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The emission spectrum of Tb3+ substituted in YAG to various concentrations (x in Y3−xTbxAl5O12 ranges from 3×10−5 to 3×10−1) has been measured on samples prepared by two different methods. These methods are powder preparation by coprecipitation and growth of crystalline layers by liquid-phase epitaxy. The spectrum consists of two groups of lines, one group around 550 nm originating from the 5D4 level and one group around 450 nm from the 5D3 level. The intensity of the two groups of lines depends on the method of excitation. In the case of UV excitation (λ=254 nm) the emission intensity for low Tb concentrations is mainly governed by the absorption of the incident radiation. With electron excitation, the energy loss of incoming electrons to ‘‘killer sites’’ in the lattice is found to influence the emission. The ratio, however, of 5D4 to 5D3 intensity is independent of the excitation mode. This ratio tends to a constant value at low concentrations and increases strongly with increasing concentration. These effects are explained by feeding of the 5D3 and 5D4 states from 5d levels, followed by cross relaxation, i.e., 5D3 to 5D4 excitation transfer.

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