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  • 1990-1994  (13)
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  • 1
    Digitale Medien
    Digitale Medien
    Dynamics of the chemisorption of O2 on Pt(111): Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1626-1635 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have used the thermal desorption spectroscopy of the O/O2+CO→CO2 system to probe the chemical nature of oxygen that remains on a Pt(111) surface following exposure to a supersonic O2 beam under various conditions. We find that for a surface temperature of 90 K, the resulting CO2 formation thermal desorption spectrum is the same for all beam kinetic energies employed up to 1.1 eV at normal incidence, in all cases resembling that assigned to the O2+CO co-adsorbate system. This spectrum is clearly distinct from the O+CO case, where atomically chemisorbed oxygen is obtained either by thermal dissociation of O2 on the surface or by exposing the 90 K surface to a beam containing O atoms. These results imply that the dissociative chemisorption of O2 on Pt(111) proceeds by way of a molecular precursor even at relatively high incidence kinetic energies, at least as high as 1.1 eV. This interpretation readily accounts for the strong surface temperature dependence associated with dissociation under these conditions but contrasts with previous assignment of a direct (or quasidirect) dissociation process at high energies. We have also reexamined a number of previous observations in terms of this new picture, including the initial decline in dissociation probability with increasing kinetic energy. This falloff is attributed to a decrease in the trapping probability into a physisorption state, as recently suggested by Luntz et al. Considering the present results in the light of other recent studies, it now seems clear that the physisorption state is then a precursor to the molecular chemisorption state which can also be accessed directly at high kinetic energy. In this picture the molecular chemisorption state is then a precursor to dissociation even at high kinetic energy, and the dissociation probability depends on the (temperature-dependent) branching ratio between the dissociation and ultimate desorption of these species.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459966
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  • 2
    Digitale Medien
    Digitale Medien
    Trapping of molecular ethane on the Ir(110)–(1×2) surface (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3986-3988 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457808
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  • 3
    Digitale Medien
    Digitale Medien
    Molecular adsorption of ethane on the Ir(110)-(1×2) surface: Monte Carlo simulations and molecular beam reflectivity measurements (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1397-1406 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Experimental results, obtained using a molecular beam reflectivity method, for the probability of molecular physical adsorption of ethane on the Ir(110)-(1×2) surface are presented. We analyze these results using Monte Carlo simulations and show that molecular adsorption can occur either "directly'' or through a precursor state in which an ethane molecule is trapped in a second layer of molecularly adsorbed ethane with subsequent migration to a vacant site. From the Monte Carlo simulations, we are able to establish that the energy barrier for the desorption of an ethane molecule from the precursor state is approximately 4.5 kcal/mol. We also find that the energy barrier for diffusion of an ethane molecule on top of a monolayer of ethane molecularly adsorbed on the Ir(110)-(1×2) surface is approximately 3.7 kcal/mol.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458151
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  • 4
    Digitale Medien
    Digitale Medien
    Dynamics of the oxidation of CO on Pt(111) by an atomic oxygen beam (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8649-8651 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have determined the angular and velocity distributions of the CO2 product of the interaction of gas-phase O atoms with CO adsorbed on Pt(111) at 90 K. The desorption intensity is found to be strongly peaked along the surface normal, falling approximately as cos12 θf with increasing final angle, θf. The velocity distributions are found to be highly non-Boltzmann with mean energies of ∼0.6 eV, decreasing slightly with increasing θf.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461244
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  • 5
    Digitale Medien
    Digitale Medien
