RUCHIRA SILVA AND DONG-SHENG YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.
Slow electron velocity-map imaging (SEVI) was used to study the structures and electronic states of La2(C6H6) and La(C6H6)2 complexes formed in a metal cluster beam source. Electron spectra obtained from SEVI have the energy resolution of E/eKE ~ 2% at eKE=400cm-1. The SEVI technique offers much higher data collection efficiency than pulsed-field ionization zero electron kinetic energy spectroscopy. From the SEVI spectra, adiabatic ionization energies were measured to be 32141(5) cm-1 for La2(C6H6) and 39033(5) cm-1 for La(C6H6)2. The most active vibrational transition for both complexes was identified to be a metal-ligand stretching mode with a frequency of 180 cm-1 in the ion state. In addition, a benzene ring out-of-plane deformation mode was measured to be 303 cm-1 for [La2(C6H6)]+ and 408 cm-1 for [La(C6H6)2]+. By combining the spectra with theoretical calculations, we identified the 2Ag 1Ag transition of La2(C6H6) (C2h) and 3A 2A of La(C6H6)2 (C1). By measuring the anisotropy parameter ( ) from photoelectron angular distribution, we found that the outgoing electron was from a largely La 6s-based molecular orbital in both complexes.