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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 Magnetosphere Exosphere**

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

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**Introduction Sputtering q Target (Z2, m2, T)**

Y = atoms or molecules ejected incoming ion Elastic Sputtering Electronic Sputtering q Linear Cascade Theory (P. Sigmund 1969) Primary excitation Secondary electrons Exciton/Hole Dynamics Ion beam (Z1, m1, E, Q, q)

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**Introduction Linear Cascade Theory Mono-Atomic Targets**

FD: Distribution of deposited-energy L: Target Parameters P. Sigmund, Phys. Rev. 184 (1969) 383 Normal Incidence Sn: Nuclear-stopping cross section (U) C0 Differential cross section for elastic scattering (B-M) U0: Surface binding energy a is an energy-independent function of the ratio between the mass of the target m2 and of the projectile m1 Differential Yield Maximum at ES = U0 / 2 ES-2 for ES >> U0

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**Introduction Linear Cascade Theory Limitations Mono-atomic targets**

Amorphous materials It works satisfactorily at intermediate and high energies (> 1keV) It doesn’t consider local U0 U’ > U0

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**Introduction Linear Cascade Theory Example #1: Si**

Sigmund’s C0 = 1.8 x cm2 C0 = (x0 N)-1 Sublimation Energy ~U0 = 4.7 eV Ycalc. Yexp. 1 keV H+ 0.11 0.008 4 keV He+ 0.28 0.09 Problem partially solved by M. Vicanek et al., NIM B36 (1989) 124 refine calculation for C0 Empirical Fit 4He Si W. Eckstein & R. Preuss, J. Nucl. Mater. 320 (2003) 209

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**Introduction Linear Cascade Theory Example #2: H2O (ice)**

M. Famá et al., Surf. Sci. 602 (2008) 156 Sigmund’s C0 = 1.8 x cm2 Water Ice C0 = 1.3 x cm2 Sublimation Energy ~U0 = 0.45 eV

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** Y = Introduction CASSINI**

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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**Sputtering of Compounds**

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 Z = h(x,y) P A O YR c YL(0)**

M.A. Makeev & A.L. Barabási, NIM B222 (2004) 316 O Maximum enhancement in the yield ~200% T.A. Cassidy & R.E. Johnson, Icarus 176 (2005) 499 Monte Carlo simulations of sputtering within a regolith YR c YL(0) with 0.2 < c < 1

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**TRIM - Binary Collision Approximation**

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

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**DisplacementEnergy (eV)**

Computer Modeling Monte Carlo TRIM – He+ (4 keV) Albite NaAlSi3O8 DisplacementEnergy (eV) Surface Binding Energy (eV) Lattice Na 25 1.12 3 Al 3.36 Si 15 4.7 2 O 28 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

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**Computer Modeling Molecular Dynamics**

No assumptions or approximations other than V(r) and Se Complete description of the projectile-surface interaction Complete description of energy dissipation Local surface binding energy, Sn, Tm are naturally included Surface topography can be easily considered

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**Total Sputtering Yield for Minerals**

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 Df - Microgravimetry

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**Energy Distributions of Sputtered Species**

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

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**Complementary Techniques @ Virginia**

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

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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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**Secondary ions energy distribution**

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

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**Modeling + + Yi Sn / (C0 U0) Ei E / (E + U0)3 Yi+**

Instrument Magnetosphere Exosphere + + Yi Sn / (C0 U0) Ei E / (E + U0)3 Yi+ Ei+ exp(-b/E) E / (E + U0)3 f (Z, E) Sn U0 C0 - Surface Composition - Morphology Mercury

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**Modeling Mercury boundary conditions Laboratory Simulations Molecular**

Dynamics Sputtering of Minerals Magnetosphere Exosphere simulators Theory

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**Questions & Suggestions**

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