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RECOMBINATION OF HYDROGEN ATOMS ON LOW-TEMPERATURE SURFACES:

RECOMBINATION OF HYDROGEN ATOMS ON LOW-TEMPERATURE SURFACES: MOLECULAR BEAM EXPERIMENTS COMBINING BOLOMETRY AND MASS-SPECTROMETRY REVISITED Thomas R.Govers*, Lorenzo Mattera**, Giacinto Scoles***

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RECOMBINATION OF HYDROGEN ATOMS ON LOW-TEMPERATURE SURFACES:

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  1. RECOMBINATION OF HYDROGEN ATOMS ON LOW-TEMPERATURE SURFACES: MOLECULAR BEAM EXPERIMENTS COMBINING BOLOMETRY AND MASS-SPECTROMETRY REVISITED Thomas R.Govers*, Lorenzo Mattera**, Giacinto Scoles*** Guelph-Waterloo Centre for Graduate Work in Chemistry and Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. Present addresses: (*) Aecono Consulting, 59 rue de Prony, 75017 Paris, France; (**) IMEM-CNR Genova and Physics Department, University of Genova, I-16146 Genova, Italy; (***) Frick Chemical Laboratory, Princeton University, Princeton, NJ 08544, USA

  2. Doped amorphous silicon bolometer serves as substrate and as energy detector Mass spectrometer measures density ( =flux / velocity) of molecules released from or reflected by the bolometer surface Surface is covered by polycrystalline water ice deposited during cooldown of the cryostat and is well controlled in terms of temperature ( 3 to 10 °K ) and of hydrogen coverage (H2 or D2)

  3. For additional information see: L. Mattera, Ph.D. Thesis, University of Waterloo, 1978; T.R. Govers, L.Mattera, G. Scoles, J. Chem. Phys.72, 5446 (1980); also: www.aecono.com.

  4. MOLECULAR HYDROGEN ON INITIALLY HYDROGEN-FREE SURFACE Changes in bolometer signal E and MS signal q reflect changes in sticking coefficient S, accomodation coefficient α, and binding energy ε

  5. For H2 , T = 293 K , Ts = 3 K E = S ( 724 + ε ) + α ( 1 – S ) 721 q(2) = ( 1 – S ) / [ 1 – 0.99 α ]1/2 We measure E and q(2) as a function of time, seek to know ε, α and S as function of time. Two equations, 3 unknowns : treat ε as adjustable parameter, impose constant value at short and long times, and absence of discontinuities in time derivatives of α and S. Analogous approach for D2 .

  6. MOLECULAR HYDROGEN ON INITIALLY HYDROGEN-FREE SURFACE

  7. FROM MOLECULAR HYDROGEN EXPERIMENTS ONE CONCLUDES: • Strong initial changes of ε, α and S over narrow coverage range : ½ ML H2increases α and S and decreases ε by a factor 2 or more ( 1 ML ≡ 1015 molecules/cm2 ) • H2build-up limited to ½ ML when Ts is raised above 3.5 K, confirming that ε has already decreased to 120 K at such limited coverage • Should expect strong influence of molecular hydrogen coverage on sticking and binding of atomic hydrogen, and therefore on recombination probability

  8. ATOMIC HYDROGEN ON INITIALLY HYDROGEN-SATURATED SURFACE • Low E compared to recomb. energy 25 980 K / atom • First order in H • No mass 3 • Recombination ejects D2 • Strong influence ofD2 coverage • No significant impact of coverage by N2, CO2, H2O: only H2 or D2matter

  9. H RECOMBINATION "BOILS OFF" PRE-ADSORBED D2 • Response to D2 beam is used to determine D2 coverage: intially 3 x 1015 molecules cm-2 ; arrows (1) 9 x 1014 cm-2, (2) 1.9 x 1014 cm-2, (3) 1.6 x 1014 cm-2, (4) 4.5 x 1013 cm-2 • Incident H flux is 3.8 x 1014 atoms cm-2 s-1 • Recombining fraction is about 0.15 or less (see below) • Each H2 molecule formed ejects at least 1 D2 molecule

  10. LOWER LIMIT TO Eint FROM S at t = 0 (D2 saturated surface)

  11. UPPER LIMIT TO Eint FROM S at t = ∞ (D2-free surface)

  12. ATOMIC HYDROGEN ON INITIALLY D2-SATURATED SURFACE • Maximum in bolometer signal reflects maximum in recombination probability Sβ • At "high" D2 coverage, S is rather large ( 0.5 to 0.8), but β is small, because residence time of H is low ( small binding energy ) • At low D2 coverage, S is small ( 0.06 to 0.16), while β approaches 1 • Optimum at about 0.25 ML D2 coverage, with S = 0.15 to 0.3 and β~ 0.5 • Recombination produces internally excited H2 • (most likely about 15 000 K, possibly more)

  13. BINDING ENERGIES • Al-Halabi et al., J.Phys.Chem. 2002 • Hollenbach & Salpeter, J.Chem.Phys. 1970 with revised • gas phase well depth ( Duquette 1977 : 130 i.o. 100 K ) • (3) Crampton et al., Phys.Rev.B 1982 • (4) Vidali et al., Surf.Sci.Reports 1991 • (5) Silvera, Rev.Mod. Phys. 1980

  14. On interstellar grains,H2will accumulate until the binding energy becomes low enough for evaporation to balance renewal 1013 exp – ε/kTs = ( S x nv / 4 ) / 1015 For T = 10 K, v (H2) = 3.25 x 104 cm/s ε / kTs = ln ( 1. 23 x 1024 / n ) gives limiting ε ε / kTs≈ ( 55.5 – ln n)

  15. In dense clouds (10 K, nH2~ 5 x 103 ) find ε≈ 480 K, i.e. 0.25 ML H2 coverage, and close to optimum H recombination efficiency : S = 0.15 to 0.4 for present T = 350 to 400 K, expect S ~ 1 for T = 10 K ( see El Halabi et al. ) while β~ 0.5 ► S β~ 0.5 for ice-covered grains in dense clouds With warmer grains / lower densities recombination is less efficient because molecular hydrogen coverage is lower and sticking coefficient small With colder grains / higher densities recombination is less efficient because higher H2 coverage results in very short H residence time

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