A new simple model for high-power pulsed gas lasers

An analysis of a modified electrode geometry for pulsed gas-laser excitation circuits, generating shorter excitation pulses than those normally obtained, is reported. Results from an atmospheric N₂ laser obtained with this electrode geometry are compared with others available in the literature.Paper partially presented at the XIII Intern. Quantum Electronics Conf., Los Angeles, CA, USA (1984)     

Alpha particle bias ionization in a pulsed nitrogen laser

This work reports the role of alpha particles generated by Americium (²⁴¹Am) stripes placed inside the discharge channel in providing a bias ionized background plasma before, during, and after the discharge of the N₂ TE UV laser (337.1 nm) circuit. The enhancement in stimulated radiation output characteristics in terms of gas pressure, charging voltage, and pulse width, with and without alpha particles, are shown. The increased laser yield is interpreted qualitatively through plasma impedance in the discharge circuit. PACS 42.60.By; 42.60.Lh     

Photolysis of methane revisited at 121.6 nm and at 118.2 nm: quantum yields of the primary products, measured by mass spectrometry

Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH(2) and CH(3) fragments. The radical products, CH(3)(X), CH(2)(X), CH(2)(a) and C((1)D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH(2) and CH(3) radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH(2)(X) and CH(2)(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH(3)(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH(3)(X) and CH(2)(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1, 171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: Φ[CH(3)(X)] = 0.42 ± 0.05, Φ[CH(2)(a)] = 0.48 ± 0.05, Φ[CH(2)(X)] = 0.03 ± 0.08, Φ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate Φ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH(2)(X) fragment: Φ[CH(3)(X)] = 0.26 ± 0.04, Φ[CH(2)(a)] = 0.17 ± 0.05, Φ[CH(2)(X)] = 0.48 ± 0.06, Φ[CH(X)] = 0.09 ± 0.01, Φ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.     

Rational design of polymer colloids

Recent advances in understanding of the fundamental mechanistic events in emulsion polymerization give the potential for rational design of new materials based on polymer colloids. It is now possible to design a new industrial process from first principles, based on well‐understood mechanistic principles. An overview of recent developments in the fundamental science of emulsion polymerization is given, with examples of the application of this knowledge to topologically‐controlled synthesis of novel materials based on natural rubber and polybutadiene seed latexes.     

Analysis of the explosive 2,4,6-trinitrophenylmethylnitramine (tetryl) in bush bean plants

Previous attempts to delineate the metabolism of the explosive 2,4,6-trinitrophenylmethylnitramine (tetryl) in plants have been unsuccessful. Development of an appropriate analytical methodology has been thwarted due to the extreme thermal and base lability of tetryl as well as its propensity to undergo photodecomposition. This study presents a methodology based on solvent extraction of plant tissue followed by fractionation of the organic extract on silica gel with subsequent determination of tetryl by HPLC. This methodology allowed 82.70 ± 5.54% recovery of tetryl from fortified bush bean leaves. The developed methodology was applied to study tetryl uptake and metabolism in bush bean plants exposed to tetryl-amended hydroponic cultures.     

Effect of branching and molecular weight on the viscoelastic properties of poly(butyl acrylate)

A series of poly(butyl acrylate) samples were prepared by emulsion polymerization with a range of molecular weights and degrees of chain branching. Characterization was performed with NMR (giving the fraction of branching, ranging from approximately 0 to 7%), gel permeation chromatography, viscometry, and determination of the gel fraction. The dynamic mechanical response, that is, the frequency dependence of the storage and loss moduli G′(ω) and G″(ω) was measured from 0.02 to 200 Hz. The occurrence of a significant insoluble fraction in the sample meant that full characterization of the molecular weight distribution was not possible, and so an unambiguous separation of the dependencies of the mechanical response on the degree of long‐chain branching (LCB) and short‐chain branching (SCB) and the molecular weight could not be made; however, trends dependent on the molecular weight alone were insufficient to model the results. At high frequencies, all trends in G′(ω) and G″(ω) could be ascribed to molecular weight dependencies; at low frequencies, the effects of both the molecular weight and total degree of branching could be inferred, with more highly branched samples showing lower storage and loss moduli. Although the relative amounts of SCB and LCB could not be determined, no dynamic features attributable to LCB were observed. The low‐frequency trends could be semiquantitatively fitted with reptation and retraction theory if it was assumed that an increased degree of SCB led to an increased tube size. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3335–3349, 2002     

Towards fully multivariate algorithmics: Parameter ecology and the deconstruction of computational complexity

The aim of this article is to motivate and describe the parameter ecology program, which studies how different parameters contribute to the difficulty of classical problems. We call for a new type of race in parameterized analysis, with the purpose of uncovering the boundaries of tractability by finding the smallest possible parameterizations which admit FPT-algorithms or polynomial kernels. An extensive overview of recent advances on this front is presented for the Vertex Cover problem. Moving even beyond the parameter ecology program we advocate the principle of model enrichment, which raises the challenge of generalizing positive results to problem definitions with greater modeling power. The computational intractability which inevitably emerges can be deconstructed by introducing additional parameters, leading towards a theory of fully multivariate algorithmics.     

Potentiometrie

Ohne Zusammenfassung     

The cavity-enhanced absorption spectrum of NH3 in the near-infrared region between 6850 and 7000 cm

In this paper, we describe new high-resolution measurements of the absorption spectrum of ¹⁴NH₃ in the 6850-7000 cm⁻¹ region using cavity-enhanced absorption spectroscopy (CEAS), and Fourier-transform spectroscopy (FTS) between ∼6400 and 6900 cm⁻¹. The CEAS measurements were used to determine line positions, line intensities (cross-sections) and pressure-broadening parameters, the latter in three different bath gases. A total of 1117 NH₃ lines were observed. The accuracy of the line positions is about 0.001 cm⁻¹, and absorptions cross-sections as low as 1×10⁻²³ cm² molecule⁻¹ are reported.