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  • 1
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    Seasonality in UV-absorbing compounds of cyanobacterial mat communities from an intertidal mangrove flat (1998)
    In:  EPIC3Aquatic Microbial Ecology 16, pp. 37-44
    Details
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , peerRev
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  • 2
    Electronic Resource
    Electronic Resource
    Electronic spectra of carbon chain anions: C2nH− (n=5–12) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9280-9286 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic absorption spectra of mono-hydrogenated carbon chain anions C2nH− (n=5–10) have been measured in the gas-phase and in 6 K neon matrices (n=8–12). The techniques of resonant two-color electron photodetachment in the gas-phase and absorption spectroscopy of mass-selected anions in neon matrix were used. A homologous series is observed, with band system origins shifting from 304 nm for C10H− to 590 nm for C20H−. In conjunction with ab initio calculations the band systems are attributed to a 1Σ+←X 1Σ+ transition of linear acetylenic anions. Another near lying electronic transition due to a second isomer is also apparent for C10H− up to C24H−. Comparison with tables of the known diffuse interstellar bands indicates possible matches for the origin bands of the C18H− and C20H− isomers. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479842
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  • 3
    Electronic Resource
    Electronic Resource
    Charge carrier interactions in ionic conductors: A classical molecular-dynamics and Monte Carlo study on AgI (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6416-6423 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equilibrium concentration of ionic and electronic charge carriers in ionic crystals as a function of temperature, concentration of dopants, and chemical environment is phenomenologically well understood as long as these point defects can be considered sufficiently dilute. However, there are cases, usually at temperatures close to the melting point, where the defects appear in higher concentrations. In these cases interactions come into play and cause anomalous increases in the conductivity or even phase transitions. Recently Hainovsky and Maier showed that for various Frenkel disordered materials this anomalous conductivity increase at high temperature can be described by a cube root term in the chemical potential of the defects. This quasi-Madelung approach does not only allow ionic conductivities and heat capacities to be computed, it also leads to a phenomenological understanding of the solid–liquid or superionic transition temperatures. In the present study we analyze this approach on the atomistic level for AgI: The defect concentrations as well as defect energies, including excess energies, are computed as a function of temperature by molecular-dynamics and Monte Carlo simulations based on a classical semiempirical potential. The simulations support the cube-root model, yield approximately the same interaction constants and show that the corrections in the chemical potential are of an energetic nature. In agreement with structural expectations, the simulations reveal that two different kinds of interstitials are present: Octahedral interstitials, which essentially determine the ionic transport at higher temperature, and tetrahedral ones, which remain substantially associated with the vacancies. It is shown how these refinements have to be introduced into the cube root. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481205
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  • 4
    Electronic Resource
    Electronic Resource
    Mid-infrared spectra of He–HN+2 and He2–HN+2 (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 3876-3885 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mid-infrared vibrational spectra of He–HN+2 and He2–HN+2 have been recorded by monitoring their photofragmentation in a tandem mass spectrometer. For He–HN+2 three rotationally resolved bands are seen: the fundamental ν1 transition (N–H stretch) at 3158.419±0.009 cm−1, the ν1+νb combination band (N–H stretch plus intermolecular bend) at 3254.671±0.050 cm−1, and the ν1+νs combination band (N–H stretch plus intermolecular stretch) at 3321.466±0.050 cm−1. The spectroscopic data facilitate the development of approximate one-dimensional radial intermolecular potentials relevant to the collinear bonding of He to HN+2 in its (000) and (100) vibrational states. These consist of a short range potential derived from an RKR inversion of the spectroscopic data, together with a long range polarization potential generated by considering the interaction between the He atom and a set of multipoles distributed on the HN+2 nuclei. The following estimates for binding energies are obtained: D0″=378 cm−1 [He+HN+2(000)], and D0′=431 cm−1 [He+HN+2(100)]. While the ν1 band of He2–HN+2 is not rotationally resolved, the fact that it is barely shifted from the corresponding band of He–HN+2 suggests that the trimer possesses a structure in which one of the He atoms occupies a linear proton-bound position forming a He–HN+2 core, to which a second less strongly bound He is attached. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471244
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  • 5
    Electronic Resource
    Electronic Resource
    The infrared spectrum of the H2–HCO+ complex (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 5152-5164 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A combined experimental and theoretical study of the structural properties of the H2–HCO+ ion-neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H2–HCO+ complexes in the 2500–4200 cm−1 range display several vibrational bands, the most intense arising from excitation of the C–H and H2 stretch vibrations. The latter exhibits resolved rotational structure, being composed of Σ–Σ and Π–Π subbands as expected for a parallel transition of complex with a T-shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD(T)/6–311G(2df,2pd) level. The complex is composed of largely undistorted H2 and HCO+ subunits, has a T-shaped minimum energy geometry with an H2...HCO+ intermolecular bondlength of approximately 1.75 A(ring). Broadening of the higher J lines in the P and R branches of the Π–Π subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H2 internal rotation. The lower J lines in the Σ–Σ and Π–Π subbands have widths of 0.06 cm−1, around three times larger than the laser bandwidth, corresponding to a decay time of ≈90 ps for the upper level. The absence of discernible rotational structure in the ν2 band suggests that it has predissociation lifetime of less than 1 ps. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469240
