Anion Photoelectron Spectroscopy and CASSCF/CASPT2/RASSI Study of Lan- (n=1, 3-7)

We present a combined experimental and theoretical study of small lanthanum clusters. The experimental photoelectron spectra of La-n(-) (n = 1, 3-7) were obtained using negative ion photoelectron spectroscopy. Electron affinities for these clusters were found to be in a range of 0.49 eV (La) to 1.5 eV (La-7). Our computational, tour de force in exploring the electronic structure and its consequences for the lanthanum atom and its anion as well as for lanthanum trimer and its anion shows the multiconfigurational method and large basis set with spin orbit corrections: CASSCF/CASPT2/RASSI/ANO-RCC-L level of theory is needed to reproduce experimental accuracy. The most stable structure for La-3(-) was established to be an equilateral triangle ((1)A(1)'). Chemical bonding analysis of the La-3(-) global minimum reveals that this is the first experimentally observed species with d-AO double sigma and pi aromaticity.