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Notes: Exact three-dimensional volume current solutions of the magnetohydrodynamic (MHD) equations are presented. The configurations are infinitely extended along a straight axis and have neither cylindrical nor helical nor mirror symmetry. All field lines lie in planes orthogonal to the axis and are closed around it. The surfaces of constant pressure have elliptical cross sections, whose ellipticity and orientation are arbitrary functions along the axis. © 1995 American Institute of Physics.

Notes: The present work explores the existence of nonaxisymmetric toroidal magnetohydrodynamic equilibria when all lines of force of the magnetic field close after a single revolution about a given magnetic axis. It is assumed that the magnetic axis is a closed curve with arbitrary curvature and zero torsion, implying that it is constrained to lie in a plane surface. In addition, it is assumed that the closed magnetic field lines lie in planes that are orthogonal to the magnetic axis. Subject to these conditions, the existence of toroidal magnetic surfaces, F(r)=const, is explored. The governing equilibrium equations of magnetohydrodynamics place a constraint on the function F(r) in the form of two nonlinear partial differential equations that must be simultaneously satisfied. It is demonstrated that this is not always possible without axisymmetry. Two specific cases where toroidal magnetic surfaces do not exist are identified: (I) isodynamic configurations, which implies that the magnetic field strength is constant on each magnetic surface, and (II) configurations with "bumpy'' surfaces, which implies that the size but not the geometrical shape of the poloidal section containing the closed field lines depends on s. © 1995 American Institute of Physics.

Notes: An enhanced supersonic carbon source produces carbon atoms in their C(3Pj) electronic ground states via laser ablation of graphite at 266 nm. The 30 Hz (40±2) mJ output of a Nd-YAG laser is focused onto a rotating graphite rod with a 1000 mm focal length UV-grade fused silica plano-convex lens to a spot of (0.5±0.05) mm diameter. Ablated carbon atoms are subsequently seeded into helium or neon carrier gas yielding intensities up to 1013 C atoms cm−3 in the interaction region of a universal crossed beam apparatus. The greatly enhanced number density and duty cycle shift the limit of feasible crossed beam experiments down to rate constants as low as 10−11–10−12 cm3 s−1. Carbon beam velocities between 3300 and 1100 m s−1, with speed ratios ranging from 2.8 to 7.2, are continuously tunable on-line and in situ without changing carrier gases by varying the time delay between the laser pulse, the pulsed valve, and a chopper wheel located 40 mm after the laser ablation. Neither electronically excited carbon atoms nor ions could be detected within the error limits of a quadrupole-mass spectrometric detector. Carbon clusters are restricted to ∼10% C2 and C3 in helium, minimized by multiphoton dissociation, and eliminating the postablation nozzle region. © 1995 American Institute of Physics.

Notes: In our laboratory a novel and convenient technique has been developed to generate an intense pulsed cyano radical beam to be employed in crossed molecular beam experiments investigating the chemical dynamics of bimolecular reactions. CN radicals in their ground electronic state 2Σ+ are produced in situ via laser ablation of a graphite rod at 266 nm and 30 mJ output power and subsequent reaction of the ablated species with molecular nitrogen, which acts also as a seeding gas. A chopper wheel located after the ablation source and before the collision center selects a 9 μs segment of the beam. By changing the delay time between the pulsed valve and the choppper wheel, we can select a section of the pulsed CN(X2Σ+) beam choosing different velocities in the range of 900–1920 ms−1 with speed ratios from 4 to 8. A high-stability analog oscillator drives the motor of the chopper wheel (deviations less than 100 ppm of the period), and a high-precision reversible motor driver is interfaced to the rotating carbon rod. Both units are essential to ensure a stable cyanogen radical beam with velocity fluctuations of less than 3%. The high intensity of the pulsed supersonic CN beam of about 2–3×1011 cm−3 is three orders of magnitude higher than supersonic cyano radical beams employed in previous crossed molecular beams experiments. This data together with the tunable velocity range clearly demonstrate the unique power of our newly developed in situ production of a supersonic CN radical beam. This versatile concept is extendible to generate other intense, pulsed supersonic beams of highly unstable diatomic radicals, among them BC, BN, BO, BS, CS, SiC, SiN, SiO, and SiS, which are expected to play a crucial role in interstellar chemistry, chemistry in the solar system, and/or combustion processes. © 1999 American Institute of Physics.

