Search

To obtain a fundamental understanding of the population behaviour ofAcidithiobacillus ferrooxidansat chalcopyrite and pyrite surfaces, the early stage attachment behaviour and biofilm formation by this bacterium on chalcopyrite (CuFeS2) and pyrite (FeS2) were studied by optical microscopy, Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and electron backscatter diffraction (EBSD). The results indicate there was no significant difference in selectivity of bacterial attachment between chalcopyrite and pyrite. However, the result of ToF-SIMS analysis suggests that the surface of the pyrite was covered more extensively by biofilm than that of the chalcopyrite, which may indicate more extracellular polymeric substances (EPS) formation by bacterial cells growing on pyrite. EBSD and optical image analysis indicated that selectivity of bacterial attachment to chalcopyrite was not significantly affected by crystal orientation. The results also suggest that the bacterial population in defective areas of chalcopyrite was significantly higher than on the polished surfaces. [ABSTRACT FROM AUTHOR]

Sulfide minerals contain sulphur in a large variety of coordination environments. Consequently, the S 2p XPS of various mineral surface states undergo different shifts in binding energy (BE) relative to the bulk, depending on the charge distribution on the surface. This in turn depends on the number, type and position of the atoms on the fracture surface, which is determined by the fracture mechanism. We have investigated three sulfide minerals: pyrite (tetrahedrally-coordinated S), chalcopyrite (tetrahedrally-coordinated S) and molybdenite (layered structure with trigonally-coordinated S). Comparison of conventional with surface sensitive synchrotron XPS shows that the S 2p spectrum displays two additional doublets at lower BE than the bulk signal for pyrite, and one doublet each at lower and at higher BE for chalcopyrite. Each of these signals is derived from surface states. Molybdenite shows no additional states. A BE shift to lower (higher) BE suggests a charge increase (decrease) on the S atoms relative to those in the bulk because of higher (lower) charge screening. We have used ab initio density functional calculations to validate this interpretation of the experimental evidence, obtaining Mulliken population analyses for the possible fracture surfaces and comparing their charge distribution with the corresponding bulk charge distribution. Our calculations support the assignments of S 2p surface contributions as follows: the lower BE peak of chalcopyrite (160.84 eV) as under-coordinated surface S states, the higher BE peak of chalcopyrite (161.88 eV) as surface S polymers, the lowest BE peak of pyrite (161.3 eV) as surface S monomers, and the next lowest BE peak of pyrite (162.0 eV) as under-coordinated surface S dimers. The absence of any surface states in molybdenite is also confirmed by the models. [ABSTRACT FROM AUTHOR]

A simple and general method of using a photon detector to directly distinguish between two or three different polarizations is described. The method is suitable for a pre-selected limited wavelength region, for example as with encoded fibre optic communication. Alternatively, wavelength discrimination via wavelength selective polarization is possible. [ABSTRACT FROM AUTHOR]

A variant route to lock-in amplifier detection of optical emission from radio pulsars is considered. The method uses the radio pulsar frequency signals as a timing clock but splits the optical signals into equal portions of one third the pulsar period. Differences in their intensities offer evidence for an optical signal at the same frequency as the pulsar without the need to know the phase differences between optical and radio signals. The route benefits from sensitivity to pulsed optical signals seen against a steady background and also allows detection of periodic reductions in intensity. The method is insensitive to changes in observational conditions. When used to acquire star images with the viewing field phase split onto three detector areas, the phase block method should provide intensity differences which identify the location of an optical signal with the frequency of the radio pulsar. Supportive data for the method from a simple laboratory based proof of concept experiment are cited. The three phase block route can equally be applied to searches for polarized emission. [ABSTRACT FROM AUTHOR]

Laser surface patterning has recently been used to form black silicon. For photon energies above the band gap energy (1.07 eV corresponding to a wavelength of 1.1 µm) the origin of the very low refiectivity can be accurately modelled as arising from enhanced absorptance from the microstructure on the silicon. However, at lower photon energies the decreased reflectance cannot be attributed to the structured surface alone. For these lower photon energies an excellent theoretical description of the experimentally observed absorptance can be achieved by the addition of an increased imaginary part of the complex permittivity. This term increases linearly with wavelength in the energy range 1.07 eV down to 0.496 eV (corresponding to a wavelength of 2.5 µm). The implication is that the structuring has introduced a wide range of new surface and sub-surface states (potentially from impurities) which absorb the light. [ABSTRACT FROM AUTHOR]