Insights into molecular chemistry of Chiapas amber using infrared-light microscopy,PIXE/RBS,and sulfur K-edge XANES spectroscopy

Chiapas amber is a natural occurring fossil resin structurally composed of long macromolecule chains with semicrystalline phases associated with both fossil and polymerization process. The most conspicuous characteristic of this fossil polymer is that it preserves ancient organic inclusions. In the present work, PIXE/RBS spectrometry (particle-induced X-ray emission/Rutherford backscattering) were combined with complementary K-edge XANES spectroscopy (X-ray absorption near-edge structure) to identify the amount of sulfur in Chiapas amber. Initially, the amber samples were examined using infrared reflected photomicrography. Amber is transparent to infrared light and so embedded plants and animals are easily visible, showing them in extraordinary detail, as if they were immersed in a water-like solution. The PIXE/RBS data show that the proportion of sulfur in amber is significantly higher than that found in recently formed resins, consistent with the biogeochemical process that transforms the resin into amber during long-term burial in geological deposits. The sulfur K-edge XANES spectra from amber confirm the sulfur abundance and reveal sulfur species in the reduced and intermediate oxidation states in amber. Almost no oxidized sulfur was found, whereas the recent resins show mostly oxidized sulfur fractions. This indicates that labile oxidized sulfur decays during fossilization and resin maturation must occur under conditions of oxygen depletion. The implications of the presence of sulfur in amber for organic preservation is also discussed here. Sulfur compounds work as a polymer additive that promotes intense resin solidification. This restricts the early oxidant-specific biodegradation of the embedded biomatter and, over geological time, provides greater stability against chemical changes.     

A bi‐prism interferometer for hard X‐ray photons

Micro‐fabricated bi‐prisms have been used to create an interference pattern from an incident hard X‐ray beam, and the intensity of the pattern probed with fluorescence from a 30 nm‐thick metal film. Maximum fringe visibility exceeded 0.9 owing to the nano‐sized probe and the choice of single‐crystal prism material. A full near‐field analysis is necessary to describe the fringe field intensities, and the transverse coherence lengths were extracted at APS beamline 8‐ID‐I. It is also shown that the maximum number of fringes is dependent only on the complex refractive index of the prism material.     

Simultaneous XAFS measurements of multiple samples

A four‐channel ionization chamber has been designed, constructed and tested. This ionization chamber allows X‐ray absorption spectra to be collected in transmission from up to four samples simultaneously. This results in spectra that are free of systematic uncertainty in relative energy alignment introduced by scan‐to‐scan stability of the monochromator or of numerical uncertainty associated with a post‐processing alignment algorithm, allowing, in a single shot, an absolute measure of edge shift between four samples of different valence. As four samples can be measured in parallel, the time expended over the course of an experiment to cycle the measurement environment between its rest state and the measurement condition is substantially reduced. The ionization chamber is simple in design and could be implemented at virtually any XAFS beamline with a horizontal fan of radiation such as that provided by a bend magnet or wiggler.     

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.     

Effects of the hyperfine interactions on the decay of the collective nuclear excited states in α-hematite

In this paper the time dependence of the coherent decay of nuclear excited state in an α-⁵⁷Fe₂O₃ single crystal is presented. The experiment was carried out in diffraction geometry. A highly monochromatized and collimated beam of synchrotron radiation was used for the excitation of nuclear levels. Quantum beat spectra taken below and above the (7, 7, 7) pure nucleation reflection of hemative show a characteristic pattern corresponding to the magnetic an quadrupole hyperfine interactions. This work was performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC02-76CH00016.     

The amide oxygen donor. Metal ion coordinating properties of the ligand nitrilotriacetamide. A thermodynamic and crystallographic study

The metal ion coordinating properties of ntam (nitrilotriacetamide) are reported. The protonation constant (pK) for ntam is 2.6 in 0.1 M NaClO(4) at 25 degrees C. Formation constants (log K(1)) in 0.1 M NaClO(4) at 25 degrees C, determined by (1)H NMR and UV-Vis spectroscopy are: Ca(II), 1.28; Mg(II), 0.4; La(III), 2.30; Pb(II), 3.69; Cd(II), 3.78; Ni(II), 2.38; Cu(II), 3.16. The measured log K(1) values for the ntam complexes are discussed in terms of the low basicity of the N-donor, as evidenced by the pK, and the effect of metal ion size on complex stability. The amide O-donors of ntam lead to the stabilization of complexes of large metal ions (Pb(II), Cd(II), La(III), Ca(II)) relative to log K1 for the NH3 complexes, while for small metal ions (Ni(II), Cu(II)) the amide O-donors lead to destabilization. This is discussed in terms of the role of chelate ring size in controlling metal ion size-based selectivity. The structures of [Pb(ntam)(NO3)2]2 (1) and [Ca2(ntam)3(H2O)2](ClO4)4.3H2O (2) are reported. For 1: triclinic, space group P1, a = 7.4411(16), b = 9.0455(19), c = 11.625(3) A, alpha = 69.976(4), beta = 79.591(4), gamma = 67.045(3) degrees, Z = 2, R = 0.0275. For 2: monoclinic, space group P2(1)/c, a = 10.485(2), b = 11.414(2), c = 38.059(8) A, beta = 92.05(3) degrees, Z = 4, R = 0.0634. Structure 1 is dimeric with two Pb atoms linked by bridging O-donors from the two ntam ligands. The coordination sphere consists of one N-donor and 3 O-donors from the ntam ligand, two O-donors from nitrates, and one bridging O-donor. The variation in bond length suggests a stereochemically active lone pair of electrons on the Pb. Structure 2 consists of two Ca(II) ions held together by 3 bridging O-donors from ntam groups. One Ca is 9-coordinate with two ntam ligands present, plus one bridging O-donor from the other Ca(II) ntam complex. The other Ca is 8-coordinate, with a single coordinated ntam, plus two coordinated H2O molecules, and two bridging O-donors from the other half of the complex. The role of M-O=C bond angles in controlling selectivity for metal ions on the basis of their size is discussed.     

Carrier-density-dependent lattice stability in InSb

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.