Aluminum cluster anions: Photoelectron spectroscopy and ab initio simulations

Atomic structures and geometries, electronic structure, and temperature-dependent photoelectron spectra of AlN 2 (N5192102) clusters are studied both theoretically via ab initio local-density-functional simulations, and experimentally with high-resolution measurements. The use of a theoretically well-defined energy shift in conjunction with a generalized Koopmans’ theorem enables direct comparisons between the calculated density of states and the experimental photoelectron spectrum. Such comparisons, using photoelectron spectra calculated for various relaxed cluster geometries, enables a determination of the optimal structures of the clusters. The atomic arrangements in the ground-state structures of Al19 , Al20 , and Al23 2 are found and they exhibit oblate, prolate, and octupole deformed shapes, respectively. In addition to the low-temperature spectra, hightemperature calculated spectra of these clusters obtained via ab initio molecular-dynamics simulations reproduce the experimentally observed trends measured for hot clusters. For larger cluster anions (N>36), the level of agreement between the theoretically calculated spectra and the measured ones is less satisfactory, indicating that while certain structural motifs are identified, the optimal ground-state structures may not have been fully determined. fcc-like low-energy structures are found in the larger cluster-size range, with decreasing distortions of the internal structures when the cluster size is increased. Indications are found pertaining to an octahedral growth pattern of larger aluminum clusters.