High-Resolution Angle-Resolved Photoelectron Spectroscopy of Cu 5 - Clusters

Angle-resolved photoelectron spectroscopy (ARPES) has been used to probe the electronic and vibrational structure of the ${Cu}_5^{-}$ cluster. High-resolution spectra recorded near the detachment threshold reveal well-resolved vibrational progressions, enabling a precise determination of the adiabatic electron affinity (EA = 1.8947(12) eV). Measured vibrational frequencies are in good agreement with density-functional-theory predictions, allowing the assignment of the observed modes. The anisotropy parameter $\beta$ of the photoelectron angular distribution from the highest occupied molecular orbital follows the universal momentum-dependent behaviour recently predicted. In some cases $\beta$ exhibits a dependence on the final vibrational excitation of the cluster which deviates from the prediction, revealing a possible influence of autoionizing excited states.