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 D2h 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 (1)A(g), and those of Y2O2 and La2O2 are (3)B(1u). Ionization of both (1)A(g) and (3)B(1u) neutral states yields the (2)A(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).