Structural Isomers of Ti2O4 and Zr2O4 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 Zr2O4–. 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 C2v and C3v isomers as the ground states of Ti2O4– and Zr2O4– anions, respectively. Minor contributions from the C2h isomer of Ti2O4– and the C2v isomer of Zr2O4– 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 Zr2O4 species.