Direct Measurements of the Fundamental Rotational Transitions of CD and 13CH (X2Πr)

The lowest energy rotational transitions of CD and 13 CH in their 2 Π r ground electronic states have been directly measured using submillimeter direct absorption spectroscopy. These two radicals were produced in an electrical discharge of either CD 4 (or CH 4 and D 2 ) or 13 CH 4 . The <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00001.gif" ALT="J=3/2≤ftarrow 1/2" /> J = 3/2← 1/2, <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00002.gif" ALT="N=1≤ftarrow 1" /> N = 1← 1 transition at 439 GHz and the <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00003.gif" ALT="J=3/2≤ftarrow 3/2" /> J = 3/2← 3/2 and <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00004.gif" ALT="5/2≤ftarrow 3/2" /> 5/2← 3/2 fine structure lines of the <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00005.gif" ALT="N=2≤ftarrow 1" /> N = 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 <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00006.gif" ALT="I=1" /> I = 1 were measured in several doublets, although some hyperfine components were blended together at higher frequency. For 13 CH, the lambda doublets of the fundamental <IMG SRC="http://ej.iop.org/images/0004-637X/687/1/731/eq-00007.gif" ALT="N=1≤ftarrow 1" /> N = 1← 1 line near 532-536 GHz were recorded; in this case, hyperfine interactions arising from both 13 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 13 CH the hydrogen Fermi contact and dipolar terms were established for the first time. These measurements will enable definitive searches for CD and 13 CH in interstellar gas, in particular with the upcoming Herschel Space Observatory .