Far-Infrared detection of methylene

We present a clear detection of CH2 in absorption towards the molecular cloud complexes Sagittarius B2 and W49 N using the ISO Long Wavelength Spectrometer. These observations represent the first detection of its low excitation rotational lines in the interstellar medium. Towards Sagittarius B2, we detect both ortho and para transitions allowing a determination of the total CH2 column density of N(CH2)=(7.5+/-1.1)x10^14 cm^-2. We compare this with the related molecule, CH, to determine [CH/CH2]=2.7+/-0.5. Comparison with chemical models shows that the CH abundance along the line of sight is consistent with diffuse cloud conditions and that the high [CH/CH2] ratio can be explained by including the effect of grain-surface reactions.