CO ON COPPER CLUSTERS - ORBITAL SYMMETRY RULES

We have investigated the size-dependent reactivity of Cu clusters with CO in the size range 14-34 atoms. A beam of neutral clusters was prod uced in a laser vaporization source cooled with liquid nitrogen, and p assed through a low-pressure reaction cell containing CO. The reaction products were detected by laser ionization and time-of-flight mass sp ectrometry. The experiment was complemented with electronic-structure calculations, in which the adsorption of one CO molecule on Cu-15-Cu-2 1 was calculated using a spherical-jellium description of the clusters . We find that the CO reactivity of Cu clusters is strongly size selec tive in the size range 15-20 atoms, with Cu-16 exhibiting the highest reactivity. A comparison between the experimental and theoretical find ings suggests that the size variations in the CO reactivity can be und erstood in terms of symmetry rules, involving the delocalized valence states of the clusters. For example, the high reaction probability of Cu-16 is rationalized by an efficient symmetry matching between the hi ghest occupied molecular orbital and lowest unoccupied molecular orbit al of the cluster and the 2 pi and 5 sigma orbitals of CO.