The observation of electron trap liberation in MgF$_{2}$ doped with   Yb$^{2+}$ using a two-color excitation experiment
  We utilize the optical transitions of Yb$^{2+}$ excited by an ultraviolet optical parametric amplifier to probe electron trap liberation in MgF$_{2}$ via the observation of a photoluminescence enhancement effect induced by a subsequent infrared pulse from a free-electron laser. The temperature dependence of the enhancement suggests that we liberate very shallow traps having a depth of approximately 17 cm$^{-1}$. The observed `trap spectrum' is consistent with a simple model of a Coulomb trap. 
P. S. Senanayake, J. P. R. Wells, M. F. Reid, G. Berden, A. Meijerink,   R. J. Reeves
cond-mat.mtrl-sci physics.chem-ph