Millimeter/submillimeter spectroscopy of Tio (X³Δ_r): the rare titanium isotopologues

Pure rotational spectra of the rare isotopologues of titanium oxide, ⁴⁶TiO, ⁴⁷TiO, ⁴⁹TiO, and ⁵⁰TiO, have been recorded using a combination of Fourier transform millimeter-wave (FTmmW) and millimeter/submillimeter direct absorption techniques in the frequency range 62–538 GHz. This study is the first complete spectroscopic characterization of these species in their X ³Δ_r ground electronic states. The isotopologues were created by the reaction of N₂O or O₂ and titanium vapor, produced either by laser ablation or in a Broida-type oven, and observed in the natural Ti isotopic abundances. Between 10 and 11 rotational transitions J + 1 $\leftrightarrow$ J were measured for each species, typically in all 3 spin–orbit ladders Ω = 1, 2, and 3. For ⁴⁷TiO and ⁴⁹TiO, hyperfine structure was resolved, originating from the titanium-47 and titanium-49 nuclear spins of I = 5/2 and 7/2, respectively. For the Ω = 1 and 3 components, the hyperfine structure was found to follow a classic Landé pattern, while that for Ω = 2 appeared to be perturbed, likely a result of mixing with the nearby isoconfigurational a ¹Δ state. The spectra were analyzed with a case (a) Hamiltonian, and rotational, spin–orbit, and spin–spin parameters were determined for each species, as well as magnetic hyperfine and electric quadrupole constants for the two molecules with nuclear spins. The most abundant species, ⁴⁸TiO, has been detected in circumstellar envelopes. These measurements will enable other titanium isotopologues to be studied at millimeter wavelengths, providing Ti isotope ratios that can test models of nucleosynthesis.