Mechanisms of Energy Transfer Involving Trivalent Dy and Tb

Energy may readily transfer from the 4F9/2 manifold of Dy to the 5D4 manifold of Tb in the Na0.5(Y, Tb, Dy)0.5WO4 tungstates at 295°, 77°, or 4.2°K. The reverse transfer may occur at 295°K but not at lower temperatures. This is due to the requirement that nonzero levels of the ground manifold of Dy must be thermally populated to permit transfer from Tb to Dy. The net flow of excitation at 295°K is from Dy to Tb for low Dy concentrations and from Tb to Dy for high concentrations. The reversal in direction is due to a rise in Dy self-quenching as concentration increases. The intersections are dipole—dipole for either direction of flow.