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  • 1
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    Subsidence, mixing and denitrification of Arctic polar vortex air measured during POLARIS (1999)
    In:  EPIC3Journal of Geophysical Research, 104, pp. 26611-26623
    Details
    Publication Date: 2019-07-16
    Description: We determine the degree of denitrification that occurred during the 1996-1997 Arctic winter using a technique that is based on balloon and aircraft borne measurements of NOy, N2O, and CH4. The NOy/N2O relation can undergo significant change due to isentropic mixing of subsided vortex air masses with extravortex air due to the high nonlinearity of the relation. These transport related reductions in NOy can be difficult to distinguish from the effects of denitrification caused by sedimentation of condensed HNO3. In this study, high-altitude balloon measurements are used to define the properties of air masses that later descend in the polar vortex to altitudes sampled by the ER-2 aircraft (i.e., ~20 km) and mix isentropically with extravortex air. Observed correlations of CH4 and N2O are used to quantify the degree of subsidence and mixing for individual air masses. On the basis of these results the expected mixing ratio of NOy resulting from subsidence and mixing, defined here as NOy**, is calculated and compared with the measured mixing ratio of NOy. Values of NOy and NOy** agree well during most parts of the flights. A slight deficit of NOy versus NOy** is found only for a limited region during the ER-2 flight on April 26, 1997. This deficit is interpreted as indication for weak denitrification (~2-3 ppbv) in that air mass. The small degree of denitrification is consistent with the general synoptic-scale temperature history of the sampled air masses, which did not encounter temperatures below the frostpoint and had relatively brief encounters with temperatures below the nitric acid trihydrate equilibrium temperature. Much larger degrees of denitrification would have been inferred if mixing effects had been ignored, which is the traditional approach to diagnose denitrification. Our analysis emphasizes the importance of using other correlations of conserved species to be able to accurately interpret changes in the NOy/N2O relation with respect to denitrification.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 2
    Electronic Resource
    Electronic Resource
    Toward the understanding of ethylene photodissociation: Theoretical study of energy partition in products and rate constants (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 10810-10820 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energy partition in the products of ethylene photodissociation (including C2H4, C2D4, D2CCH2, cis- and trans-HDCCDH) at 193 and 157 nm and the rate constants of H loss channels were computed based on ab initio ethylene ground-state surfaces of which most were reported earlier. In the calculations of the energy partitions, a simple model was used in which the excess energy above the transition state is distributed statistically and the energy released by the exit barrier is described by the modified impulsive model. The rate constants of the ethylene H(D) elimination were calculated according to the variational RRKM (Rice–Ramsperger–Kassel–Marcus) theory, and the RRKM rate constants with tunneling corrections were obtained for vinyl decomposition at 193 nm. In contrast with previous conclusions drawn by LIF (laser induced fluorescence) studies, the rate constant calculations suggest that the H loss may be a nonstatistical process. However, the computed variational transition states for H loss appear reasonable as indicated by the translational energy. That with present investigation indicates that the atomic elimination proceeds via the predicted transition states though the process is nonstatistical. Analysis of the H2 translational energy measured at 193 and 157 nm by molecular beam experiments gives evidence that the overall mechanisms of the molecular elimination are different at the two wavelengths, which is also in disagreement with previous belief. At 193 nm, both H2 elimination channels may occur through the predicted transition states. On the other hand, further comparison of the theoretical and experimental translational energy of hydrogen molecule at 157 nm suggests that the observed (1,1E) reaction path is most likely of much higher "exit barrier" than the one computed. For the (1,2E) channel, the calculations are still in support of the computed transition state being the one along the experimentally observed pathway at 157 nm. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478999
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  • 3
    Electronic Resource
    Electronic Resource
    Crossed beam reaction of cyano radicals with hydrocarbon molecules. I. Chemical dynamics of cyanobenzene (C6H5CN; X 1A1) and perdeutero cyanobenzene (C6D5CN; X 1A1) formation from reaction of CN(X 2Σ+) with benzene C6H6(X 1A1g), and d6-benzene C6D6(X 1A1g) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 7457-7471 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The chemical reaction dynamics to form cyanobenzene C6H5CN(X 1A1), and perdeutero cyanobenzene C6D5CN(X 1A1) via the neutral–neutral reaction of the cyano radical CN(X 2Σ+), with benzene C6H6(X 1A1g) and perdeutero benzene C6D6(X 1A1g), were investigated in crossed molecular beam experiments at collision energies between 19.5 and 34.4 kJ mol−1. The laboratory angular distributions and time-of-flight spectra of the products were recorded at mass to charge ratios m/e=103–98 and 108–98, respectively. Forward-convolution fitting of our experimental data together with electronic structure calculations (B3LYP/6−311+G**) indicate that the reaction is without entrance barrier and governed by an initial attack of the CN radical on the carbon side to the aromatic π electron density of the benzene molecule to form a Cs symmetric