Rotation of ammonium groups by infrared irradiation far from obvious absorption bands

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Abstract
Direct irradiation of any of the four distinct N–D stretching bands of the ammonium nickel Tutton salt [(NH4)2Ni(H2O)6(SO4)2] that has been doped with a few percent deuterium, produces a persistent hole in that band. The hole and the accompanying antiholes indicate rotation of the ammonium ion. Indirect irradiation at up to a few hundred wave numbers away from the stretching bands—in a region of the spectrum that appears to be featureless background—also rotates the ion, although this requires considerably more power. The most effective indirect irradiation takes place at the positions of the combination bands between the stretches and the librations of the ammonium ions. Other effective irradiation positions are in the tails of the stretching bands or in combinations with other unassigned modes. Sufficient energy is transferred through the vibrational degrees of freedom to the librations without being dissipated to drive the rotation of the ammonium ion.
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