Inversion Frequency of Ammonia and Molecular Interaction

Several experiments on the shape and position of the inversion lines of NH₃ have indicated an effective decrease in the inversion frequency with increasing pressure of the gas. As suggested in a former paper, this is to be expected on theoretical grounds. It is here shown, first with the use of a simple linear model, that the interaction of two NH₃ molecules causes the doublet levels to change their spacing in a manner which produces two possible transitions of different frequencies, one smaller and one larger than the normal value. When transition probabilities are calculated it is seen that the smaller frequency gains in strength upon approach of the molecules, at the expense of the other. In Section 2 the effect of rotation upon the distortion of the levels is calculated, and Section 3 contains a schematic calculation of the interaction of three molecules. The pressure for which the reduction in the resonance frequency is experimentally observed agrees surprisingly well with the distance of separation between molecules at which the effect here calculated sets in.