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We present a detailed investigation of the dynamics of impurity-trapped excitons in CaF 2 :Yb 2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing a UV optical parametric amplifier (OPA) synchronized to an infrared free electron laser (FEL). The temporal behaviour is well approximated by a multi-level rate equation model as relaxation between excited states of the exciton as well as a small contribution from local lattice heating by the FEL which becomes evident due to the 40 cm −1 splitting of the exciton excited states giving rise to the transient photoluminescence enhancement itself.

Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystal scintillation through their fast d–f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators. We report high-resolution X-ray-excited optical (IR to UV) luminescence spectra of CaF2:Yb and SrF2:Yb samples excited at 8949 eV and 80 K. Ionizing radiation excites the known anomalous emission of ytterbium in the CaF2 host but not in the SrF2 host. Wave function-based ab initio calculations of host-to-dopant electron transfer and Yb2+/Yb3+ intervalence charge transfer explain the difference. The model also explains the lack of anomalous emission in Yb-doped SrF2 excited by VUV radiation.

The relaxation pathways of states in the energy region of the fundamental absorption of NaMgF3:Yb2+ and MgF2:Yb2+ are investigated using time-resolved vacuum ultraviolet spectroscopy. For NaMgF3:Yb2+, excitation into intrinsic free exciton states or above the band gap results in emission features associated with self-trapped excitons and impurity-trapped excitons. Excitation into host-related states of MgF2:Yb2+ similarly exhibits self-trapped exciton emission as well as emission from Yb2+ 4f135d states. The excitation features related to Yb2+ 4f14→4f135d transitions are interpreted using semi-empirical crystal field models. For both materials, the interplay between intrinsic distortions and Yb impurity centres, as the excitation wavelength and sample temperature are varied, is presented and analysed. © 2019 Elsevier B.V.

Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystal scintillation through their fast d-f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators. We report high-resolution X-ray-excited optical (IR to UV) luminescence spectra of CaF

Results of a vacuum ultraviolet spectroscopic characterization of NaMgF3:Yb2+ are presented. The material demonstrates emission features associated with self-trapped excitons and impurity-trapped excitons. The emission features noticeably overlap giving rise to a broad emission band from 17 000 to 35 000 cm−1 at a sample temperature of 8 K. To identify the true profiles of the emission features we have used a deconvolution procedure. The deconvolution was possible due to the thermal quenching of self-trapped excitons at room temperature that allowed for direct observations of the impurity trapped exciton emission band. Energy transfer between host electronic excitations (excitons and e–h pairs) and Yb2+ ions leading to the formation of impurity-trapped excitons is evident from excitation spectra.

Luminescence spectroscopic and temporal dynamic properties of high energy elementary excitations in CdF2-CaF2 superlattices have been studied utilising excitation with vacuum ultraviolet and X-ray synchrotron radiation while comparing the results with those obtained for CdF2 and CaF2 bulk crystals. It is shown that the optical properties of the superlattice structures are determined by exciton emission in the CdF2 monolayers. The experimental manifestations of exciton confinement phenomena are discussed. © 2016 AIP Publishing LLC.

Y3Al5O12 (YAG) transparent ceramics doped with Pr3+ have been obtained by vacuum sintering of spray dried commercial powders and characterized by XRD and SEM techniques. A comprehensive spectroscopic investigation has then been carried out including: absorption spectra and decay time measurements in the visible-IR spectral region and x-ray and VUV excitation and emission properties studied with synchrotron radiation. The main processes responsible for the excited states dynamics have been identified and characterized. Comparison with the properties of the single crystal reveals that the investigated material has interesting perspectives for applications in optics and photonics.

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${}^{\ensuremath{-}1}$ (448 nm) and 24000 cm${}^{\ensuremath{-}1}$ (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${}^{\ensuremath{-}1}$.

Ytterbium-doped wide-bandgap fluoride crystals CaF2, SrF2 and NaMgF3 have been measured using x-ray absorption near edge structure (XANES) on the L3 edge to determine the ratio of trivalent to divalent Yb ions present in the crystals. This study improves upon previous XANES measurements of dopant ion valency by taking into account the x-ray emission transition probabilities for the divalent and trivalent species instead of simply assuming that the relative concentrations may be determined by the ratio of the x-ray excitation band areas. Trivalent to divalent ratios as high as 5 are inferred even at low total dopant ion concentrations of 0.05 mol% Yb.