MECHANISMS OF ENERGY TRANSPORT IN RUBY

The experimental results of Imbusch on energy transfer between single ions and pairs in 0.003%-1% Cr3+:Al2O3 are re-examined. It is shown that the energy transfer can only be plausibly explained on the basis of orbitally dependent exchange rather than the electric quadrupole-quadrupole interaction as has been postulated. This result, besides being of intrinsic interest, also has important consequences for other energy-transfer experiments. In particular, the experimental concentration dependence of the intensities seems to imply an R5mean law for the interaction operative in the energy transfer rather than the much more rapid falloff characteristic of exchange. It is suggested that this discrepancy arises from the propagating nature of the single ion excitation which tends to enhance the importance of short-range interactions. This same anomaly should be present in many other systems in which energy transfer has been studied.