Velocity modulation spectroscopy of molecular ions I: The pure rotational spectrum of TiCl+ (X3Φr)

The pure rotational spectrum of the TiCl+ ion in its X3Φr 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 TiCl4 and argon. Ten, eleven, and nine rotational transitions were recorded for the 48Ti35Cl+, 48Ti37Cl+, and 46Ti35Cl+ 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 3Δr 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.