Generation and measurement of 200 femtosecond optical pulses

Pulses as short as 0.2 ps have been obtained from a passively mode locked dye laser. The technique is simpler than those previously used, but one must pay the price of a loss of tunability. The pulse durations were determined from their second order correlation functions measured with a highly stable interferometer which is described.     

Excitonic effects and optical spectra of single-walled carbon nanotubes

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an ab initio many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by approximately 1 eV in the semiconducting (8,0) tube and by approximately 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.     

Quasiparticle energies,excitonic effects and optical absorption spectra of small-diameter single-walled carbon nanotubes

We present a first-principles study of the effects of many-electron interactions on the optical properties of single-walled carbon nanotubes. Motivated by recent experiments, we have carried out ab initio calculations on the single-walled carbon nanotubes (3, 3), (5, 0) and (8, 0). The calculations are based on a many-body Greens function approach in which both the quasiparticle (single-particle) excitation spectrum and the optical (electron–hole excitation) spectrum are determined. We show that the optical spectrum of both the semiconducting and metallic nanotubes studied exhibits important excitonic effects due to their quasi-one-dimensional nature. Binding energies for excitonic states range from zero for the metallic (5, 0) tube to nearly 1 eV for the semiconducting (8, 0) tube. Moreover, the metallic (3, 3) tube possesses exciton states bound by nearly 100 meV. Our calculated spectra explain quantitatively the observed features found in the measured spectra. PACS 78.67.Ch; 71.35.Cc; 73.22.-f     

Gas and Liquid Phase Imaging of Foam Flow Using Pure Phase Encode Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a non-invasive and non-optical measurement technique, which makes it a promising method for studying delicate and opaque samples, such as foam. Another key benefit of MRI is its sensitivity to different nuclei in a sample. The research presented in this article focuses on the use of MRI to measure density and velocity of foam as it passes through a pipe constriction. The foam was created by bubbling fluorinated gas through an aqueous solution. This allowed for the liquid and gas phases to be measured separately by probing the ¹H and ¹⁹F behavior of the same foam. Density images and velocity maps of the gas and liquid phases of foam flowing through a pipe constriction are presented. In addition, results of computational fluid dynamics simulations of foam flow in the pipe constriction are compared with experimental results.     

An Inverted‐Sandwich Diuranium μ‐η5:η5‐Cyclo‐P5 Complex Supported by U‐P5 δ‐Bonding

Reaction of [U(Tren^TIPS)] [ 1 , Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃] with 0.25 equivalents of P₄ reproducibly affords the unprecedented actinide inverted sandwich cyclo‐P₅ complex [{U(Tren^TIPS)}₂(μ‐η⁵:η⁵‐cyclo‐P₅)] ( 2 ). All prior examples of cyclo‐P₅ are stabilized by d‐block metals, so 2 shows that cyclo‐P₅ does not require d‐block ions to be prepared. Although cyclo‐P₅ is isolobal to cyclopentadienyl, which usually bonds to metals via σ‐ and π‐interactions with minimal δ‐bonding, theoretical calculations suggest the principal bonding in the U(P₅)U unit is polarized δ‐bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo‐P₅ unit to give a cyclo‐P₅ charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo‐P₅ compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium δ‐symmetry 5f orbitals.     

A particle‐in‐cell method for the solution of two‐layer shallow‐water equations

A particle‐in‐cell (PIC) numerical method developed for the study of shallow‐water dynamics, when the moving fluid layer is laterally confined by the intersection of its top and bottom surfaces, is described. The effect of ambient rotation is included for application to geophysical fluids, particularly open‐ocean buoyant vortices in which the underlying density interface outcrops to the surface around the rim of the vortex. Extensions to include the dynamical effect of a second moving layer (baroclinicity) and the presence of a lateral rigid boundary (sidewall) are also described. Although the method was developed for oceanographic investigations, applications to other fluid mechanics problems would be straightforward. Copyright © 2000 John Wiley & Sons, Ltd.     

Total synthesis of the cytotoxic cyclopeptide mollamide, isolated from the sea squirt Didemnum molle

Full details of a total synthesis of the novel reverse prenyl substituted cyclic peptide mollamide, isolated from the ascidian Didemnum molle, are described.     

Furo[3,2‐b]pyridine: A Privileged Scaffold for Highly Selective Kinase Inhibitors and Effective Modulators of the Hedgehog Pathway

Reported is the identification of the furo[3,2‐b]pyridine core as a novel scaffold for potent and highly selective inhibitors of cdc‐like kinases (CLKs) and efficient modulators of the Hedgehog signaling pathway. Initially, a diverse target compound set was prepared by synthetic sequences based on chemoselective metal‐mediated couplings, including assembly of the furo[3,2‐b]pyridine scaffold by copper‐mediated oxidative cyclization. Optimization of the subseries containing 3,5‐disubstituted furo[3,2‐b]pyridines afforded potent, cell‐active, and highly selective inhibitors of CLKs. Profiling of the kinase‐inactive subset of 3,5,7‐trisubstituted furo[3,2‐b]pyridines revealed sub‐micromolar modulators of the Hedgehog pathway.