ROTATION OF AMMONIUM GROUPS BY INFRARED IRRADIATION FAR FROM OBVIOUS ABSORPTION-BANDS

Direct irradiation of any of the four distinct N-D stretching bands of the ammonium nickel Tutton salt [(NH4)(2)Ni(H2O)(6)(SO4)(2)] that has been doped with a few percent deuterium, produces a persistent hole i n 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 th at appears to be featureless background-also rotates the ion, although this requires considerably more power. The most effective indirect ir radiation takes place at the positions of the combination bands betwee n the stretches and the librations of the ammonium ions. Other effecti ve irradiation positions are in the tails of the stretching bands or i n combinations with other unassigned modes, Sufficient energy is trans ferred through the vibrational degrees of freedom to the librations wi thout being dissipated to drive the rotation of the ammonium ion. (C) 1996 American Institute of Physics.