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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.