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Abstract: Transitions between the spin–rotational levels of the 12CH radical in the v =1 level of the X 2Π state have been studied by the technique of laser magnetic resonance at far-infrared wavelengths. The data have been combined with a measurement of lambda-doubling transition frequencies at 7GHz to determine an improved set of molecular parameters for CH in the v =1 level. The parameters provide information on the effects of vibrational excitation on the structural properties of CH. Accurate predictions of the transition frequencies between the low-lying levels of the radical in the absence of a magnetic field have also been made. Small inconsistencies in the least-squares fit of the laser magnetic resonance data prompted re-measurement of three far-infrared laser frequencies, the 122.5μm line of CH2F2 pumped by 9R(22), the 122.5μm line of CH2F2 pumped by 9P(8) and the 554.4μm line of CH2CF2 pumped by 10P(14). The new measurements differ by as much as 3.8MHz from those made previously and are more accurate; they also remove the inconsistencies in the fit. The re-measured frequencies of the two 122.5μm lines are identical within experimental error which suggests that the far-infrared lasing transition is the same, namely the rR23(32) transition in the v 9=1 level of CH2F2. [Copyright &y& Elsevier]

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon rich Asymptotic Giant Branch (AGB) stars. The increased sensitivity and angular resolution of high altitude ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra - both in the ground and vibrationally excited states of the normal and rare isotopic species - of the principal molecules in the inner region which appear to contain only two or three heavy atoms Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeters-wave rotational lines that are observed in the inner envelope of carbon rich AGB stars. As an illustration we refer to the example of an initial laboratory approach for establishing whether vibrationally excited SiC 2 and HCN are the carriers of some of the unassigned features observed in the prototypical carbon rich AGB star IRC+10216 with ALMA. Also highlighted are ongoing laboratory studies of the silicon carbides SiC 2 and SiCSi in their ground and excited vibrational states, and SiC 3 in the ground vibrational state. Following the initial detection of SiC 3 and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with J ≤ 48, K a ≤ 3 and upper level energies E u ≤ 484 K in the range from 178 - 391 GHz, and 35 new transitions of SiC 3 with J ≤ 38, K a ≤ 20 and E u ≤ 875 K between 315 and 440 GHz were measured in the laboratory. In addition five to six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi, and the two lowest rotational transitions in the previously unexplored C-C stretching mode ( ν 1 ) of SiCC were measured in the normal and doubly substituted 13 C isotopic species.