Another type of band, relevant to microwave spectra, is the inversion band of ammonia. The arrangement of NH3 has the N atom slightly above or below the plane of the three H atoms and the inversion band arises from symmetry considerations of whether the N atom is above or below this plane. This gives rise to a cluster of absorption lines centered at a wavelength of 1.3 cm, which is the dominant feature of microwave spectra of Jupiter and Saturn, although not Uranus and Neptune, which is how the low abundance of ammonia above ^80 bar in these atmospheres is inferred. In addition to the microwave inversion band, this molecular structure also gives rise to a splitting of the rotational energy levels of ammonia, known as inversion doubling (Herzberg, 1945).
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