Kutner and Thaddeus 1971
Kutner, M., and P. Thaddeus, 1971: 6-centimeter formaldehyde absorption and emission in the Orion Nebula. Astrophys. J., 168, L67-L71, doi:10.1086/180786.
Weak 6-cm H2CO absorption has been found over much of the Orion Nebula, and is attributed to "anomalous" absorption of the universal microwave radiation by H2CO behind the H II continuum source. In the Kleinmann-Low infrared nebula the line is self-reversed, with a core of about 0.10 K that is probably emission from the H2CO observed there at 2 mm. These observations are interpreted as indicating that neutral-particle collisions suppress the pumping mechanism responsible for anomalous absorption.