M. L. Rappaport

Tel Aviv University, Tel Aviv, Tel Aviv, Israel

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Publications (5)11.32 Total impact

  • Source
    A. Bar-nun, G. Herman, D. Laufer, M.L. Rappaport
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    ABSTRACT: The trapping and release of H2, CO, CO2, CH4, Ar, Ne, and N2 by amorphous water ice was studied experimentally under dynamic conditions, at low temperatures starting at 16°K, with gas pressure of 5 × 10−8−10−6 Torr. CO, CH4, Ar, and N2 were found to be released in three or four distinct temperature ranges, each resulting from a different trapping mechanism: (a) 30–55°K, where the gas frozen on the water ice evaporates; (b) 135–155°K, where gas is squeezed out of the water ice during the transformation of amorphous ice to cubic ice; (c) 165–190°K, where gas and water are released simultaneously, probably by the evaporation of a clathrate hydrate, and, occasionally (d) 160–175°K, where deeply buried gas is released during the transformation of cubic ice to hexagonal ice. If the third range is indeed due to clathrate formation, CO was found to form this compound. CO2 did not form a clathrate under the experimental conditions. Excess hydrogen did not affect the occlusion of other gases. Hydrogen itself was trapped only at 16°K. Neon was not trapped at 25°K. With cubic ice, the only trapping mechanism is freezing of gas on the ice surface. No fractionation between the gas phase and the ice was observed with a mixture of CO and Ar. Massive ejection of ice grains was observed during the evaporation of the gas in three (a,c,d) out of the four ranges. The experimental results are used to explain several cometary phenomena, especially those occurring at large heliocentric distances, and are applied also to Titan's atmospheric composition and to the possible ejection of ice grains from Enceladus.
    Origins of Life and Evolution of Biospheres 01/1985; · 1.83 Impact Factor
  • A. Bar-Nun, G. Herman, M. L. Rappaport, Yu Mekler
    Applied Surface Science 01/1985; 150:143-156. · 2.54 Impact Factor
  • A. Bar-Nun, G. Herman, M.L. Rappaport, Yu. Mekler
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    ABSTRACT: The ejection of H2O, O2, H2 and H from water ice at 30–140 K, bombarded by 0.5–6 keV H+ and Ne+ was studied experimentally. Neon ions in this energy range deposit their energy in the ice by nuclear collisions, whereas with protons of 0.5 to 6 keV the energy deposition mechanism shifts gradually from predominantly nuclear collisions to predominantly electronic processes. The existing theory of nuclear sputtering predicts very well the yield of ejected water molecules and the experimental results in the region of electronic processes agree well with the experimental results of Lanzerotti, Brown and Johnson. However, the major mass loss from water by ion bombardment is via the ejection of O2, H2 and H atoms, which exceed the ejection of water molecules. O2 and H2 production is markedly enhanced at temperatures exceeding ~100 K, whereas H2O and H production are temperature independent, suggesting that O2 and H2 are produced in the bulk of the ice whereas H2O and H atoms are ejected from the surface or near surface layers.
    Surface Science 01/1985; · 1.87 Impact Factor
  • A. Bar-Nun, G. Herman, M. L. Rappaport, Yu. Mekler
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    ABSTRACT: The sputtering from water ice at 30-140 K by H(+) ions of 0.5-6.0 keV was studied experimentally. The sputtering of water molecules was found to be temperature-independent over the whole temperature range. Neon ions deposit their energy in the ice by nuclear collisions, while for protons the energy deposition mechanism shifts gradually from predominantly nuclear collisions to predominantly electronic processes from 0.5 to 6.0 keV. The existing theory of nuclear sputtering predicts very well the yield of ejected water molecules and the experimental results in the region of electronic processes agree well with the experimental results of Lanzerotti, Brown and Johnson. However, the major mass loss of water bombarded by ions is via the ejection of O2 and H2 molecules and of H atoms, which exceed the ejection of water molecules. O2 and H2 production is markedly enhanced at temperatures exceeding 100 K whereas H2O and H production are temperature-independent, suggesting that O2 and H2 are produced in the bulk of the ice whereas H2O and H atoms are ejected from the surface or near surface layers. About 2 percent of the mass loss is due to the ejection of positive ions and clusters.
    01/1985; -1:287-298.
  • Source
    A. Bar-Nun, M. Litman, M. L. Rappaport
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    ABSTRACT: The formation of methane and other hydrocarbons by the reaction of hydrogen atoms with graphite when both are at T greater than or equal to 7 K is demonstrated experimentally. Thus, hydrocarbon formation by impinging H-atoms on graphite grains should be operable also in dark and cold interstellar clouds. The observed hydrocarbon formation at these low temperatures suggests that quantum-mechanical tunnelling is effective also in the H-atom graphite reaction and not only in the recombination of H-atoms on graphite surfaces. At T less than or equal to 20 K a monolayer of hydrocarbons, mostly of methane, would be formed on the graphite grains. This mantle could be polymerized by short UV photons and low energy cosmic rays into carbonaceous substances similar to those found in carbonaceous chondrites.
    Astronomy and Astrophysics 04/1980; 85:197-200. · 5.08 Impact Factor