Negative ion photoelectron spectroscopy of the group 5 metal trimer monoxides V3O, Nb3O, and Ta3O

The vibrationally resolved 488 nm negative ion photoelectron spectra of V3O, Nb3O, and Ta3O provide measurements of their electron affinities, vibrational frequencies, and low-lying electronic state energies. The electron affinities increase smoothly down the triad with values (eV) of 1.218 +/-0.008 for V3O, 1.393 +/-0.006 for Nb3O, and 1.583 +/-0.010 for Ta3O. The metal-oxygen symmetric stretching fundamental frequencies are 750 +/- 20, 710 +/- 15, and 710 +/- 15 cm(-1), respectively, for the neutral clusters and 770 +/- 20 cm(-1) for V3O-. Lower frequency symmetric modes are also active, with frequencies (+/- 15 cm(-1)) of 415 and 340 cm-1 for V3O, 320 cm(-1) for Nb3O, 225 cm(-1) for Ta3O, and of 355, 300, and 215 cm(-1) (+/- 20 cm(-1)), respectively, for their anions. Weaker transitions also showing vibrational structure are observed to excited states at 3350 +/- 30 cm(-1) for V3O, 4300 +/- 100 and 6190 +/- 120 cm(-1) for Nb3O, and 3180 +/- 40 and 3870 +/- 50 cm(-1) for Ta3O. The spectra indicate that the extra electrons in the anions occupy essentially nonbonding orbitals, and that the neutral and anionic clusters have planar structures with doubly bridging oxygen atoms. Results are discussed in light of previous studies of Nb3O and the Group 5 M-3 and MO molecules, and tentative assignments for the observed M3O and M3O- states are proposed. (C) 2001 American Institute of Physics.