Impurity-trapped excitons and electron traps in CaF$_2$:Yb$^{2+}$ and   SrF$_2$:Yb$^{2+}$ probed by transient photoluminescence enhancement
  CaF$_2$:Yb$^{2+}$ and SrF$_2$:Yb$^{2+}$ crystals have been investigated by a two-color UV + IR transient photoluminescence enhancement technique. The enhancement gives information about both changes in internal energy levels of the excitons and liberation of electrons from traps in the crystals. 
P. S. Senanayake, J. P. R. Wells, M. F. Reid, G. Berden, A. Meijerink   and R. J. Reeves
cond-mat.mtrl-sci physics.chem-ph