Fundamental Excitations of the Shared Proton in the H3O2 - and H5O2 + Complexes

We exploit recent advances in argon predissociation spectroscopy to record the spectroscopic signature of the shared proton oscillations in the H3O2 - system and compare the resulting spectrum with that of the H5O2 + ion taken under similar conditions. Very intense 1 ← 0 transitions are observed below 1100 cm-1 in both cases and are surprisingly sharp, with the 697 cm-1 transition in H3O2 - being among the lowest in energy of any shared proton system measured to date. The assignments of the three fundamental transitions associated with the three-dimensional confinement of the shared proton in H3O2 - are carried out with full-dimensional (DMC) calculations to treat this strongly anharmonic vibrational problem.