===== On the Detection of the CH2 in the ISM =====

The methylene radical CH<sub>2</sub> had been identified in spectra recorded toward Orion KL and W51 M:\\
J. M. Hollis, P. R. Jewell, and F. J. Lovas,\\
**[[https://ui.adsabs.harvard.edu/abs/1995ApJ...438..259H|Confirmation of Interstellar Methylene]]**\\
//Astrophys.// **438**, 259–264 (1995).\\
Even though only one transition was detected, namely the //N<sub>K<sub>a</sub>,K<sub>c</sub></sub>// = 4<sub>0,4</sub> – 3<sub>1,3</sub> 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\\
**[[https://doi.org/10.1051/0004-6361:20041598|Far-Infrared Detection of Methylene]]**\\
//Astron. Astrophys.// **431**, 203–213 (2005).


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Contributor(s): H. S. P. Müller; 02, 2006


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