			15min:

VIBRATIONAL AND GEOMETRIC STRUCTURES OF La₃C₂O AND La₃C₂O⁺ FROM MASSE-ANALYZED THRESHOLD IONIZATION.

ROUDJANE MOURAD, LU WU AND D. S. YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.

La₃C₂O is produced for the first time by laser vaporization in a pulsed cluster source and identified by photoionization time-of-flight mass spectrometry. Vibrationally-resolved ion spectra are obtained with mass-analyzed threshold ionization (MATI) spectroscopy. The adiabatic ionization energy of La₃C₂O is measured to be 30891(5) cm^-1. The spectra display several short vibrational progressions, and these progressions are associated mainly with La-La, La-C and La₃C₂O stretching excitations. The electron-spin multiplicities and molecular symmetries of La₃C₂O and La₃C₂O⁺ are determined by combining the experimental measurements with ab initio calculations at MP2 level. Preliminary data analysis shows that the ¹A₁ leftarrow ²A₁ transition is responsible for the observed MATI spectra. The cluster has C_2v symmetry with La₃C₂O in a bi-pyramid structure and oxygen being attached to the La₃ plane.