Aluminum cluster anions:: Photoelectron spectroscopy and ab initio simulations

Atomic structures and geometries, electronic structure, and temperature-dependent photoelectron spectra of Al-N(-) (N = 19-102) 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 Al-19(-), Al-20(-), and Al-23(-) are found and they exhibit oblate, prolate, and octupole deformed shapes, respectively. In addition to the low-temperature spectra, high temperature 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 greater than or equal to 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. fee-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.