    On the nature of trapping and desorption at high surface temperatures. Theory and experiments for the Ar–Pt(111) system (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1516-1527 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We report molecular dynamics calculations and molecular beam experiments on trapping and desorption as a function of surface temperature and initial gas conditions for the Ar–Pt(111) system. The trapping process involves very rapid equilibration of the normal component of incidence velocity but extremely slow accommodation of the parallel component. At high surface temperatures for which the residence time of Ar is sufficiently short (e.g., roughly 40 ps at 273 K), trapped atoms desorb before their incident parallel velocity is thermalized. Thus trapping in the usual sense of complete equilibration with the surface does not occur; instead these quasitrapped atoms are characterized by full accommodation of only the normal velocity component. In both simulations and experiments there is a range of temperatures for which the desorbing flux associated with quasitrapping is distinguishable from the flux due to direct inelastic scattering. These results are qualitatively reproduced by a simple model for the time-varying velocity distribution of quasitrapped atoms, which treats the different components of velocity independently.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460695
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  • 6
    Digitale Medien
    Digitale Medien
    Dynamics of recombinative desorption: Angular distributions of H2, HD, and D2 desorbing from Cu(111) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7499-7501 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have determined the angular distributions of H2, HD, and D2 desorbing from Cu(111) for surface temperatures in the range 370–800 K. These are found to be strongly peaked and symmetric about the surface normal in every case. Results for all three isotopes are found to be indistinguishable, being close to a cos 12θf distribution at 600 K, slightly narrower at 370 K, and slightly broader at 800 K. Results are discussed in terms of other previous desorption measurements and related to adsorption data via detailed balance.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460181
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  • 7
    Digitale Medien
    Digitale Medien
    Trapping-mediated dissociative chemisorption of ethane on Ir(110)-(1×2) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4508-4512 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Evidence is presented to support a trapping-mediated dissociative chemisorption mechanism for ethane interacting with an Ir(110)-(1×2) surface. The data were obtained from supersonic molecular-beam measurements with an incident kinetic energy Ei ranging between 1.2 and 24.1 kcal/mol, a surface temperature Ts between 154 and 500 K, and an incident angle θi between 0° and 45°. For Ei less than approximately 13 kcal/mol, the probability of dissociative chemisorption S0 decreases rapidly with increasing Ts. For a surface temperature of 154 K, S0 decreases with increasing Ei for 1.2≤Ei ≤13.4 kcal/mol, consistent with a trapping-mediated chemisorption mechanism. Indeed, the data also support quantitatively a kinetic model consistent with a trapping-mediated chemisorption mechanism. The difference in the activation energies for desorption and chemisorption from the physically adsorbed, trapped state Ed −Ec is 2.2±0.2 kcal/mol. In the trapping-mediated regime, S0 is found to be rather insensitive to incident angle, scaling with Ei cos0.5 θi .
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457762
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  • 8
    Digitale Medien
    Digitale Medien
    Ion-beam-assisted etching of Si with fluorine at low temperatures (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7562-7566 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The ion-assisted etching of Si with F atoms has been studied over the temperature range from 77 K to room temperature. Separately controllable beams of F atoms and 1 keV Ar+ ions are used in an ultrahigh-vacuum environment. Neutral etch products are measured with modulated beam mass spectrometry. The ion-assisted etch rate is seen to increase as the temperature is decreased whereas the spontaneous etch rate goes to zero at low temperatures. The nature of the etch products is essentially independent of temperature over this temperature range. Evidence is presented indicating that the spontaneous etching of Si by F atoms at 77 K is blocked by the formation and condensation of Si2F6. © 1994 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.357990
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  • 9
    Digitale Medien
    Digitale Medien
    Interaction of ammonia with chemically modified ruthenium (001) surfaces (1991)
    s.l. : American Chemical Society
    Langmuir 7 (1991), S. 1689-1694 
    Details
    ISSN: 1520-5827
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/la00056a021
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  • 10
    Digitale Medien
    Digitale Medien
    Penetrometer resistance, root penetration resistance and root elongation rate in two sandy loam soils (1991)
    Springer
    Plant and soil 131 (1991), S. 59-66 
    Details
    ISSN: 1573-5036
    Schlagwort(e): maize ; mechanical stress ; penetrometer ; root growth ; sandy loam
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft
    Notizen: Abstract Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00010420
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