Structural isomers of Ti₂O₄ and Zr₂O ₄ anions identified by slow photoelectron velocity-map imaging spectroscopy

High-resolution anion photoelectron spectra are reported for the group 4 metal dioxide clusters Ti₂O₄^- and Zr ₂O₄^-. Slow photoelectron velocity-map imaging (SEVI) spectroscopy of cryogenically cooled, mass-selected anions yields photoelectron spectra with submillielectronvolt resolution, revealing extensive and well-resolved vibrational progressions. By comparison of the spectra with Franck-Condon simulations, we have identified the C_2v and C _3v isomers as the ground states of Ti₂O₄ ^- and Zr₂O₄^- anions, respectively. Minor contributions from the C_2h isomer of Ti₂O ₄^- and the C_2v isomer of Zr₂O ₄^- are also seen. The SEVI spectra yield upper bounds for the adiabatic detachment energies, as well as vibrational frequencies for various modes of the neutral Ti₂O₄ and Zr ₂O₄ species.