Precise Rest Frequencies for AlH and AlD ( X 1 + ): A Reassessment of the Aluminum Hyperfine Structure

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Abstract
The J = 0 → 1 transition of AlH at 377 GHz and the J = 1 → 2 line of AlD at 393 GHz have been recorded using submillimeter direct absorption methods. The two species were created by the reaction of aluminum vapor and H2 or D2 gas in a DC discharge. The 27Al quadrupole hyperfine splittings, which are critical for the astronomical identification of these molecules, were measured for both AlH and AlD. In the case of AlH, the observed hyperfine pattern was not consistent with previous measurements. Values of the hyperfine parameters have thus been revised to be eQq = -48.59 MHz and CI = 0.306 MHz, which agree very closely with recent ab initio predictions. For AlD, eQq was established to be -48.48 MHz with CI = 0.156 MHz—the first determination of the hyperfine parameters for the molecule. These measurements should enable viable astronomical searches to be conducted for AlH and AlD, presumably with space-borne platforms. Diatomic hydrides containing a cosmically abundant element such as aluminum are fundamental for interstellar and circumstellar chemistry.