University of Groningen Hollow-atom probing of surfaces Limburg, Johannes IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 1996 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Limburg, J. (1996). Hollow-atom probing of surfaces Groningen: s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 03-05-2018
Contents Preface v 1 Objectives 1 2 Fundamentals 7 2.1 Introduction..... 7 2.2 Charge exchange......... 8 2.2.1 One-electron transitions........ 8 2.2.2 Auger transitions..... 10 2.2.3 Collective excitations... 11 2.2.4 Radiative decay... 12 2.3 The classical over-the-barrier model... 12 2.3.1 Capture distance and image charge acceleration.... 13 2.4 Hollow atom formation and deexcitation... 15 2.5 Experimental approaches... 16 2.5.1 Scattered projectiles... 17 2.5.2 Sputtered material.... 17 2.5.3 Electron emission..... 18 2.5.4 Photon emission... 20 2.5.5 Computer simulations......... 20 3 Experiment 21 3.1 ECR ion source... 21 3.2 Sir... 24 3.2.1 Collimator and deceleration system......... 26 3.2.2 Electrostatic analyzer... 28 3.2.3 Time of ight system... 30 3.3 Targets... 33 xi
xii Contents 4 KLL Auger spectra of hydrogenic ions 35 4.1 Introduction...... 36 4.2 Targets... 36 4.3 Electron spectra... 38 4.4 Identication of the peaks..... 41 5 Atomic structure calculations 47 5.1 Introduction...... 48 5.2 Limited observation time... 48 5.3 Calculations for hollow atoms... 53 6 Coster-Kronig transitions in hollow atoms 61 6.1 Introduction...... 62 6.2 Experimental results........ 62 6.3 Coster-Kronig transitions and rate equations... 64 6.4 Discussion....... 67 7 An L-shell lling model 69 7.1 Introduction...... 70 7.2 Experiment... 70 7.3 Ion trajectory simulations..... 72 7.4 Modelling KLL Auger emission......... 75 7.5 Discussion....... 83 8 Hollow atom formation on an insulator surface 85 8.1 Introduction...... 86 8.2 Experiments... 86 8.2.1 The N 6+ series....... 88 8.2.2 The N, O, Ne series... 89 8.3 Discussion....... 91 9 Ion scattering: He on Al(110) 93 9.1 Introduction...... 94 9.2 Experiment... 94 9.2.1 Energy loss experiments... 94 9.2.2 Measurement ofcharge state fractions........ 96 9.2.3 Trajectory simulations... 96 9.3 Charge exchange... 97 9.3.1 He ++ projectiles...... 97 9.3.2 He + projectiles....... 99 9.3.3 Modelling charge state fractions...100 9.3.4 Discussion...100 9.4 Energy loss...102
xiii 9.4.1 Modelling energy loss...102 9.4.2 Discussion.........104 9.4.3 Results...106 9.4.4 Stopping power...106 10 Ion scattering: O q+ on Al(110) 109 10.1 Introduction.....110 10.2 Scattered charge state fractions........110 10.3 Energy loss...112 11 Negative ion formation 115 11.1 Introduction.....116 11.2 Experiment...116 11.3 A simple model...119 11.4 Results and discussion......125 12 Conclusions and outlook 129 12.1 Timescales of hollow atom deexcitation...129 12.2 Target eects.... 130 12.3 The evolution of hollow atoms.........133 12.4 Hollow-atom probing of surfaces...135 Appendices 136 A Atomic Units 137 B Chopper pulse width 139 B.1 Bunching eects...139 B.2 Lens system.....141 B.3 Geometry eect...142 C Solution of Coster-Kronig rate equations 143 Bibliography 147 List of publications 159 Samenvatting 161 Dankwoord 165
List of Tables 4.1 Measured and calculated K 1 LL Auger energies for C up to F 43 5.1 He KLL Auger transition energies and rates........ 52 5.2 Calculated KLL-Auger transition rates for Li-like systems.. 53 5.3 Calculated KLL Auger transition energies for Li-like systems 55 7.1 Simulated trajectory data.... 76 7.2 N 6+ hollow atom congurations, rates, energies...... 78 9.1 Comparison of AN rates A...101 11.1 C, O and F ionizaton and anity energies.........119 11.2 Electron conguration of O 0 3 P, 1 D and 1 S states.....124 11.3 Fit results using v? (1)...125 11.4 Fit results using v? (z c )...125 11.5 Fit results for and f;s 0...126 xv
