Interstellar Formaldehyde. I. The Collisional Pumping Mechanism for Anomalous 6-CENTIMETER Absorption
"Anomalous" absorption by formaldehyde in diffuse dust clouds-6-cm line absorption in the absence of a continuum source-is generally conceded to result from a pump which maintains the 6-cm excitation temperature below 2 K, producing thereby absorption of the microwave background. A quantum- mechanical treatment is given here of the collisional-pumping process which Townes and Cheung suggest will cool the 6-cm levels by the required amount. It is shown that quantum effects probably limit the proposed process to regions where the kinetic temperature Tk > 45 K, and may even suppress it altogether. (Hydrogen molecules are taken for the colliding neutral particles, but essentially the same results are expected from H or He collisions.) Electron impact, moreover, is shown to heat the 6-cm transition at essentially all Tk, and would suppress the proposed collisional cooling in regions having n /na2 even if it were to occur in less ionized regions. Since radio observations of OH, H I, and CO Tk to be well below 45 K in a typical dark nebula where anomalous absorption is observed (Heiles's Cloud 2), and since the required combination of high Tk and low n /na2 is difficult to reconcile with thermal and ionization equilibrium, it is concluded that collisions are probably not the cause of anomalous absorption.