CO on copper clusters: Orbital symmetry rules

We have investigated the size-dependent reactivity of Cu clusters with CO in the size range 14–44 atoms. A beam of neutral clusters was produced in a laser vaporization source cooled with liquid nitrogen, and passed through a low-pressure reaction cell containing CO. The reaction products were detected by laser ionization and time-of-flight mass spectrometry. The experiment was complemented with electronic-structure calculations, in which the adsorption of one CO molecule on ${\mathrm{Cu}}_{15}$–${\mathrm{Cu}}_{21}$ was calculated using a spherical-jellium description of the clusters. We find that the CO reactivity of Cu clusters is strongly size selective in the size range 15–20 atoms, with ${\mathrm{Cu}}_{16}$ exhibiting the highest reactivity. A comparison between the experimental and theoretical findings suggests that the size variations in the CO reactivity can be understood in terms of symmetry rules, involving the delocalized valence states of the clusters. For example, the high reaction probability of ${\mathrm{Cu}}_{16}$ is rationalized by an efficient symmetry matching between the highest occupied molecular orbital and lowest unoccupied molecular orbital of the cluster and the 2π* and 5σ orbitals of CO. © 1996 The American Physical Society.