Low-Energy Photoelectron Imaging Spectroscopy of La_n(benzene) (n = 1 and 2)

La_n(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La₂(benzene) displays a strong origin band, a short metal-ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La₂(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring. The ground electronic state of the inverse sandwich is ¹A_1g (D_3d) with (5d_xy,x²-y² + π∗)⁴6s² electron configuration. Ionization removes a La-based 6s electron and yields a ²A_1g ion. The spectrum of La(benzene) is similar to the zero-electron kinetic energy spectrum reported previously by our group, although the spectral line width is somewhat broader. The measurement of the photoelectron angular distribution of La(benzene) confirms that the ejected electron has largely a p wave character. The metal-ligand bonding of La₂(benzene) is considerably stronger than that of La(benzene) due to the threefold binding of each La atom in the dilanthanum species and the twofold binding in the monolanthanum complex.