2014ApJ...791...65H

The J = 2 ⟵ 1 rotational transition of AlH (X ¹Σ⁺) near 755 GHz and the J = 4 ⟵ 3 line of AlD (X ¹Σ⁺) near 787 GHz have been measured using terahertz direct absorption spectroscopy. Both species were created in an AC discharge of Al(CH₃)₃ and H₂or D₂in the presence of argon. This work is the first direct measurement of both transitions. These data were combined with previous submillimeter transition frequencies for both molecules recorded by Halfen & Ziurys in global analyses to refine their spectroscopic parameters. The constants B and D for AlH were determined for the first time based on pure rotational data only, improving their accuracy, while those for AlD were slightly refined. Predictions for higher-lying transitions of AlH must consequently be revised by at least 50 MHz, a significant difference. AlH has been observed via its A ¹Π-X ¹Σ⁺ electronic transition in stellar photospheres, suggesting that this species may be present in circumstellar gas surrounding late-type stars, where five aluminum-bearing molecules have already been detected.