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  • 1
    Monograph available for loan
    Monograph available for loan
    Introduction to paleolimnology (1968)
    Amsterdam [u.a.] : Elsevier
    Associated volumes
    Details Availability
    Call number: 9/G 9223
    In: Developments in sedimentology
    Type of Medium: Monograph available for loan
    Pages: XII, 228 S. : Ill., graph. Darst., Kt.
    Series Statement: Developments in sedimentology 11
    Location: Reading room
    Branch Library: GFZ Library
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  • 2
    Electronic Resource
    Electronic Resource
    Dissociative chemisorption of methane on Ir(111): Evidence for direct and trapping-mediated mechanisms (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 10229-10241 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with room-temperature methane gas over Ir(111) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, these calculations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to the results of a previous study of methane dissociation on Ir(110), although the reactivity of thermalized methane is approximately an order of magnitude higher on the (110) surface of iridium. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475306
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  • 3
    Electronic Resource
    Electronic Resource
    Trapping dynamics of ethane on Si(100)-(2×1): Molecular beam experiments and molecular dynamics simulations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 7567-7575 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The trapping probability, or physical adsorption probability, of ethane on a clean Si(100)-(2×1) surface has been measured as a function of the incident translational energy and incident polar angle of the molecule at a surface temperature of 65 K. At all incident angles the trapping probability decreases as the translational energy of the incoming ethane molecule is increased from 0.05 to 1.3 eV. As the incident polar angle, with respect to the surface normal, is increased, the trapping probability decreases. This decrease in trapping probability with increasing polar angle contradicts the idea of normal energy scaling and has been seen in very few cases. Classical molecular dynamics calculations have been employed to study the cause of this unusual angular dependence. This simulation predicts trapping probabilities in good agreement with the experimental data. Analysis of the computed trajectories indicates that the initial site of impact within the unit cell, as well as energy exchange on initial impact with the surface, is important in determining the fate of an incident molecule. Normal momentum of the incident molecule is dissipated during the first impact much more efficiently than is parallel momentum. The simulations also indicate that the observed angular dependence can be explained in terms of parallel momentum accommodation. Large amounts of parallel momentum remaining after initial impact may be converted to normal momentum on subsequent impacts, causing molecules to scatter from the surface. Therefore, molecules that impact the surface at glancing angles and high translational kinetic energies are more likely to scatter from the surface than those at normal incidence or with lower translational kinetic energy. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480083
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  • 4
    Electronic Resource
    Electronic Resource
    Oxygen adsorption on Si(100)-2×1 via trapping-mediated and direct mechanisms (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11574-11584 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the results from a molecular beam study of the initial adsorption probability (S0) of O2 on Si(100)-2×1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trapping-mediated mechanism is dominant, exhibiting a strong increase in S0 with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are well-described by a simple precursor model, which gives a difference in activation barrier heights of (Ed−Ec)=28 meV, and a ratio of preexponentials νd/νc=24.2. Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in S0 with kinetic energy indicates that a direct chemisorption mechanism is active, with the observed energy scaling proportional to cos θi. There is also an unusual increase in S0 with surface temperature, with only a weak increase below 600 K, and a stronger increase above 600 K. The direct mechanism trends are discussed in terms of a possible molecular ion intermediate with thermally activated charge transfer. The molecular beam measurements are also used in calculating the reactivity of a thermalized gas with a clean surface. The precursor model is combined with a two-region fit of the direct adsorption data to predict chemisorption probabilities as a function of the incident conditions. These functions are then weighted by a Maxwell-Boltzmann distribution of incident angles and energies to calculate the adsorption probability for a thermal gas. These calculations indicate that the predominant mechanism depends strongly on temperature, with trapping-mediated chemisorption accounting for all of the adsorption at low temperatures, and direct adsorption slowly taking over at higher temperatures. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478005
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  • 5
    Electronic Resource
    Electronic Resource
    Experimental study of CO oxidation by an atomic oxygen beam on Pt(111), Ir(111), and Ru(001) (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 3057-3063 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Impinging O-atoms react with adsorbed CO on Pt(111), Ir(111), and Ru(001), to form CO2 at surface temperatures as low as 77 K. The initial reaction probability is measured on these three surfaces using reflectivity techniques and is much lower on Pt(111) than previously supposed. The reaction probability is measured as a function of surface temperature, incident O-atom flux, kinetic energy, and angle. Interestingly, a significant dependence on incident angle is observed on all surfaces (the reaction probability is ∼2.5 times greater at normal incidence than at glancing angles), and a kinetic energy effect is noted at the higher incident angles studied. Also, surface temperature is shown to have an effect on the reaction probability in measurements performed on Pt(111) and Ir(111) at normal incidence. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475693
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  • 6
    Electronic Resource
    Electronic Resource
