15min:

LU WU, BENI DANGI, MOURAD ROUNJANE AND DONG-SHENG YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.

M₂O₂ ( M = Ce and Pr) is produced in a pulsed laser-vaporization metal-cluster source and studied by mass-analyzed threshold ionization (MATI) spectroscopy. From the MATI spectra, the adiabatic ionization energy is determined to be 37300(5) cm^-1 for Ce₂O₂, and 37885 (5) cm^-1 for Pr₂O₂. Like group 3 transition metal M₂O₂ (M=Sc, Y, and La) clusters we reported previously, these lanthanide clusters have a D_2h planer structure and the vibrational modes observed are from the in-plane motions. However, the ground and other low-energy electronic states of the lanthanide oxides have a much higher electron spin multiplicity due to the existence of 4f electrons in the Ce and Pr atoms. The 4f electron of Ce atom has significantly lower energies than the 5d or 6s electrons and remain uncoupled in Ce₂O₂. On the other hand, the energy differences between the 4f and 5d/6s electrons of Pr atom are relatively small, and a 4f 5d electron promotion is required in the formation of Pr₂O₂. The electronic transitions responsible for the observed MATI spectra are tentatively determined to be ⁴B_1u ⁵A_g for Ce₂O₂ and ⁶B_1u ⁷B_2g and ⁶B_1u ⁵B_1u for Pr₂O₂.