2016ApJ...833...89H

New laboratory measurements of hydrides have been carried out using terahertz direct absorption spectroscopy. Spin components of the N=2←1 transition of the free radical CrH (X ⁶Σ⁺) have been recorded in the range 730-734 GHz, as well as a new measurement of the J=2←1 line of AlH (X ¹Σ⁺) near 755 GHz. Both species were created in an AC discharge of H₂, argon, and metal vapor. For CrH, the chromium source was Cr(CO)₆, while AlH was produced from Al(CH₃)₃. The J=4.5←3.5 and 3.5←2.5 fine-structure components were recorded for CrH, each which consists of resolved proton hyperfine doublets. For AlH, the two main quadrupole components, F=4.5←3.5 and 3.5←2.5, of the J=2←1 transition were observed as blended features. These data were analyzed with previous 1←0 millimeter/submillimeter measurements with ⁶Σ and ¹Σ Hamiltonians for chromium and aluminum hydrides, respectively, and rotational, fine-structure (CrH only), and hyperfine constants were derived. The new measurements have resulted in refined spectroscopic parameters for both species, as well as direct measurement of the respective 2←1 rotational transitions. This work also resolves a 10 MHz discrepancy in the frequency of the AlH line. CrH and AlH have already been observed in the photospheres of stars via their electronic transitions. These data will facilitate their discovery at submillimeter/terahertz wavelengths in circumstellar envelopes and perhaps in diffuse clouds.