Notes: The cosmic ray simulator consists of a 50 l cylindrical stainless steel chamber. A rotable cold finger milled of a silver (111) monocrystal optimizes heat conductivity and is connected to a programmable, closed cycle helium refrigerator allowing temperature control of an attached silver wafer between 10 and 340 K (±0.5 K). Oil-free ultrahigh vacuum (UHV) conditions of ≈10−10 mbar are provided by a membrane, drag, and cryopump, hence guaranteeing a vacuum system free of any contamination. Ice layers of defined crystal structures and reproducible thickness of (5±1) μm are achieved by depositing gases, e.g., CH4, CD4, CD4/O2, and CH4/O2, with a computer-assisted thermovalve on the cooled wafer. These frosts are irradiated at 10 and 50 K with 7.3 MeV protons and 9 MeV α particles of the compact cyclotron CV28 in Forschungszentrum Jülich up to doses of 150 eV per molecule, i.e., simulating the distribution maximum of galactic cosmic ray particles interacting with primordial matter in space during 0.7×109 yr. During the experiments, gas phase and solid state are monitored for the first time quantitatively on line and in situ by a quadrupole mass spectrometer (QMS) via matrix interval arithmetic and a Fourier transform infrared spectrometer (FTIR) in an absorption-reflection mode at 62.5°.For the first time, a cosmic ray simulator allows detailed and reproducible mechanistic studies on the interaction of cosmic ray particles with frozen gases in space based on pressure conditions (hydrocarbon free UHV conditions, the limitation of condensations of residual gases during an experiment to less than one monolayer), temperature regime (the use of silver monocrystals, FTIR in reflection, optimized ion currents, and target thicknesses 〈5 μm restrict temperature increasing to 14 K), and defined target systems. In combination with two on line and in situ analyses techniques, i.e., FTIR and QMS, the machine yields unprecedented options such as computing the heating of the ice surfaces directly exposed to the ion beam by a calibrated QMS and a complete quantification of product distribution. Preliminary results indicate a strong temperature-dependent component of the reaction mechanisms in the frosts: surface layers are heated by impinging ions to (14±1) K and yield (70%–100%) of higher molecular weight species, such as C11D24, whereas 10 K regions produce majority of simpler hydrocarbons, e.g., C3D8. Second, O2 contaminations influence the experiments dramatically by trapping of diffusive H atoms as O2H and, thus, yield oxygen-containing yellow to brown residues after heating to 293 K. Irradiation of pure methane targets, however, produce no residues. But an increasing concentration of H atoms exceeding (6%±3%) leads to ejection of up to 90% of the frosts into vacuum. © 1995 American Institute of Physics.

Notes: A novel, efficient technique to identify and quantify complex gas mixtures is described. This approach can be applied on line and in situ and is extendible to samples with reactive and thermally labile species. Complex hydrocarbon mixtures are prepared in test experiments by irradiating frozen methane targets with 9 MeV α particles in an ultrahigh vacuum chamber and releasing them during successive heating of the solid samples from 10 to 293 K after each ion bombardment. A quadrupole mass spectrometer monitors time-dependent ion currents of selected m/z values, which are proportional to partial pressures in the case of a nonoverlapping fragmentation pattern. Predominantly, parent molecules and fragments of different molecular species add to a specific m/z value, i.e., C2H+4, N+2, and CO+ contribute to m/z=28. Programmed m/z ratios are chosen to result in an inhomogeneous system of linear equations including the measured ion current (right-hand vector), partial pressures (unknown quantity), and the calibration factors of fragments of individual gases determined in separate experiments. Since all quantities are provided with experimental errors, matrix interval algebra, i.e., an IBM high accuracy arithmetic subroutine defining experimental uncertainties as intervals, is incorporated in the computations to extract individual, calibrated components of complex gas mixtures. This proceeding enables the quantitative sampling of calibrated hydrocarbons, and, especially, H2 and D2 without further time-consuming preseparation devices on line and in situ, hence justifying the use of this approach in space missions to elucidate the chemical composition of, e.g., planetary atmospheres without payload wasting gas chromatographs. © 1995 American Institute of Physics.

