Study of the effect of implantation temperature on the migration behaviour of Xe implanted into glassy carbon.

The effect of implantation temperature on the migration behaviour of xenon (Xe) implanted into glassy carbon and the effect of annealing on radiation damage retained by ion implantation were investigated. Glassy carbon substrates were implanted with 320 keV Xe+ to a fluence of 2 × 1016 cm-2. The implantation process was performed at room temperature (RT) and 100 °C Some of the as-implanted samples were isochronally annealed in vacuum at temperatures ranging from 300 °C to 700 °C in steps of 100 °C for 10 h. The as-implanted and annealed samples were characterized using Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. The RT implanted depth profiles indicated that the migration of Xe towards the surface of glassy carbon was accompanied by a loss of Xe ions. The samples implanted at 100 °C indicated no diffusion or loss of Xe after annealing at 300 °C. However, annealing at temperatures ranging from 400 °C to 700 °C resulted in a slight shift in the Xe profile tail-end towards the bulk of glassy carbon. The diffusion coefficients (D) in the temperature range of 300 °C-700 °C for the RT and 100 °C implanted samples, activation energies (Ea), and pre-exponential factors (Do), were extracted. The values of D ranged from (9.72 ± 0.48) × 10-21 to (1.87 ± 0.09) × 10-20 m2/s with an activation energy of (6.25 ± 0.31) × 10-5 eV for RT implanted samples, and the samples implanted at 100 °C, D ranged from (3.85 ± 0.19) × 10-21 to (6.96 ± 0.34) × 10-20 m2/s with activation energy of (4.10 ± 0.02) × 10-5 eV. The Raman analysis revealed that implantation at the RT amorphised the glassy carbon structure while the samples implanted at 100 °C showed mild damage compared to RT implantation. Annealing of the RT-implanted sample resulted in some recovery of the damaged region as a function of increasing annealing temperature.

Energy dissipation mechanisms in the FCPb light-harvesting complex of the diatom Cyclotella meneghiniana.

Transient absorption spectroscopy has been applied to investigate the energy dissipation mechanisms in the nonameric fucoxanthin-chlorophyll-a,c-binding protein FCPb of the centric diatom Cyclotella meneghiniana. FCPb complexes in their unquenched state were compared with those in two types of quenching environments, namely aggregation-induced quenching by detergent removal, and clustering via incorporation into liposomes. Applying global and target analysis, in combination with a fluorescence lifetime study and annihilation calculations, we were able to resolve two quenching channels in FCPb that involve chlorophyll-a pigments for FCPb exposed to both quenching environments. The fast quenching channel operates on a timescale of tens of picoseconds and exhibits similar spectral signatures as the unquenched state. The slower quenching channel operates on a timescale of tens to hundreds of picoseconds, depending on the degree of quenching, and is characterized by enhanced population of low-energy states between 680 and 710 nm. The results indicate that FCPb is, in principle, able to function as a dissipater of excess energy and can do this in vitro even more efficiently than the homologous FCPa complex, the sole complex involved in fast photoprotection in these organisms. This indicates that when a complex displays photoprotection-related spectral signatures in vitro it does not imply that the complex participates in photoprotection in vivo. We suggest that FCPa is favored over FCPb as the sole energy-regulating complex in diatoms because its composition can more easily establish the balance between light-harvesting and quenching required for efficient photoprotection.