LU WU, MOURAD ROUDJANE, YANG LIU AND DONG-SHENG YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.

Lu2 is produced in a pulsed laser-vaporization metal-cluster source and studied by mass-analyzed threshold ionization (MATI) spectroscopy. The MATI spectrum displays several long progressions from the transitions between various vibrational levels of the neutral and ion electronic states. From the spectrum, the upper limit of the ionization energy of the dimer is determined to be 43996 cm-1, and the vibrational frequencies are measured to be 121 cm-1 in the neutral state and 90 cm-1 in the ion state. By combining with ab initio calculations at CASPT2 level, the ground state of Lu2 is identified as 3 Sigmag-. The 3 Sigmag- state has an electron configuration of 6s sigmag25d piu15d piu16s sigmau2, which is formed by the interactions of two Lu atoms in the 2D(5d6s2) ground state. Ionization of the neutral state removes a 5d piu bonding electron and yields a ion state with a considerably longer bond distance. Lu2 has a very different bonding feature from La2, for which a 1 Sigmag+ ground state was previously identified with an electron configuration of 5d piu46s sigmag2 formed by the interactions of two La atoms in the 4F(5d26s) excited state.