Nuclear resonance in crystals

Abstract

In crystals, the nuclear spin-lattice relaxation time, t 1, is generally very long while the spin-spin relaxation time, t 2, is very short. In very few instances, however, is t 1 as long as it ought to be in a perfect crystal. Usually the dominant relaxation process involves not the lattice vibrations but internal molecular or ionic rotations, or possibly paramagnetic impurities. The temperature dependence of t 1 gives considerable information about internal rotation. Experiments on a number of crystals, including the ammonium halides, are described, and the relation of these studies to line-shape investigations is discussed. The coupling of a nuclear electric quadrupole to a vibrating lattice can be much stronger than the dipole effect. Pound has found examples in which this effect is dominant at room temperature, and has identified the mechanism as quadrupole coupling by selectively saturating one line of a quadrupole multiplet while observing the intensity of another. The relaxation time is not far from that predicted by Waller's theory, adapted to this case. The availability of crystals with a long relaxation time at room temperature has led to studies of the persistence of nuclear polarization when the crystal is removed from the magnet — in effect, “adiabatic demagnetization” from room temperature. It is possible to reserve the field in the crystal quickly, without reversing the polarization; the spin temperature is then negative, and the crystal emits, rather than absorbing, at resonance. These experiments raise a number of interesting questions about the state of the system when H 0 is comparable to, or less than, the internal fields, and make possible resonance studies in this new region.