We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF 2 :Yb 2+ . The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation. 14,15 Recent ab initio calculations have given valuable insight into the quantum physics of exciton formation.9 However, the broad bands provide no detailed information, and experimental information on the energy-level structure of impurity-trapped excitons is largely deduced from indirect measurements such as temperature dependencies, 16,17 pressure dependencies, 12 and photoconductivity. 18In this Brief Report, we report on an investigation of the internal structure of impurity-trapped excitons using twofrequency measurements of single-crystal CaF 2 doped with Yb 2+ . By applying IR radiation to the crystal after exciting it in the UV, we induce transitions between exciton states. Since some of the exciton-excited states have much higher radiative rates than the lowest exciton state, we can detect the excited state absorption by monitoring photoluminescence enhancement.CaF 2 :Yb 2+ crystals were grown using the vertical Bridgmann technique. The UV component of our two-frequency excitation was from a Quantronix TOPAS traveling-wave optical parametric amplifier (OPA) providing 3 ps pulses tunable in the 250-400 nm region of interest in this work at a repetition rate of 1 kHz. Pulsed infrared excitation was achieved using the Dutch free electron laser (FEL) FELIX in Nieuwegein. The IR output of FELIX consists of a 4-6 μs macropulse at a repetition rate of 10 Hz, containing micropulses at 25 MHz. FELIX is continuously tunable from 3 to 250 μm. The OPA was synchronized to the FEL, and the electronic timing between the two lasers could be varied. The UV and IR beams were spatially (but not temporally) overlapped on the sample, held at cryogenic temperatures within an Oxford instruments microstat helium flow cryostat. Visible fluorescence was detected using a TRIAX 320 spectrometer equipped with a C31034 photomultiplier. Our results for 365-nm-pulsed UV excitation of CaF 2 :Yb 2+ are consistent 113110-1 1098-0121/2011/84(11)/113110(4)

The excited-state structure of impurity-trapped excitons are measured in a multisite system. We use a two-color (UV-IR) pulsed photoluminescence enhancement technique, which probes the interlevel transitions and dynamics of impurity-trapped excitons in doped insulating phosphor materials. The technique is applied to NaMgF 3 :Yb 2+ , which exhibits emission from two charge-compensation centers with peaks at 22300 cm −1 (448 nm) and 24000 cm −1 (417 nm). The observed photoluminescence enhancement is caused by a combination of intraexcitonic excitation and electron trap liberation. The electron traps are inferred to have a depth of approximately 800 cm −1 .

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.

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.

We model the dynamic behaviour observed for impurity-trapped excitons in SrF 2 :Yb 2þ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF 2 . The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states. V C 2015 AIP Publishing LLC. [http://dx