High-resolution terahertz spectrum of C[H.sub.2]--low J rotational transitions near 2 T[Hz.sup.1]

The methylene radical (C[H.sub.2]) was very important to Gerhard Herzberg. We have carried out high-resolution spectroscopic measurements on two energetically low-lying, pure rotational transitions of methylene in its ground vibrational--electronic state at frequencies near 2 THz. One of the transitions--the [N.sub.KaKc] = [2.sub.11] [left arrow] [2.sub.02] multiplet--belongs to ortho-C[H.sub.2] and is centered at 1.954 THz. The other rotational transition--the [N.sub.KaKc] = [l.sub.10] [left arrow] [l.sub.01] multiplet--belongs to para-C[H.sub.2] and is centered at 1.915 THz. Since the ground electronic state of methylene has [sup.3][B.sub.1] symmetry, the rotational transitions are split into three fine-structure components, while the ortho transitions are split additionally by hyperfine structure. In total, we have measured 29 new lines, six for the para transition and the remainder for the ortho transition. The newly measured lines can be added to high-resolution spectral data obtained by ourselves and others at lower frequencies. Progress towards a global fit is discussed. Key words: spectroscopy, THz frequencies, radicals, methylene, interstellar medium.