Velocity modulation spectroscopy of molecular ions I: The pure rotational spectrum of TiCl⁺ (X³Φ_r)

The pure rotational spectrum of the TiCl⁺ ion in its X ³Φ_r ground state has been measured in the frequency range 323-424 GHz, using a combination of direct absorption and velocity modulation techniques. The ion was created in an AC discharge of TiCl ₄ and argon. Ten, eleven, and nine rotational transitions were recorded for the ⁴⁸Ti³⁵Cl⁺, ⁴⁸Ti³⁷Cl⁺, and ⁴⁶Ti ³⁵Cl⁺ isotopomers, respectively; fine structure splittings were resolved in every transition. The rotational fine structure pattern was irregular with the Ω = 4 component lying in between the Ω = 2 and 3 lines. This result is consistent with the presence of a nearby ³Δ_r state, which perturbs the Ω = 2 and 3 sub-levels, shifting their energies relative to the Ω = 4 component. The data for each isotopomer were analyzed in a global fit, and rotational and fine structure parameters were determined. The value of the spin-spin constant was comparable to that of the spin-orbit parameter, indicating a large second-order spin-orbit contribution to this interaction. The bond length established for TiCl⁺, r₀ = 2.18879 (7) Å, is significantly shorter than that of TiCl, which has r₀ = 2.26749 (4) Å. The shorter bond length likely results from a Ti²⁺Cl^- structure in the ion relative to the neutral, which is thought to be represented by a Ti ⁺Cl^- configuration. The higher charge on the titanium atom shortens the bond.