C6H6CN(C6D6CN) complex. At all collision energies, the center-of-mass angular distributions are forward–backward symmetric and peak at π/2. This shape documents that the decomposing intermediate has a lifetime longer than its rotational period. The H/D atom is emitted almost perpendicular to the C6H5CN plane, giving preferentially sideways scattering. This experimental finding can be rationalized in light of the electronic structure calculations depicting a H–C–C angle of 101.2° in the exit transition state. The latter is found to be tight and located about 32.8 kJ mol−1 above the products. Our experimentally determined reaction exothermicity of 80–95 kJ mol−1 is in good agreement with the theoretically calculated one of 94.6 kJ mol−1. Neither the C6H6CN adduct nor the stable iso cyanobenzene isomer C6H5NC were found to contribute to the scattering signal. The experimental identification of cyanobenzene gives a strong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn's moon Titan. This reaction might further present a barrierless route to the formation of heteropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrestrial environments as well as hydrocarbon rich flames. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480070
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  • 4
    Electronic Resource
    Electronic Resource
    Ab initio/RRKM approach toward the understanding of ethylene photodissociation (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 2748-2761 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C2H4, C2D4, D2CCH2, and cis- and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G(d,p) level. The coupled cluster method, CCSD(T)/6-311+G(3df,2p), was employed to calculate the corresponding energies with the zero-point energy corrections by the B3LYP/6-311G(d,p) approach. Ethylidene was found to be an intermediate in the 1,2-H2 elimination channel. The barrier for the 1,1-H2 elimination was computed to be the lowest (4.10–4.16 eV), while the 1,2-H2 elimination and H loss channels have barriers of a similar height (4.70–4.80 eV). The rate constant for each elementary step of ethylene photodissociation at 193 and 157 nm was calculated according to the RRKM theory based on the ab initio surfaces. The rate equations were subsequently solved, and thus the concentration of each species was obtained as a function of time. The concentrations at t→∞ were taken for calculating branching ratios or yields. In accord with previous experimental findings, the calculated branching ratio for the 1,1-H2 elimination process is higher than that for the 1,2-H2 elimination, and the atomic elimination channel is predicted to be favored at increasing excitation energy when competing with the molecular elimination. The significant discrepancies between theoretical and experimental results in the magnitude of the yields and their dependence on the wavelength for the molecular elimination channels suggest the dynamics of either 1,2-H2, or 1,1-H2 elimination, or both channels may be nonstatistical in nature. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476877
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  • 5
    Electronic Resource
    Electronic Resource
    Crossed beam reaction of cyano radicals with hydrocarbon molecules. II. Chemical dynamics of 1-cyano-1-methylallene (CNCH3CCCH2; X 1A′) formation from reaction of CN(X 2Σ+) with dimethylacetylene CH3CCCH3 (X 1A1′) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 7472-7479 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction dynamics to form the 1-cyano-1-methylallene isomer CNCH3CCCH2 in its 1A′ ground state via the radical–closed shell reaction of the cyano radical CN(X 2Σ+) with dimethylacetylene CH3CCCH3 (X 1A1′) are unraveled in a crossed molecular beam experiment at a collision energy of 20.8 kJ mol−1 together with state-of-the-art electronic structure and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. Forward convolution fitting of the laboratory angular distribution together with the time-of-flight spectra verify that the reaction is indirect and proceeds by addition of the CN radical to the π orbital to form a cis/trans CH3CNC(Double Bond)CCH3 radical intermediate. This decomposes via a rather lose exit transition state located only 6–7 kJ mol−1 above the products to CNCH3CCCH2 and atomic hydrogen. The best fit of the center-of-mass angular distribution is forward–backward symmetric and peaks at π/2 documenting that the fragmenting intermediate holds a lifetime longer than its rotational period. Further, the hydrogen atom leaves almost perpendicular to the C5H5N plane resulting in sideways scattering. This finding, together with low frequency bending and wagging modes, strongly support our electronic structure calculations showing a H–C–C angle of about 106.5° in the exit transition state. The experimentally determined reaction exothermicity of 90±20 kJ mol−1 is consistent with the theoretical value, 80.4 kJ mol−1. Unfavorable kinematics prevent us from observing the CN versus CH3 exchange channel, even though our RRKM calculations suggest that this pathway is more important. Since the title reaction is barrierless and exothermic, and the exit transition state is well below the energy of the reactants, this process might be involved in the formation of unsaturated nitriles even in the coldest interstellar environments such as dark, molecular clouds and the saturnian satellite Titan. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480071
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  • 6
    Electronic Resource
    Electronic Resource
