Al4- cluster anion: electronic structure, excited states, and electron detachment

Full-valence complete-active-space self-consistent-field and coupled-cluster methods are used to investigate excitation and electron detachment energies of the Al(4)(-) cluster anion. Both, Al(4) and Al(4)(-), have electronic structures with many low-energy electron configurations and exhibit accordingly pronounced configuration mixing. The complete-active-space self-consistent-field calculations show that all low-energy states of the neutral and the cluster anion can nevertheless be characterized by a single configuration, and in contrast to Møller-Plesset perturbation theory, coupled-cluster methods are found to recover enough static correlation to describe these states accurately. Coupled-cluster methods are then used to find minimal-energy structures of four states of the anion and four states of the neutral cluster, the respective ground states are identified, and vertical and adiabatic detachment energies are computed. The computed vertical detachment energies suggest that the first peak in the experimental photoelectron spectrum consists of at least three different transitions, and the computed excitation energies corroborate the conclusions from the analysis of the recently observed delayed detachment from excited Al(4)(-) ions in an electrostatic trap.