We utilize the optical transitions of Yb²⁺ excited by an ultraviolet optical parametric amplifier to probe electron trap liberation in MgF₂via 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.