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  Journal of Spectroscopy and Dynamics 2014, 4: 21
  Research Article
VUV photoionization measurements and electronic structure calculations of the ionization energies of gas-phase tantalum oxides TaOx (x=3-6)
  Manori Pereraa, Kevin M. Roenitza, Ricardo B. Metzb, Oleg Kostkoc, Musahid Ahmedc  
a Department of Chemistry, Illinois Wesleyan University, Bloomington, IL 61702 USA
b Department of Chemistry, University of Massachusetts Amherst, Amherst, MA 01003 USA
c Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA

  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.
  Keywords: Photoionization; Tantalum oxides; Superoxide  

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