Microwave Studies of Collision-Induced Transitions between Rotational Levels. IV. Steady-State Measurements in NH3

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Abstract
The technique of high-power microwave double resonance has been applied to the study of collision-induced transitions between rotational levels of ammonia. The inversion doublets for selected J values have been “pumped” by high microwave power, and the increase of absorption in other inversion doublets have been observed. Because of the abundance and intensities of ammonia lines, a systematic choice of 18 four-level systems was possible. The observed results were analyzed by steady-state equations, and relative values of rate constants have been determined. The experiments and analysis have led to the following qualitative conclusions: (i) Collision-induced transitions with dipole selection rules (ΔJ = 0, ± 1, parity + ↔ −) are “preferred.” (ii) The ΔJ = 0 dipole-type transitions have much greater probability than the ΔJ = ± 1 dipole-type transitions for levels with J∼K, but they have probabilities of equal order of magnitude for levels with J≫K. (iii) The ΔJ > 1 transitions have much smaller probabilities than the ΔJ = ± 1 transitions. (iv) The ΔK ≠ 0 transitions have much smaller probabilities than the ΔK = 0 transitions. A comparison of the observed results with values calculated by using Anderson's theory suggests that the probabilities of the ΔJ = ± 1 quadrupole-type transition (parity +↔ +) are the same order of magnitude as those of the corresponding dipole-type transitions.