VUV Photoionization Measurements and Electronic Structure Calculations of the Ionization Energies of Gas-Phase Tantalum Oxides TaOx (x=3-6)


Photoionization efficiencies (PIE) were measured for TaOx (x=3-6) via single-photon ionization using tunable VUV from a synchrotron. The PIEs were compared to simulations based on candidate structures for neutrals and cations calculated at the B3LYP/aug-cc-pVTZ level to assign ground state structures for the neutrals and determine ionization energies. TaO3 shows a sharp photoionization onset, which a harmonic normal mode analysis fails to reproduce. Calculations predict that TaO3 and TaO3+ have pseudo-C3v symmetry, with distortions along two degenerate antisymmetric Ta-O stretches. Modeling this distortion using calculated coupled anharmonic 2D potentials leads to a much sharper predicted PIE onset, in excellent accord with the experiment, with IE(TaO3)=10.65±0.05 eV. The PIE of TaO4 shows a more gradual onset, and is due to ionization of the ground, superoxide structure to a similar superoxide form of TaO4+, with an ionization energy of 10.53±0.05 eV. This is calculated to lie 0.17 eV above the ground state of TaO4+, in which O2 is electrostatically bound to TaO2+. The adiabatic IE(TaO4) is thus 10.36±0.1 eV. TaO5 and TaO6 show gentle onsets and slightly lower ionization energies. The calculations predict a (η2-O2) TaO3 superoxide structure for TaO5. However, this is not consistent with the measured PIE, which supports a (η1-O2) TaO3 dioxygen structure, with IE(TaO5)=10.0±0.2 eV. A similar situation occurs for TaO6. Although our calculations predict a bis(η2-O2) TaO2 structure, the relatively sharp observed PIE clearly favors a (η2-O2)(η1-O2) TaO2 structure, with an ionization energy of 10.10±0.05 eV.