The a 4Σ−–X 2Π transition in CF: A measurement of the term energy and bond length of a fluoromethylidyne metastable

We report the first observation of the (0,0) and (0,1) vibrational bands of the weak, spin forbidden a 4Σ−–X 2Π transition of the CF radical. The spectra, obtained in a flowing afterglow, yield values for T0, B0, and r0 of the a 4Σ− state of 28 564±1 cm−1, 1.3035±0.002 cm−1, and 1.326±0.001 Å, respectively. The intensity distribution in the rotational subbands is consistent with the mixing of the a 4Σ− state predominantly with the X 2Π state and the uncharacterized C 2Σ+. This result is discussed with regard to previous calculations.