Role of radicals in UV-initiated postplasma grafting of poly-ε-caprolactone: an electron paramagnetic resonance study

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Structure of F2(-) centers in K2YF5 and their relation to thermoluminescence below room temperature

X-ray irradiation at 77 K produces signals from several trapped hole and/or electron centers in the electron-paramagnetic-resonance (EPR) spectra of rare-earth-doped K2YF5. Five of the spectral components have a structure typical of a center with electron spin S=1/2 exhibiting a strong hyperfine interaction with two nearly equivalent F-19 nuclei. They are identified as V- or H-type intrinsic-trapped hole centers, having the F-2(-) molecular anion as their core. Three centers are characterized by monoclinic g and F-19 self-hyperfine tensors, i.e., with one principal axis along the crystal's twofold screw axis, the two others are triclinic. Plausible models for these five centers are discussed and via thermal-annealing experiments, in which EPR and thermoluminescence (TL) spectra are simultaneously monitored, their role in the TL processes below room temperature is investigated