Fluorescence-yield Spectroscopy Research Articles

How microbial biofilms impact the interactions of Quantum Dots with mineral surfaces?

The increasing use of Quantum Dots (QDs) - nanoparticles exhibiting unique optical properties – and their incorporation in multiple engineering products is likely to result in the release of this new class of contaminants into natural systems. In soils, bacterial biofilms and mineral surfaces form highly reactive interfaces, which may control QDs' environmental fate. However, little is known regarding QDs' stability in, and modes of interactions with, biofilm/mineral interfaces. This study examines the interactions, distributions and stability of thioglycolic acid-capped CdSe/ZnS QDs at the corundum (α-Al2O3)/Shewanella oneidensis MR-1 interface, for exposure times ranging between 1 h to 24 h. Long Period – X-ray Standing Wave – Fluorescence Yield spectroscopy and Grazing Incidence – X-ray Absorption Spectroscopy were used. Results indicate increases in Zn and Se concentrations within the biofilm/crystal system with time, demonstrating its high accumulation capacity over 24 h. In addition, dissolution of a part of the ZnS shell occurs within 1 h, highlighting the potential degradation of QDs when exposed to the biofilm/crystal compartment. Once released, Zn(II) migrates toward the biofilm-crystal interface and interacts preferentially with the crystal surface. In contrast, the remaining CdSe core is mostly preserved, and stays within the biofilm thickness. However, at 24 h, Se and Zn present similar distribution profiles indicating a general reduction in ZnS shell dissolution at this longer exposure time.

Application of microprobe soft X-ray fluorescence and absorption spectroscopic analyses to characterize the buried multi-layered micro-structure

We have applied a recently developed scanning soft X-ray (SX) spectromicroscope to characterize the buried multi-layered micro-structure by means of microprobe X-ray fluorescence (μXRF) analyses. We could successfully visualize single element maps of O and Fe contained in a buried 500 μm square test-pattern made of SiO2 and Ni81Fe19 (permalloy) beneath 30 nm thick Au coating. For their chemical-state analyses, we measured O Kα and Fe Lα X-rays emitted from the buried micro-structure using a SX microprobe to explore the X-ray absorption spectroscopy spectra of O and Fe contained in a local area by partial fluorescence yield spectroscopy.

Soft X-ray refractive index by reconciling total electron yield with specular reflection: experimental determination of the optical constants of graphite.

The complex refractive index of many materials is poorly known in the soft X-ray range across absorption edges. This is due to saturation effects that occur there in total-electron-yield and fluorescence-yield spectroscopy and that are strongest at resonance energies. Aiming to obtain reliable optical constants, a procedure that reconciles electron-yield measurements and reflection spectroscopy by correcting these saturation effects is presented. The procedure takes into account the energy- and polarization-dependence of the photon penetration depth as well as the creation efficiency for secondary electrons and their escape length. From corrected electron-yield spectra the absorption constants and the imaginary parts of the refractive index of the material are determined. The real parts of the index are subsequently obtained through a Kramers-Kronig transformation. These preliminary optical constants are refined by simulating reflection spectra and adapting them, so that measured reflection spectra are reproduced best. The efficacy of the new procedure is demonstrated for graphite. The optical constants that have been determined for linearly polarized synchrotron light incident with p- and s-geometry provide a detailed and reliable representation of the complex refractive index of the material near π- and σ-resonances. They are also suitable for allotropes of graphite such asgraphene.

Detection of thePe1Series of Doubly Excited Helium States belowN=2via the Stark Effect

The Stark effect on the doubly excited states of helium below the N=2 threshold has been studied by vacuum ultraviolet fluorescence yield spectroscopy. Two new series of states are observed at moderate fields (<10 kV/cm), and assigned to the previously unobserved even 1pe series, and a group of 1De series. The 1Se states are observed indirectly via their mixing with nearby 1 po states. The observations at moderate field contradict theoretical predictions that field strengths about an order of magnitude greater are necessary to observe the Stark effect on He doubly excited states at low quantum numbers.

Mn valence instability inLa2∕3Ca1∕3MnO3thin films

A Mn valence instability on ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films, grown on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (001) substrates is observed by x-ray absorption spectroscopy at the Mn $L$-edge and O $K$-edge. As-grown samples, in situ annealed at $800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, ${M}_{S}$, of the films is detected. Simultaneously a ${\mathrm{Mn}}^{2+}$ spectral signature develops, in addition to the expected ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence yield spectroscopy indicates that the location of the Mn valence anomalies is not confined to a narrow surface region of the film, but can extend throughout the whole thickness of the sample. High temperature annealing at $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the ${\mathrm{Mn}}^{2+}$ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the ${\mathrm{Mn}}^{2+}$ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.

Kβ Resonant X-ray Emission Spectroscopy for Fe, Fe2O3and Fe3O4

The measurements combining K β resonant x-ray emission and partial fluorescence yield spectroscopy were performed for Fe metal, Fe 2 O 3 and Fe 3 O 4 near K absorption edge. Spin states of these materials are studied systematically with considering the origins of the K β' line and of the pre-edge peak. Theoretical calculations for the cluster model of Fe 2 O 3 were performed and reproduce well the experimental emission spectra. Resonant Raman scattering effect both in the absorption edge and in the pre-edge regions was observed and compared with the theory. Chemical effects between metal and its oxides were also discussed.

Excitonic energy transfer in Au doped and undoped Kr solids

Excitons were produced in Au doped and undoped Kr by VUV irradiation. The dependence of exciton self-trapping as well as trapping (at the Au atoms) on excitation energy and sample temperature has been studied via fluorescence yield spectroscopy. An exciton diffusion model describes the measured spectra quantitatively.