UNDERSTANDING BROAD BANDS

摘要

The luminescence properties of lanthanides have been studied extensively in the past. The understanding of the sharp intraconfigurational 4f~(n) luminescence is high, but for the broader 4f~n→4f~(n-1)5d and charge transfer luminescence there are still many unexplained observations. In this article new insight into various aspects of broad band luminescence of lanthanide ions will be presented. Spin-allowed and spin-forbidden fd emission and absorption bands have been observed. Life time measurements for the spin-allowed fd emission show that the electric dipole matrix element for the fd transition decreases through the lanthanide series. Also the splitting between the high-spin and low-spin fd states decreases through the series. A peculiar observation is the reappearance of fine structure in the excitation spectrum of Tb~(3+) in LiYF_4. Charge transfer luminescence has been found for Yb~(3+) is many compounds. Based on the results a model for quenching of the CT luminescence by photoionization is discussed. Finally, the long life time of the CT luminescence from Ce~(4+) in Sr_2CeO_4 is explained by the spin-forbidden character of this emission.