Mechanisms of energy transfer between Tb3+ions in Tb(OH)3have been isolated for several states of the Tb3+ion by means of line shape measurements of the bandhyphen;tohyphen;band fluorescence from the two lowest exciton bands of the5D4manifold to one exciton band in the7F6(ground) and7F5(first excited) manifolds. The interionic energy transfer matrix elements were determined by comparing the observed line shapes with calculations of the exciton bandhyphen;tohyphen;band transitions in which the energy dispersion was varied. A consideration of the simple nature of the singlehyphen;ion states, an examination of the selection rules on the various energy transfer mechanisms, and an analysis of the range dependence of these mechanisms has enabled us to conclude that (1) the exchange plays a major role in both the7F5and7F6bands, (2) the electric multipole interaction does not make the dominant contribution to the energy transfer matrix elements in either manifold, (3) the magnetic dipolendash;dipole interaction plays a small but significant role particularly in the7F6manifold, and (4) the virtual phonon exchange plays a role comparable to that of exchange in the7F6band, a result which is of special interest in view of the few cases in which its importance has been clearly demonstrated.
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