Observations of the 14.5-GHz transition of interstellar H2CO towards continuum sources and dark clouds.

Observations of the 2₁₁ - 2₁₂ transition of ortho-H₂CO at 14.5 GHz have been made with the Parkes radio telescope towards about 100 continuum sources and dark clouds and compared with observations of the 1₁₀ - 1₁₁ transition. The observed linewidths were similar for both transitions, with median values of 4.8 km s^-1 for the clouds towards continuum sources, and 1.7 km s^-1 for dark clouds. Absorbing clouds near H II regions have higher excitation temperature (about 7K) and median optical depths (0.045 and 0.14 at 14.5 and 4.8 GHz) than the values (3.5K, 0.01, 0.08) for clouds well-separated from the H II regions. Comparison of the observations with an LVG model based on collisional excitation is discussed.