The spectrum and structure of singlet CH2

Abstract The red absorption bands of CH2, previously shown to correspond to a transition between singlet states, have been analysed in detail. In the lower electronic state a1A1 the molecule is a strongly asymmetric top with rotational constants A000 = 20.14, B000 = 11.16 and C000 = 7.06 cm-1. From these constants a CH distance of 1.11 A and an HCH angle of 102.4° is obtained. In the upper state b1B1 the molecule is nearly linear, having an HCH angle of about 140° and a CH distance of 1.05 A. Most of the observed absorption bands correspond to excitation of the bending vibration v2 in the upper state (v2 = 6, 8, ..., 18). In some of them in addition the symmetric stretching vibration v2 is singly excited. The levels 1, v2-4, 0 cause strong perturbations in the levels 0, v2, 0 for v2 = 8, 10, 12, 14. In the lower state only one excited vibrational level 0, 1, 0 has been established which yields v2 = 1352.6 cm -1. Fragments of some other singlet absorptions in the near ultraviolet and in the vacuum ultraviolet have been observed and are briefly described. The upper state of the near ultraviolet system is probably the predicted 1A1 state corresponding to 1E+ in the linear conformation.