Terahertz spectroscopy of the CHD radical ((X)over-tilde3 A)

Context. Pure rotational spectra in the field-free condition of the CH2 and CD2 isotopologues of the methylene radical have already been reported, whereas only six rotational transitions of the mono-deuterated species, CHD, detected by FIR-LMR spectroscopy have been. Aims. Spectra of the mono-deuterated methylene radical in its ground electronic state ((X) over tilde (3) A '') were observed in the field-free condition for the first time in the frequency region below 1 THz. Methods. CHD was generated in a positive column discharge in a CH2D2 - Ar mixture cooled with liquid nitrogen. Results. Two rotational transitions, each split into three fine-structure components, were measured. The B and C rotational constants, the epsilon(bb) and epsilon(cc) spin-rotation constants, and the beta spin-spin constant were revised. Conclusions. The reported frequencies are suitable for astronomical searches.