    A flexible implementation of scanning tunneling spectroscopy utilizing C-language on the personal computer (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 3646-3651 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Scanning tunneling spectroscopy (STS) capable of measuring the local density of states dI/dV=G(V,x,y) correlated with the topography z(x,y) is implemented on a PC/AT computer system. An algorithm using dimensional decoupling and recursive linking is incorporated in a compact C-language code to achieve flexible and sophisticated STS control. Data acquisition and processing methods which provide the local density of states G(V,x,y,s) at different tip heights s in registry with z(x,y) are described. Examples of novel low-temperature experiments are given to show the capabilities of this implementation of STS.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143591
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  • 7
    Electronic Resource
    Electronic Resource
    Fabrication of gatable submicron channels in AlxGa1−xAs-GaAs heterostructures (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 1246-1248 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new, simplified process has been developed for fabricating submicron AlxGa1−xAs-GaAs heterostructure devices for low-temperature transport studies, with the advantage of gatability. This process utilizes electron beam lithography, photolithography, and wet etching techniques to laterally confine the two-dimensional electron gas, and possesses the unique feature that the metal etch mask for the electron beam defined narrow section is deposited directly on top of the heterostructure surface, allowing for use as a gate. Devices of lithographic widths from 0.4 to 2.0 μm have been successfully fabricated, where the 0.4 μm devices of both enhancement and depletion modes have been demonstrated to function down to 0.35 K in temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.99170
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  • 8
    Electronic Resource
    Electronic Resource
    Tunneling current noise in thin gate oxides (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 2885-2887 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have examined fluctuations in the tunneling current of 3.5 nm SiO2 barriers for voltages in the direct tunneling regime. We find a 1/f power law for the spectral density of the fluctuations where f is the frequency. This 1/f noise can be attributed to fluctuations of a trap assisted tunneling current through the oxide that causes current noise but is not evident in the I–V curves. We suggest that this noise may be a more sensitive probe of trap assisted tunneling and degradation in thin oxides than other measures. At voltages above a threshold of 2.5 V, we observe the reversible onset of non-Gaussian current transients in the noise. The onset of these current transients can be related to a transition in the spacial uniformity of the tunneling current density that may result in eventual breakdown of the oxide. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.117351
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  • 9
    Electronic Resource
    Electronic Resource
    Scanning Hall probe microscopy (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 61 (1992), S. 1974-1976 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We describe the implementation of a scanning Hall probe microscope of outstanding magnetic field sensitivity (∼0.1 G) and unprecedented spatial resolution (∼0.35 μm) to detect surface magnetic fields at close proximity to a sample. Our microscope combines the advantages of a submicron Hall probe fabricated on a GaAs/Al0.3Ga0.7As heterostructure chip and the scanning tunneling microscopy technique for precise positioning. We demonstrate its usefulness by imaging individual vortices in high Tc La1.85Sr0.15CuO4 films and superconducting networks, and magnetic bubble domains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.108334
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  • 10
    Electronic Resource
    Electronic Resource
    Crossed beam reaction of cyano radicals with hydrocarbon molecules. III. Chemical dynamics of vinylcyanide (C2H3CN;X 1A′) formation from reaction of CN(X 2Σ+) with ethylene, C2H4(X 1Ag) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8643-8655 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The neutral–neutral reaction of the cyano radical, CN(X 2Σ+), with ethylene, C2H4(X 1Ag), has been performed in a crossed molecular beams setup at two collision energies of 15.3 and 21.0 kJ mol−1 to investigate the chemical reaction dynamics to form vinylcyanide, C2H3CN(X 1A′) under single collision conditions. Time-of-flight spectra and the laboratory angular distributions of the C3H3N products have been recorded at mass-to-charge ratios 53−50. Forward-convolution fitting of the data combined with ab initio calculations show that the reaction has no entrance barrier, is indirect (complex forming reaction dynamics), and initiated by addition of CN(X 2Σ+) to the π electron density of the olefin to give a long-lived CH2CH2CN intermediate. This collision complex fragments through a tight exit transition state located 16 kJ mol−1 above the products via H atom elimination to vinylcyanide. In a second microchannel, CH2CH2CN undergoes a 1,2 H shift to form a CH3CHCN intermediate prior to a H atom emission via a loose exit transition state located only 3 kJ mol−1 above the separated products. The experimentally observed mild "sideways scattering" at lower collision energy verifies the electronic structure calculations depicting a hydrogen atom loss in both exit transition states almost parallel to the total angular momentum vector J and nearly perpendicular to the C2H3CN molecular plane. Since the reaction has no entrance barrier, is exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the vinylcyanide reaction product soundly implies that the title reaction can form vinylcyanide, C2H3CN, as observed in the atmosphere of Saturn's moon Titan and toward dark, molecular clouds holding temperatures as low as 10 K. In strong agreement with our theoretical calculations, the formation of the C2H3NC isomer was not observed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1289529
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