1995A&A...299..847L

We have observed galactic λ6cm and λ2mm ortho- and para-H₂CO absorption along lines of sight toward and around compact extragalactic mm-wave continuum sources. From comparison of on and off-source λ6cm profiles it is possible to derive the anomalous excitation temperature, which is surprisingly close to the temperature of the cosmic microwave background in several cases. The λ2mm absorption profiles show that the ortho/para ratio is 3-4, which (taken alone) favors formation either on grains at T_K>20K or at comparable temperatures in the gas phase. However the abundance of H₂CO, as with other species, is probably too high at low extinction to be explained by any known process; we find N(H₂CO)/N(¹³CO)∼10^–2 for N(¹³CO)∼10¹⁴cm^–2.