Possible Rotational Equilibration of Interstellar Ammonia by Radiative Δk = ±3 Transitions

Abstract

It is pointed out that the probabilities of radiative Ak = +3 transitions between metastable rotational levels of ammonia are not much smaller than those for collision-induced transitions considered by Townes and his collaborators in the discussion of rotational relaxation of NH3 in interstellar space. Calculated values of the time of spontaneous emission for such radiative transitions give the upper limits of the lifetimes of metastable levels and provide a useful time standard in interstellar space. Competition between the radiative transitions and the collision-induced transitions are discussed; the discussion uses the calculated values of the former and the laboratory results on the latter. Lower limits for the density of collision partners required to comply with the observation by Cheung et a'. are given.