J. O. Cross

End-to-End Stacking and Liquid Crystal Condensation of 6– to 20–Base Pair DNA Duplexes

Short complementary B-form DNA oligomers, 6 to 20 base pairs in length, are found to exhibit nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal ordering. Structural study shows that these phases are produced by the end-to-end adhesion and consequent stacking of the duplex oligomers into polydisperse anisotropic rod-shaped aggregates, which can order into liquid crystals. Upon cooling mixed solutions of short DNA oligomers, in which only a small fraction of the DNA present is complementary, the duplex-forming oligomers phase-separate into liquid crystal droplets, leaving the unpaired single strands in isotropic solution. In a chemical environment where oligomer ligation is possible, such ordering and condensation would provide an autocatalytic link whereby complementarity promotes the extended polymerization of complementary oligomers.

Multielement spectrometer for efficient measurement of the momentum transfer dependence of inelastic x-ray scattering

Nonresonant x-ray Raman scattering (XRS) is the inelastic scattering of hard x rays from the K shell of low-Z elements or the less tightly bound shells of heavier elements. In the limit of low momentum transfer q, XRS is determined by the same transition matrix element as is measured by x-ray absorption spectroscopies. However, XRS at higher q can often access higher order multipole transitions which help separate the symmetry of various contributions to the local density of states. The main drawback of XRS is its low cross section—a problem that is compounded for a q-dependent study. To address this issue, we have constructed a multielement spectrometer to simultaneously measure XRS at ten different values of q. By means of example, we report new measurements of the XRS from the L- and K-edges of Mg. This instrument is now available to general users at the Advanced Photon Source as the lower energy resolution inelastic x-ray scattering (LERIX) spectrometer.

Local electronic structure of dicarba-closo-dodecarboranes C2B10H12.

We report nonresonant inelastic X-ray scattering (NRIXS) measurement of core-shell excitations from both B 1s and C 1s initial states in all three isomers of the dicarba-closo-dodecarboranes C2B10H12. First, these data yield an experimental determination of the angular-momentum-projected final local density of states (l-DOS). We find low-energy resonances with distinctive local s- or p-type character, providing a more complete experimental characterization of bond hybridization than is available from dipole-transition limited techniques, such as X-ray absorption spectroscopies. This analysis is supported by independent density functional theory and real-space full multiple scattering calculation of the l-DOS which yield a clear distinction between tangential and radial contributions. Second, we investigate the isomer sensitivity of the NRIXS signal and compare and contrast these results with prior electron energy loss spectroscopy measurements. This work establishes NRIXS as a valuable tool for borane chemistry, not only for the unique spectroscopic capabilities of the technique but also through its compatibility with future studies in solution or in high-pressure environments. In addition, this work also establishes the real-space full multiple scattering approach as a useful alternative to traditional approaches for excited states calculations of aromatic polyhedral boranes and related systems.

Fine structure and chemical shifts in nonresonant inelastic x-ray scattering from Li-intercalated graphite

The authors report measurements of hard x-ray nonresonant inelastic x-ray scattering (IXS) from the Li and C 1s electrons of fully staged LiC6 Li-intercalated graphite prepared by both chemical and electrochemical methods. They find that the Li 1s orbital shifts to higher energies relative to Li metal. Relative to graphite, the C 1s IXS for LiC6 shows a shift for the σ-orbital threshold to lower energies, but no shift for the π* resonance. The findings provide bulk-sensitive evidence for substantial charge transfer from the Li intercalant to the carbon host and establish important groundwork for future in situ electrochemical studies.