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  • 6
    Electronic Resource
    Electronic Resource
    The infrared spectrum of the N2H+–He ion-neutral complex (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 5570-5571 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally resolved, vibrational predissociation spectra of the HN+2–He complex have been recorded in the region of the N–H stretch (3100–3200 cm−1). The complex appears to be linear. Fitting of the measured lines to the pseudodiatomic expression ν=ν0+(B'ν+B‘ν)m +(B'ν−B‘ν–D'ν+D'ν) m2−2(D'ν+D‘ν)m3 −(D'ν−D‘ν)m4 yields the following constants: ν0=3158.419±0.009 cm−1, B‘=0.3517±0.0005 cm−1, D‘=(5.8±0.5)×106 cm−1, B'=0.3579 ±0.0005 cm−1, D'=(3.9±0.6)×106 cm−1. The data support a proton bound He–HNN+ structure, with a 1.72 A(ring) vibrationally averaged intermolecular bondlength, and an approximate intermolecular stretching frequency of 150 cm−1. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469286
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  • 7
    Electronic Resource
    Electronic Resource
    Microcontact impedance measurements of individual highly resistive grain boundaries: General aspects and application to acceptor-doped SrTiO3 (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 2372-2381 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electrical properties of grain boundaries in polycrystalline materials can vary from boundary to boundary and conventional impedance measurements yield only averaged information. Local measurements at individual grain boundaries using microelectrodes can partly overcome this restriction. Finite element calculations on the impedance of such microelectrode measurements are performed to reveal the interrelations between the measured grain boundary impedance and the grain boundary properties (grain boundary resistivity, permittivity and geometry). We will discuss how far adjacent grain boundaries influence the results. In particular, the calculations revealed that even in the case of identical grain boundaries a considerable part of the current between two microelectrodes may flow across neighboring grain boundaries. It is shown how these additional current paths can be taken into account with respect to a reliable data analysis. Experiments on SrTiO3 polycrystals demonstrate that spatially resolved impedance investigations on single grain boundaries are possible in this way. The results are interpreted in terms of a double Schottky barrier at the grain boundaries. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.372189
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  • 8
    Electronic Resource
    Electronic Resource
    Low-temperature defect chemistry of oxides. I. General aspects and numerical calculations (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 5422-5433 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using oxides as examples, the defect chemistry is systematically analyzed for a low-temperature regime, at which the oxygen exchange equilibrium reaction is no longer reversible, while the internal defect equilibrium reactions (in particular, the electronic transfer processes) may still be reversible. For the partially frozen-in states as well as for the complete equilibrium cases, defect concentrations are numerically calculated for idealized model oxides including pure, acceptor-doped, and donor-doped oxides. Foreign ions (major/minor, shallow/deep, acceptor/donor), oxygen vacancies, and oxygen interstitials are taken into account as redox-active defects. The deep-level (redox-active) defects often dominate defect concentrations in the partially frozen-in states, while the major dopants fix the concentrations in complete equilibrium. The temperature and oxygen partial pressure dependencies of defect concentrations in the partially frozen-in states are discussed. The description does not only allow one to extend the defect chemistry to lower temperatures, such as room temperature, but also offers a quantitative basis for manipulation and prediction of defect concentrations in ionic crystals. Thereby, the physical and chemical performance of such materials may be controlled at temperatures lower than those at which the oxygen nonstoichiometry is established. The results are equally relevant for applications in solid state physics (e.g., compound semiconductors) and in solid state chemistry (e.g., solid electrolytes, mixed conductors). © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.371541
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  • 9
    Electronic Resource
    Electronic Resource
    Low-temperature defect chemistry of oxides. II. Analytical relations (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 5434-5443 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The low-temperature defect chemistry of oxides is considered, characterized by frozen-in interaction with the ambient oxygen and reversibility of internal interactions, in particular the redistribution of electronic carriers. Analytical relations describing ionic and electronic defect concentrations are derived for various conditions. The presence of redox-active, i.e., deep-level, dopants proves to be of special interest in this context. The analytical relations permit the detailed discussion of the dependencies of the charge carrier concentrations on the control parameters. Such analytical relations are useful for understanding and tailoring defect concentrations and thus related properties of electroceramics. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.371542
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  • 10
    Electronic Resource
    Electronic Resource
    Two-photon absorption spectroscopy of mass-selected ions: N2O+ and CS+2 (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3451-3455 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A technique for the spectroscopic characterization of mass-selected ions in ground and bound excited electronic states involving two-photon absorption and fragment ion detection is demonstrated. A triple quadrupole mass analyzer system is used. Collisionally relaxed ions (M+) from a high pressure source are mass selected, then excited sequentially by two laser colors (λ1,λ2) according to M+→λ1M+*→λ2M+**→F+i and fragment ions (F+i) are detected. Stable ionic states are characterized by scanning λ1 with constant λ2 and this is illustrated on the known transitions of N2O+ (A˜ 2Σ+←X˜ 2Π) and of CS+2 (B˜ 2Σ+u←X˜ 2Πg). New spectroscopic information—vibrational frequencies and rotational constants—and dynamic parameters—lifetimes and fragmentation branching ratios—on N2O+(B˜ 2Σ+) and CS+2 (C˜ 2Σ+g) are obtained by scanning λ2 when λ1 is chosen to populate several or individual rotational levels of the intermediate state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453893
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