MILLIMETER/SUBMILLIMETER SPECTROSCOPY of TiO (X ³Δ_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 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.