The methylene radical CH2 had been identified in spectra
recorded toward Orion KL and W51 M:
J. M. Hollis, P. R. Jewell, and F. J. Lovas,
Confirmation of Interstellar Methylene
Astrophys. 438, 259264 (1995).
Even though only one transition was detected, namely the NKa,Kc = 40,4 31,3 transition, the observation of resolved fine and hyperfine structure secures the detection. It is noteworthy to point out that this transition probes fairly hot methylene; the transition is more than 200 K above ground.
More recently, cold methylene has been detected in absorption
toward Sgr B2 and W49 N. The transitions had been recorded earlier with the
ISO Long Wavelength Spectrometer:
E. T. Polehampton, K. M. Menten, S. Brünken, G. Winnewisser, and J.-P. Baluteau
Far-Infrared Detection of Methylene
Astron. Astrophys. 431, 203213 (2005).