Development of a mirror backed volume phase grating with potential for large aperture and high damage threshold

The present paper proposes a new grating concept by which a volume phase grating is placed on top of a dielectric mirror stack. Instead of using a multilayer dielectric grating, wherein the uppermost layer of a thin film mirror is etched to create the desired binary phase grating we suggest to have an upper grating layer of a lower density gelatin-based volume phase grating in either sol–gel or dichromated gelatin. The key benefit is the elimination of the etching step which is the limiting factor in the production of large aperture gratings. We have investigated designs for s- and p-polarization as well as the necessary chemistry that is needed to achieve the required index modulations. Damage testing on dichromated gelatin and aerogel was performed and diffraction efficiencies for various prototypes were measured.     

Low-energy Compton scattering and the polarizabilities of the proton

Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θ^lab _γ = 59^°-155^° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5_stat.±1.3_syst.±0.3_mod.] ^. 10^-4 fm ³ and = [1.2±0.7_stat.±0.3_syst.±0.4_mod.)] ^. 10^-4 fm ³. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] ^. 10^-4 fm ³. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γ_π beyond the t-channel π⁰-exchange. Received: 9 January 2001 / Accepted: 13 February 2001     

Double- π0 photoproduction from the deuteron

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (π⁰π⁰ N) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Δ(1232)-resonance as an intermediate state. Received: 5 June 2000 / Accepted: 4 September 2000     

Search for narrow dibaryon resonances in neutral pion photoproduction from the deuteron

The reaction γd↦π⁰ X has been measured with TAPS at MAMI in the energy range E _γ = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2σ. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m? 2100 MeV/c² were deduced. They are in the range 2-5 μb averaged over the 0.8 MeV energy resolution. Received: 25 October 2000 / Accepted: 24 November 2000     

Photoproduction of neutral pion pairs from the proton

Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 μb at E _γ = 740 MeV. Dalitz plots of m ²(π⁰π⁰) versus m ²(p,π⁰) for seven bins of incident photon energy have been analysed. For E _γ > 610 MeV, a strong contribution of a sequential decay is observed with the Δ(1232)-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate from the D ₁₃(1520) and also the P ₁₁(1440)-resonance. Received: 6 June 2000 / Accepted: 18 August 2000     

Ultraviolet filamentation in air

Ultrashort UV pulses are seen to self-focus in air and form meter length filaments. Various measurements (spectrum, size, conductivity) indicate that the balance of the processes that produce the filament are different in the UV compared to the IR.     

Synthesis and Antimycobacterial Activity of 3-Phenyl-1H-indoles

Tuberculosis has been described as a global health crisis since the 1990s, with an estimated 1.4 million deaths in the last year. Herein, a series of 20 1H-indoles were synthesized and evaluated as in vitro inhibitors of Mycobacterium tuberculosis (Mtb) growth. Furthermore, the top hit compounds were active against multidrug-resistant strains, without cross-resistance with first-line drugs. Exposing HepG2 and Vero cells to the molecules for 72 h showed that one of the evaluated structures was devoid of apparent toxicity. In addition, this 3-phenyl-1H-indole showed no genotoxicity signals. Finally, time-kill and pharmacodynamic model analyses demonstrated that this compound has bactericidal activity at concentrations close to the Minimum Inhibitory Concentration, coupled with a strong time-dependent behavior. To the best of our knowledge, this study describes the activity of 3-phenyl-1H-indole against Mtb for the first time.     

Direct observation of a rho decay of the D13(1520) baryon resonance

The reaction gammap-->pi(+)pi(0)n has been measured at MAMI for photon energies up to 820 MeV. Invariant mass spectra of the particles in the final state (pi(+)n), (pi(0)n), (pi(+)pi(0)) have been determined for several bins of incident photon energy. Differences in pi(+)pi(0) and simultaneously measured pi(0)pi(0) invariant mass distributions are assigned to a rho branch of the D13(1520) nucleon resonance.     

Oligopyrrole macrocycles: receptors and chemosensors for potentially hazardous materials

Oligopyrroles represent a diverse class of molecular receptors that have been utilized in a growing number of applications. Recently, these systems have attracted interest as receptors and chemosensors for hazardous materials, including harmful anionic species, high-valent actinide cations, and nitroaromatic explosives. These versatile molecular receptors have been used to develop rudimentary colorimetric and fluorimetric assays for hazardous materials.