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Laboratory measurements of sputtering and modeling of ion-surface interaction processes Marcelo Fama Laboratory for Atomic and Surface Physics University of Virginia R.A. Baragiola R.E. Johnson SERENA-HEWG Conference - Santa Fe, NM - May 12-14, 2008

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Outline Motivation Introduction Sputtering Linear Cascade Theory Sputtering of Compounds Surface Morphology Computer modeling Monte Carlo Molecular Dynamics Laboratory simulations Discussion

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Motivation A complex scenario Electron stimulated desorption Photon stimulated desorption Thermal desorption Sputtering induced by charged particles bombardment Chemical sputtering Meteoritic impact Exosphere Mercury - f (Z, m, E, Q) - Surface Composition and Morphology Magnetosphere

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Introduction Sputtering Ion beam (Z 1, m 1, E, Q, ) Target (Z 2, m 2, T) Y = atoms or molecules ejected incoming ion Elastic Sputtering Linear Cascade Theory (P. Sigmund 1969) Electronic Sputtering Primary excitation Secondary electrons Exciton/Hole Dynamics

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Introduction Linear Cascade Theory Mono-Atomic Targets : Target Parameters F D : Distribution of deposited-energy S n : Nuclear-stopping cross section (U) C 0 Differential cross section for elastic scattering (B-M) U 0 : Surface binding energy is an energy-independent function of the ratio between the mass of the target m 2 and of the projectile m 1 Normal Incidence P. Sigmund, Phys. Rev. 184 (1969) 383 Differential Yield Maximum at E S = U 0 / 2 E S -2 for E S >> U 0

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Mono-atomic targets Amorphous materials It works satisfactorily at intermediate and high energies (> 1keV) It doesn’t consider local U 0 Introduction Linear Cascade Theory Limitations U’ 0 > U 0

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Introduction Linear Cascade Theory Example #1: Si Sigmund’s C 0 = 1.8 x 10 -16 cm 2 C 0 = (x 0 N) -1 Sublimation Energy ~ U 0 = 4.7 eV Y calc. Y exp. 1 keV H + 0.110.008 4 keV He + 0.280.09 Problem partially solved by M. Vicanek et al., NIM B36 (1989) 124 refine calculation for C 0 W. Eckstein & R. Preuss, J. Nucl. Mater. 320 (2003) 209 Empirical Fit 4 HeSi

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Introduction Linear Cascade Theory Example #2: H 2 O (ice) Sigmund’s C 0 = 1.8 x 10 -16 cm 2 Water Ice C 0 = 1.3 x 10 -16 cm 2 Sublimation Energy ~ U 0 = 0.45 eV M. Famá et al., Surf. Sci. 602 (2008) 156

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Introduction Sputtering of ice grains and icy satellites in Saturn's inner magnetosphere, Planetary and Space Science, In Press R.E. Johnson, M. Famá, M. Liu, R.A. Baragiola, E.C. Sittler Jr, H.T. Smith Y = CASSINI

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Introduction Sputtering of Compounds Preferential sputtering Different binding energies Recoil implantation Radiation induced diffusion (segregation) Surface composition bulk composition

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Introduction Surface Morphology A P O Z = h(x,y) M.A. Makeev & A.L. Barabási, NIM B222 (2004) 316 Maximum enhancement in the yield ~200% T.A. Cassidy & R.E. Johnson, Icarus 176 (2005) 499 Monte Carlo simulations of sputtering within a regolith Y R c Y L (0) with 0.2 < c < 1

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~15 eV Semicond. ~25 eV Metals Displacement Energy Surface Binding Energy Lattice Binding Energy Computer Modeling Monte Carlo TRIM - Binary Collision Approximation Heat of Sublimation ~1-3 eV p T , T pEV(r)pEV(r) Equation of Motion

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Computer Modeling Monte Carlo TRIM – He + (4 keV) Albite Reliability of a popular simulation code for predicting sputtering yields of solids and ranges of low-energy ions K. Wittmaack, J. Applied Phys. 96 (2004) 2632 NaAlSi 3 O 8 Displacement Energy (eV) Surface Binding Energy (eV) Lattice Binding Energy (eV) Na251.123 Al253.363 Si154.72 O2823

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Computer Modeling Molecular Dynamics No assumptions or approximations other than V(r) and S e Complete description of the projectile-surface interaction Complete description of energy dissipation Local surface binding energy, S n, T m are naturally included Surface topography can be easily considered

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Experimental Methods Total Sputtering Yield for Minerals Cambridge A.J.T. Jull et al., NIM 168 (1980) 357 - Ion microprobe - Interferometry R National Physical Laboratory M.P. Seah et al., SIA 39 (2006) 69 - Mesh replica Virginia Not tested in minerals yet - Microgravimetry ff

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Experimental Methods Energy Distributions of Sputtered Species Post-ionization + -Electron beams -Low energy plasmas -Penning ionization -Post-ionizing laser Time of flight Secondary ions + - Non-radiative deexcitation - Neutralization Argonne National Laboratory M. J. Pellin (1998)

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Experimental Methods Complementary Techniques @ Virginia X-rays Ultra High Vacuum (~10 -10 Torr) Quartz Crystal Microbalance (~0.1 ML) + SIMS XPS NMSe-e- or TOF Nanosecond laser pulses (micrometeorite impact)

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Some Results XPS

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Some Results Thermal depletion of Na

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Some Results Depletion of Na due to ion bombardment

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Some Results Secondary ions energy distribution Ar + (4 kev) Albite

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Modeling Exosphere Mercury Magnetosphere SnU0C0SnU0C0 - Surface Composition - Morphology f (Z, E) Y i S n / (C 0 U 0 ) E i E / (E + U 0 ) 3 Y i + E i + exp(-b/ E) E / (E + U 0 ) 3 + + Instrument

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Modeling Laboratory Simulations Molecular Dynamics Mercury boundary conditions Magnetosphere Exosphere simulators Theory Sputtering of Minerals

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