XAFS at the Pacific Northwest Consortium-Collaborative Access Team undulator beamline

The Pacific Northwest Consortium-Collaborative Access Team (PNC-CAT) has begun operating an insertion device beamline at the Advanced Photon Source. The beamline has been extensively used for XAFS studies. This paper summarizes its capabilities, and our initial operational experience. The beamline is based on APS undulator A, and incorporates full undulator scanning. The monochromator is liquid nitrogen cooled and has both Si(111) and Si(311) crystals in a side-by-side configuration. Crystal changes only take a few minutes. The crystals cover the energy range from 3-50 keV with fluxes as high as 2x10(13) ph/sec. Microbeams can be produced using Kirkpatrick-Baez mirrors (spot size 1-3 microm) or tapered capillaries (sub-microm spots). When these optics are combined with a 13-element Ge detector, the beamline provides powerful microbeam imaging and spectroscopy capabilities. Experimental examples from the environmental field and in-situ UHV film growth will be discussed.

Deconvolving instrumental and intrinsic broadening in core-shell x-ray spectroscopies

Intrinsic and experimental mechanisms frequently lead to broadening of spectral features in core-shell spectroscopies. For example, intrinsic broadening occurs in x-ray absorption spectroscopy XAS measurements of heavy elements where the core-hole lifetime is very short. On the other hand, nonresonant x-ray Raman scattering XRS and other energy loss measurements are more limited by instrumental resolution. Here, we demonstrate that the Richardson-Lucy RL iterative algorithm provides a robust method for deconvolving instrumental and intrinsic resolutions from typical XAS and XRS data. For the K-edge XAS of Ag, we find nearly complete removal of 9.3 eV full width at half maximum broadening from the combined effects of the short core-hole lifetime and instrumental resolution. We are also able to remove nearly all instrumental broadening in an XRS measurement of diamond, with the resulting improved spectrum comparing favorably with prior soft x-ray XAS measurements. We present a practical methodology for implementing the RL algorithm in these problems, emphasizing the importance of testing for stability of the deconvolution process against noise amplification, perturbations in the initial spectra, and uncertainties in the core-hole lifetime.

Background proportional enhancement of the extended fine structure in nonresonant inelastic x-ray scattering.

We report new measurements and calculations of the nonresonant inelastic x-ray scattering (NRIXS) from Mg and Al for a wide range of momentum transfers, q. Extended oscillations in the dynamic structure factor S(q,omega) due to scattering from the 2p and 2s orbitals (i.e., L edges) are observed out to more than 150 eV past the binding energy. These results are discussed in context of the recently proposed representation of S(q,{omega}) for core shells as an atomic background modulated by interference between different photoelectron scattering paths, in analogy to the standard treatment of extended x-ray absorption fine structure. In agreement with this representation, we find a strong increase in the atomic background with increasing q with a concomitant enhancement in the amplitude of the extended fine structure. This effect should be generic and hence may enable improved measurement of the extended fine structure in a wide range of materials containing low-Z elements.

The local electronic structure of α-Li3N

New theoretical and experimental investigations of the occupied and unoccupied local electronic densities of states (DOS) are reported for alpha-Li(3)N. Band-structure and density-functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS find less extreme nominal valences than have previously been proposed. Nonresonant inelastic x-ray scattering, RSFMS calculations, and calculations based on the Bethe-Salpeter equation are used to characterize the unoccupied electronic final states local to both the Li and N sites. There is a good agreement between experiment and theory. Throughout the Li 1s near-edge region, both experiment and theory find strong similarities in the s-and p-type components of the unoccupied local final DOS projected onto an orbital angular momentum basis (l-DOS). An unexpected, significant correspondence exists between the near-edge spectra for the Li 1s and N 1s initial states. We argue that both spectra are sampling essentially the same final DOS due to the combination of long core-hole lifetimes, long photoelectron lifetimes, and the fact that orbital angular momentum is the same for all relevant initial states. Such considerations may be generally applicable for low atomic number compounds.

Exciton spectroscopy of hexagonal boron nitride using non-resonant x-ray Raman scattering.

We report non-resonant x-ray Raman scattering (XRS) measurements from hexagonal boron nitride for transferred momentum from 2 to 9

Direct determination of epitaxial film and interface structure: Gd2O3 on GaAs (1 0 0)

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