Luminescence and photoionization processes of Yb2+ in CaF 2, SrF2 and BaF2

Abstract

In this paper, we analyze the fluorescence and the photoconductivity properties of Yb2+ in CaF2, SrF2 and BaF2. We present a new approach to the interpretation of the « anomalous » emission usually observed in such systems by an impurity trapped exciton which is the lowest excited state of the system. We show that the excited « d » electron of Yb2+ is delocalized in the « s » orbitals of next neighbor Ca2+ ions from where it can either autoionize or recombine with Yb3+ giving rise to the red shifted luminescence. The photoconductivity spectra show that when the electron is excited into the higher energy bands it autoionizes without any prior relaxation. The strong temperature dependence of lifetime shows that the exciton level is split into two close levels and the study of the risetime of the decays allow us to calculate the energy of Eu, T2u level and the probability of tunneling from this level into the exciton level.