Tomography of dispersive media

When waves propagate through layered structures, the phase velocity is frequency dependent (dispersive). If one wants to reconstruct the velocity variations in this medium, conventional traveltime-based tomographic methods cannot be used, since each frequency component has a different traveltime. A tomographic method is presented for reconstructing the phase velocity of guided waves in laterally varying media. The dispersive character of guided waves is explicitly accounted for by using a phase-based error criterium instead of "picked" traveltimes. Phase velocity and source waveform can be reconstructed to within a few percent, and the algorithm is shown to be robust in the presence of interference noise. When applied to seismic field data, the reconstructed phase velocity field correlates well with the topography of the area.     

Structure determination of the two-domain ( 1x4) anatase TiO2(001) surface

The reconstructed anatase TiO2(001) surface has been investigated by low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and angle-resolved mass spectroscopy of recoiled ions (AR-MSRI). Prior investigations have observed or considered only a (1x1) unreconstructed termination for this surface with no detailed structural analysis. Our LEED results indicate a previously unobserved two-domain (1x4) reconstruction after sputtering and annealing the (1x1) surface. The XPS data for this reconstruction indicate the presence of only Ti4+. Simulations of the AR-MSRI experimental data indicate a best fit for a microfaceted surface, revealing both (103) and (1;03) surface planes.     

Intense picosecond X-Ray pulses from laser plasmas by use of nanostructured "Velvet" targets

We describe the optical, radiative, and laser-plasma physics of a new type of nanostructured surface especially promising as a very high absorption target for high-peak-power subpicosecond laser-matter interaction. This oriented-nanowire material, irradiated by 1 ps pulses at intensities up to 10(17) W cm(-2), produces picosecond soft x-ray pulses 50x more efficiently than do solid targets. We compare this to "smoke" or metallic clusters, and solid nanogroove-grating surfaces; the "metal-velvet" targets combine the high yield of smoke targets with the brief emission of grating surfaces.     

Precise positioning of carbon nanotubes by ac dielectrophoresis using floating posts

Single-walled carbon nanotubes (SWNTs) have been precisely aligned and positioned in device architectures using ac dielectrophoresis by patterning floating metal posts or strips within the electrode gaps. These structures perturb the electric field, causing local enhancements in the field intensity, as seen in simulation, that guide the nanotubes along a predictable path in given directions, in zigzag patterns, or as single or a sequence of tubes along a series of posts. This method enables the assembly of SWNTs in complex multi-electrode geometries, when specifying the electrode voltages is not sufficient to direct the desired assembly. The device characteristics of the dielectrophoretically-aligned SWNTs are discussed. PACS 61.46.Fg; 73.63.-b; 87.15.Tt     

State-specific reactions and autoionization dynamics of Ar produced by synchrotron radiation

The long-lived excited states of doubly charged rare gases can markedly affect their reactivity. In this paper we demonstrate the presence of strong state-specific effects in the charge exchange of Ar²⁺ (³P, ¹D and ¹S) with several neutral targets (He, Ne, Kr, Xe, D₂, and CH₄). State sensitive measurements have been performed by producing the different Ar²⁺ electronic states via tunable synchrotron radiation (Elettra-Trieste, Italy and SuperACO-Orsay, France). From the product ion yield data of charge transfer, state-selected total cross-sections have been deduced. Using the state-specific reactivity of Ar²⁺ towards different neutral targets, it has been possible to extract the photon-energy-dependent production branching of the three doubly charged states and to investigate the autoionization dynamics of neutral or singly charged Ar in the vicinity of the double ionization threshold.     

A fast algorithm for solving a linear feasibility problem with application to Intensity-Modulated Radiation Therapy

The goal of Intensity-Modulated Radiation Therapy (IMRT) is to deliver sufficient doses to tumors to kill them, but without causing irreparable damage to critical organs. This requirement can be formulated as a linear feasibility problem. The sequential (i.e., iteratively treating the constraints one after another in a cyclic fashion) algorithm ART3 is known to find a solution to such problems in a finite number of steps, provided that the feasible region is full dimensional. We present a faster algorithm called ART3+. The idea of ART3+ is to avoid unnecessary checks on constraints that are likely to be satisfied. The superior performance of the new algorithm is demonstrated by mathematical experiments inspired by the IMRT application.     

Pyranocyclopropanyl sugar amino acids,a new class of constrained (di)peptide isosteres

The design, synthesis and application of oxabicyclo[4.1.0]heptane amino acids as conformationally restricted sugar amino acid dipeptide isosteres are reported.