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

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.