is antibonding. © 1985 The American Physical Society.