Threshold Photoionization and Density Functional Theory Investigations of Small Lanthanum Monoxide Clusters, LanO (n=2-10)

Small lanthanum monoxide clusters, LanO (n = 2-10), have been studied by laser threshold photoionization (PI) spectroscopy and density functional theory (DFT). Ionization energies (IEs) gradually decrease with cluster size with oscillatory behavior at certain sizes at n = 4, 7, and 9, and the IEs are slightly lower than the corresponding bare La-n clusters. Mulliken charge analysis of the neutral and cation clusters indicates that the threshold PI occurs from a delocalized metal-based 5d orbital. The stable geometric structures and IEs for the LanO (n = 2-8) clusters have been determined from DFT. Zero electron kinetic energy and PI spectra have been calculated based on DFT calculations and compared with the experimental PI spectra. On the basis of favorable agreement between the simulated and experimental PI spectra, reliable ground-state geometric structures have been assigned. The O atom is negatively charged, equivalent to one electronic charge mainly transferred from La atoms bonded to it. Chemical bonding between the La and O atoms is ionic, whereas La atoms are mostly covalently bonded. No direct correlation is found between the electronic properties like the IE, electron affinity, and HOMO-LUMO gap and the stability of these clusters.