Threshold ionization, structural isomers, and electronic states of M ₂O₂ (M = Sc, Y, and La)

M₂O₂ (M = Sc, Y, and La) were synthesized in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and ab initio calculations. Adiabatic ionization energies (AIEs) and several vibrational frequencies were measured accurately for the first time from the MATI spectra. Six possible structural isomers of M₂O₂ were considered in the calculations and the three converged structures were used in the spectral analysis. A planar cyclic structure in D_2h point group was predicted to be the most stable one by the theory and observed by the experiment. The cyclic structure is formed by joining two MO₂ fragments together through two shared oxygen atoms. In forming the ground state clusters, each metal atom loses two (n - 1)d electrons and as a result, has only one ns electron in the metal-based valence orbital. The ground electronic state of Sc₂O₂ is ¹A_g, and those of Y₂O₂ and La ₂O₂ are ³B_1u. Ionization of both ¹A_g and ³B_1u neutral states yields the ²A_g ion state by removing one of the two ns electrons, and the resultant ion has a similar geometry to the neutral cluster. The AIEs of the clusters are 5.5752 (6), 5.2639 (6), 4.5795 (6) eV for M = Sc, Y, and La, respectively. The vibrational frequencies of the observed modes, including O-M and M-M stretches, are in the range of 200-800 cm^-1.