Laurent Binet

Defects induced in cerium dioxide single crystals by electron irradiation

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5u2009MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0u2009MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin un-irradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f1 configuration) in a high-symmetry local environment. It is conclude...

Colour centre recovery in yttria-stabilised zirconia: photo-induced versus thermal processes

Abstract The photo-annealing of colour centres in yttria-stabilised zirconia (YSZ) was studied by electron paramagnetic resonance spectroscopy upon UV-ray or laser light illumination, and compared to thermal annealing. Stable hole centres (HCs) were produced in as-grown YSZ single crystals by UV-ray irradiation at room temperature (RT). The HCs produced by 200-MeV Au ion irradiation, as well as the F+-type centres ( centres involving oxygen vacancies) were left unchanged upon UV illumination. In contrast, a significant photo-annealing of the latter point defects was achieved in 1.4-MeV electron-irradiated YSZ by 553-nm laser light irradiation at RT. Almost complete photo-bleaching was achieved by laser irradiation inside the absorption band of centres centred at a wavelength ~550 nm. Thermal annealing of these colour centres was also followed by UV–visible absorption spectroscopy showing full bleaching at 523 K. Colour-centre evolutions by photo-induced and thermally activated processes are discussed on the basis of charge exchange processes between point defects.

EPR of Primitive Organic Matter: A Tool for Astrobiology

Insoluble organic matter (IOM) conserved in ancient sedimentary rocks and in carbonaceous meteorites can reveal valuable information about the origin of Life on Earth and on the birth of the Solar System, respectively. These IOMs are also reference materials for the search for possible organic traces of extinct life on Mars. The combination of continuous-wave and pulsed EPR of the radicals in IOM provided several markers distinguishing these materials and related to their histories. For terrestrial IOM, the EPR linewidth of the radicals is mostly determined by unresolved 1H hyperfine interactions for IOM younger than 2500 million years (H-rich), and by dipolar interactions for older (H-depleted) IOM. The age of very primitive IOM could be estimated through the lineshape, which continuously evolves from Lorentzian to stretched Lorentzian upon ageing due to a change in the dimensionality of the radical spatial distribution. Nuclear spins within or near the radicals and the hyperfine interactions probed by pulsed EPR (through ESEEM and HYSCORE sequences) clearly distinguish meteoritic from terrestrial IOM. Radicals in meteorites are massively enriched in deuterium compared to terrestrial radicals, as a result of specific deuterium enrichment processes in the outer early Solar System. Meteoritic and terrestrial IOMs are also distinguished by the isotropic vs dipolar relative contributions in the 1H hyperfine interactions and by the 13C/1H HYSCORE signal ratio. Strong 31P and 14N HYSCORE signals were detected in terrestrial IOM, which point to possible P and N rich biological precursors. The spin states of the radicals could also be determined either indirectly from the temperature dependence of the EPR intensity or directly by transient nutation spectroscopy. Meteoritic IOM, in addition to S = 1/2 radicals, specifically contains species with either triplet ground state or thermally excited triplet states, which are lacking in terrestrial IOM.