List of Figures 2.1 Charge exchange processes.... 9 2.2 Ion-surface distances....... 13 2.3 Ion-surface potential curve... 15 3.1 Atomic physics hall........ 22 3.2 Electron Cyclotron Resonance Ion Source ECRIS4.... 23 3.3 Photograph of Sir set-up.... 25 3.4 Schematic of the Sir set-up... 26 3.5 Lens system, target manipulator and ESA......... 28 3.6 Beam prole... 30 3.7 Time of ight tube... 31 3.8 Chopper-sweeper system..... 32 3.9 TOF spectra for contaminated and clean Al........ 33 4.1 Auger spectrum of hydrogenic N on Ni(110)........ 37 4.2 KLL Auger spectra of O 7+ on Si(100)... 37 4.3 Doppler shift of sharp KLL peaks....... 39 4.4 Intensity of sharp Auger peaks versus velocity... 40 4.5 Auger spectra of hydrogenic C,N,O,F and Ne on Si(100)... 40 4.6 KLL Auger spectra of hydrogenic C, N and O on Ni(110).. 42 4.7 KLL Auger spectra of hydrogenic C,N,O and Ne on W.... 44 4.8 Auger spectra of O 7+ onw,niandsi... 45 5.1 Calculated and measured He KLL Auger spectra..... 49 5.2 He KLL-Auger peak intensities vs. observation time... 51 5.3 Calculated spectra for Li-like and hollow atom congurations 54 5.4 Calculated carbon KLL spectra........ 56 5.5 Relative energy shift of KLL-Auger peaks......... 58 5.6 KLL Auger spectrum for O 7+ on Si(100)... 59 6.1 Auger spectra of hydrogenic and helium-like ions on Si(100). 63 6.2 KLL Auger and Coster-Kronig transitions......... 64 6.3 Solutions of rate equations, O 7+... 65 xvii
xviii List of Figures 6.4 Solutions of rate equations, O 6+... 66 6.5 Calculated and measured spectra, O 7+ and O 6+... 67 7.1 KLL Auger spectra of N 6+ on Si(100), constant v?... 71 7.2 Auger spectra of N 6+ on Al(110), constant v?... 71 7.3 Auger spectra of N 6+ on Al(110), specular reection.... 73 7.4 Doppler shifts of sharp peaks and of FWHM... 73 7.5 Simulated trajectories for constant v?... 74 7.6 Simulated trajectories for specular reection......... 75 7.7 Calculations of Auger energies and lifetimes for hollow atoms 77 7.8 Calculated KLL Emission probability... 83 8.1 KLL Auger spectra of N 6+ on LiF(100)... 87 8.2 KLL Auger spectra of N 6+ on Si(100)... 87 8.3 KLL Auger spectra of N 6+,O 7+ and Ne 9+ on LiF(100)... 88 8.4 L and M electron binding energies for N, O and Ne hollow atoms... 90 9.1 Energy spectra of He scattered o Al(110)... 95 9.2 Fractions He + /(He + +He 0 ) vs. v... 95 9.3 Simulated trajectory lengths.... 97 9.4 He +,He 2+ neutralization scheme........ 98 9.5 Experimental and calculated loss curves...105 9.6 Fit results from the energyloss model......105 9.7 Stopping power versus v...107 10.1 Scattered charge state fractions O q+ /O tot...111 10.2 Energy loss distributions of scattered O r+ r = 1; 1; 2; 3...113 11.1 Negative ion fractions for C 4+ and C + scattered on Al(110). 117 11.2 Negative ion fractions for O 6+ and O +...117 11.3 Negative ion fraction for F 4+...118 11.4 Position of C, O, F anity and ionization levels...120 12.1 A hydrogenic ion approaches a conducting surface...131 12.2 Hydrogenic ion approaching an insulating surface......132 12.3 diabatic vs. adiabatic picture of hollow-atom evolution... 134 12.4 Ion-surface potential V t (z), diabatic and adiabatic picture.. 135 B.1 Pulse width and time spread of chopper-sweeper system... 140