Direct Measurements of the Fundamental Rotational Transitions of Cd And

The lowest energy rotational transitions of CD and CH in their 2 r ground electronic states have been directly measured using submillimeter direct absorption spectroscopy. These two radicals were produced in an electrical discharge of either CD4 (or CH4 and D2) or CH4. The J 1⁄4 3/2 1/2, N 1⁄4 1 1 transition at 439 GHz and the J 1⁄4 3/2 3/2 and 5/2 3/2 fine structure lines of theN 1⁄4 2 1 rotational transition near 885 GHz and 916 GHz were recorded for CD (Hund’s case b notation), each of which consist of lambda doublets. In addition, hyperfine splittings due to the deuterium nuclear spin of I 1⁄41 were measured in several doublets, although some hyperfine components were blended together at higher frequency. For CH, the lambda doublets of the fundamental N 1⁄4 1 1 line near 532–536 GHz were recorded; in this case, hyperfine interactions arising from both C and H nuclei were resolved. These data were fit with a case b effective Hamiltonian, and spectroscopic parameters were derived. In particular, the deuterium hyperfine constants for CD were improved by about an order of magnitude, while for CH the hydrogen Fermi contact and dipolar terms were established for the first time. These measurements will enable definitive searches for CD and CH in interstellar gas, in particular with the upcoming Herschel Space Observatory. Subject headingg s: astrochemistry — ISM: molecules — methods: laboratory — molecular data — submillimeter — techniques: spectroscopic