Velocity modulation spectroscopy of molecular ions II: The millimeter/submillimeter-wave spectrum of TiF⁺ (X³Φ_r)

The pure rotational spectrum of the molecular ion TiF⁺ in its ³Φ_r ground state has been measured in the range 327-542 GHz using millimeter-wave direct absorption techniques combined with velocity modulation spectroscopy. TiF⁺ was made in an AC discharge from a mixture of TiCl₄, F₂ in He, and argon. Ten transitions of this ion were recorded. In every transition, fluorine hyperfine interactions, as well as the fine structure splittings, were resolved. The fine structure pattern was found to be regular with almost equal spacing in frequency between the three spin components, in contrast to TiCl⁺, which is perturbed in the ground state. The data were fit with a case (a) Hamiltonian and rotational, fine structure, and hyperfine constants were determined. The bond length established for TiF⁺, r₀ = 1.7775 Å, was found to be shorter than that of TiF, r₀ = 1.8342 Å-also established from mm-wave data. The hyperfine parameters determined are consistent with a δ¹π¹ electron configuration with the electrons primarily located on the titanium nucleus. The nuclear spin-orbit constant a indicates that the unpaired electrons are closer to the fluorine nucleus in TiF⁺ relative to TiF, as expected with the decrease in bond length for the ion. The shorter bond distance is thought to arise from increased charge on the titanium nucleus as a result of a Ti²⁺F^- configuration. A similar decrease in bond length was found for TiCl⁺ relative to TiCl.