Pressure induced shifts of the (J,K)=(12,12) inversion line in the microwave spectrum of ammonia

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Abstract
Experimental observations of the frequency shifts of the (J,K)=(12,12) line in the inversion spectrum of NH3 due to collisions with the foreign perturbers, He, A, CH4, N2, Co2, COS, CH3CN and CH3F have been reported: an attempt is made in the present paper to interpret these observations in terms of the impact theory of Anderson. Dipole-induced dipole, dispersion exchange dipole-dipole, dipole-quadrupole, quadrupole-dipole, quadrupole-quadrupole, quadrupole-dipole induced dipole and dipole-quadrupole induced dipole interactions are considered: all but the first three interactions make no contribution to the line shift according to the theory, and even these give nonzero contributions only if the dipole moment and/or the polarizability of an ammonia molecule change when the molecule undergoes a transition between the inversion states. Provided the polarizability of NH3 is greater in the upper level of the inversion doublet than in the lower, the dipole induced dipole and the dispersion interactions give shifts towards lower frequencies, in qualitative agreement with experiment, but the estimated magnitudes, with the exception of shifting by He and A, were too small by factors of up to approximately 100; the exchange interaction gives a contribution to the shift of the opposite sign to that observed.