Notes: Raman spectra in the region of the pentagonal pinch mode Ag(2) of C60 were taken in situ during the deposition of C60 on the GaAs(100) surface at different temperatures. For very low coverages, only the feature corresponding to the pentagonal pinch mode of pristine C60 is visible. The onset of polymerization under laser irradiation occurred at thicknesses of about 15 nm which is attributed to a suppressive effect on the polymerization process due to the interaction of C60 with the substrate surface. The line shape for the feature due to photopolymerized C60 was different at each temperature indicating distinct polymeric states at different temperatures. These different states are discussed in comparison to recent theoretical calculations. Additionally, the photopolymerization due to irradiation after growth was investigated in situ. © 1998 American Institute of Physics.

Notes: The reaction between ground state carbon atoms and propylene, C3H6, was studied at average collision energies of 23.3 and 45.0 kJ mol−1 using the crossed molecular beam technique. Product angular distributions and time-of-flight spectra of C4H5 at m/e=53 were recorded. Forward-convolution fitting of the data yields a maximum energy release as well as angular distributions consistent with the formation of methylpropargyl radicals. Reaction dynamics inferred from the experimental results suggest that the reaction proceeds on the lowest 3A surface via an initial addition of the carbon atom to the π-orbital to form a triplet methylcyclopropylidene collision complex followed by ring opening to triplet 1,2-butadiene. Within 0.3–0.6 ps, 1,2-butadiene decomposes through carbon–hydrogen bond rupture to atomic hydrogen and methylpropargyl radicals. The explicit identification of C4H5 under single collision conditions represents a further example of a carbon–hydrogen exchange in reactions of ground state carbon with unsaturated hydrocarbons. This versatile machine represents an alternative pathway to build up unsaturated hydrocarbon chains in combustion processes, chemical vapor deposition, and in the interstellar medium. © 1997 American Institute of Physics.

Notes: Iron doping of InP and GaInAsP(λg=1.05 μm) layers grown by metalorganic molecular beam epitaxy was studied using elemental source material in combination with a conventional effusion cell. This study was aimed at the creation of semi-insulating optical waveguides under growth conditions compatible with selective area growth. Secondary ion mass spectroscopy measurements revealed a reproducible and homogeneous incorporation behavior of the iron dopant in the materials investigated. Resistivities in excess of 109 Ω cm were obtained for both compositions at medium doping levels. GaInAsP/InP waveguide structures grown at 485 °C—the minimum temperature necessary for selective deposition—exhibited averaged resistivities of 5×107 Ω cm in combination with optical losses of 2.5±0.5 dB/cm. © 1998 American Institute of Physics.

Notes: The reaction between ground state carbon atoms, C(3Pj), and acetylene, C2H2(1∑+g), was studied at an average collision energy of (8.4±0.3) kJ mol−1 using the crossed molecular beam technique. The product angular distribution and time-of-flight spectra of m/z=37, i.e., C3H, were recorded. Only m/z=37 was detected, but no signal from the thermodynamically accessible C3(1∑+g)+H2(1∑+g) channel. Forward-convolution fitting of the results yielded a center-of-mass angular flux-distribution forward scattered in respect to the carbon beam, whereas the translational energy flux distribution peaked at only (5.4±1.2) kJ mol−1, suggesting a simple C–H-bond-rupture to H+C3H. The reaction likely proceeds on the triplet surface with an entrance barrier to the C3H2–PES of 〈(8.4±0.3) kJ mol−1 via addition of the carbon atom to two bonding π-orbitals located both at C1 or at C1 and C2 of the acetylene molecule. The explicit identification of C3H product under single collision conditions strongly demands incorporation of atom-neutral reactions in reaction networks simulating chemistry in the interstellar medium, in interstellar shock waves, and in outflows of carbon stars. © 1995 American Institute of Physics.