    Silicon deposition from disilane on Si(100)-2×1: Microscopic model including adsorption (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 4981-4989 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present a model for the calculation of homoepitaxial film growth rates during silicon deposition on Si(100)-2×1 from disilane. Central to this model is the use of thermalized gaseous disilane adsorption probabilities that have been determined as a function of gas and surface temperature by convoluting supersonic molecular beam adsorption probability data with a Maxwell–Boltzmann distribution of incident kinetic energies and angles. These calculations show that the primary adsorption pathway over the entire range of conditions investigated is the so-called trapping-mediated mechanism, in which dissociative chemisorption occurs via a physisorbed intermediate. A second adsorption mechanism, direct chemisorption, is activated by translational energy and does in fact contribute somewhat to adsorption, but only at high gas and surface temperatures. Hydrogen coverages and silicon film growth rates are calculated from a simple surface decomposition kinetic model together with a phenomenological thermal desorption model and compare favorably to experimental measurements. Under conditions of high flux or low surface temperature, the growth rate is limited by hydrogen desorption and therefore increases with increasing surface temperature. In the flux-limited or adsorption-limited growth regime, the growth rate is predicted to decrease with increasing surface temperature due to a drop in the adsorption probability, resulting in a maximum in the growth rate for a given set of deposition conditions. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1402141
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  • 7
    Electronic Resource
    Electronic Resource
    Hypothesis Testing and Proof by Contradiction: An Analogy (1980)
    Oxford, UK : Blackwell Publishing Ltd
    Teaching statistics 2 (1980), S. 0 
    Details
    ISSN: 1467-9639
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mathematics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1467-9639.1980.tb00387.x
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  • 8
    Electronic Resource
    Electronic Resource
    Trapping-mediated chemisorption of disilane on Si(100)-2×1 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 2470-2478 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Disilane adsorption probabilities have been measured on Si(100)-2×1 over a wide range of incident kinetic energies, incident angles, and surface temperatures using supersonic molecular beam techniques. The trapping-mediated chemisorption mechanism is shown to be the dominant adsorption pathway under the conditions investigated. The first step in such a mechanism, namely trapping into the physical adsorption well, has been studied directly via measurements at a surface temperature of 77 K. As expected, the trapping probability drops with increasing kinetic energy, but nearly 50% of incident molecules trap at 1 eV incident energy, indicating that trapping is quite efficient over a wide range of translational energies. Chemisorption probability values measured at higher surface temperatures are fit to a simple trapping-mediated chemisorption model that can be used to predict adsorption probabilities over a wide range of conditions. Measurements of the chemisorption probability at 500 K are independent of incident angle at kinetic energies of 0.75 eV and below. However, trapping probabilities measured at 77 K are shown to decrease with increasing angle of incidence at kinetic energies of 0.6 eV and above. This unusual effect is discussed in terms of molecular scattering during parallel momentum accommodation. In order to investigate the effect of surface hydrogen formed as a result of disilane decomposition, adsorption probabilities were measured as a function of monohydride coverage as well. On a monohydride-saturated surface the trapping probability is found to be lower than on a bare surface, most likely due to a decreased disilane physical adsorption binding energy compared to the bare surface. Also, the trapping probability varies linearly with hydrogen coverage between bare-surface and monohydride-saturated values. On the other hand, the hydrogen coverage dependence of the chemisorption probability is found to follow a simple second-order kinetic scheme based on chemisorption occurring at two vacant surface sites. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.482064
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  • 9
    Electronic Resource
    Electronic Resource
    Nitrate, Chloride and Dissolved Solids, Ogallala Aquifer, West Texas (1978)
    Oxford, UK : Blackwell Publishing Ltd
    Ground water 16 (1978), S. 0 
    Details
    ISSN: 1745-6584
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Notes: The distribution of nitrate, chloride and dissolved solids from ground water from the Ogallala aquifer in a 27-county area of west Texas illustrates widespread areas of poor water quality. Elements studied increased in concentration from northwest to southeast across the Southern High Plains. Northeast of a line from about Clovis, New Mexico to Lubbock, Texas, nitrate tends to be 〈45 mg/l, chloride is 〈20 mg/l, and dissolved solids are 〈400 mg/l. However, southwest of this line nitrate may exceed 60 mg/l and in some areas exceeds 170 mg/l. Chloride commonly exceeds 500 mg/l and may be 〉2000 mg/l, and dissolved solids usually exceed 1000 mg/l and may be 〉8000 mg/l. Regional distribution may be the result of long-term migration of Ogallala ground water but the present water quality and distribution, as well as time, distance, stratigraphy and permeability, suggest contamination of Ogallala ground water by vertical rather than lateral migration. Most of the high nitrate values (〉45 mg/l) occur in areas having sandy soils which have been intensively cultivated, thus leaching of nitrogen-based fertilizers is suspect. However, the high chloride and dissolved solids, which exist in essentially the same geographic area, probably represent vertical to local lateral seepage of saline water from large alkali lake basins and local vertical migration from saline Cretaceous aquifers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1745-6584.1978.tb03218.x
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  • 10
    Electronic Resource
    Electronic Resource
    Automatic Fundamental Calculations of Molecular Structure (1956)
    [s.l.] : Nature Publishing Group
    Nature 178 (1956), S. 1207-1209 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] IN principle, it is possible to predict the structure and properties of molecules with malimited accuracy merely from Schrodingers equation and the numbers and properties of the electrons and nuclei defining the molecule. The complicated mathematical formulation, the very intractable intermediate ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/1781207a0
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