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TOF-SIMS Machine and software.
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TOF-SIMS Machine and software.

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  • 1. Fragmentation of Organic Molecules Desorbed from a Surface using Secondary-Ion Mass Spectrometry Alger Pike Nicholas Winograd
  • 2. Secondary-Ion Mass Spectrometry SIMS
    • Imagine shooting a bullet into sand.
    • Shrink the bullet and sand to atomic scale.
  • 3. SIMS in Combinatorial Chemistry An optical image of a bead-holder specially fabricated for 60  m polystyrene beads (capacity: 10,000 beads/cm 2 ) The TOF image of Sasrin-Biotin beads loaded into the bead-holder. The molecular mass of biotin 245.3 amu is shown in green. R. M. Braun, A. Beyder, J. Xu, M. C. Wood, A. G. Ewing and N. Winograd, Anal. Chem. 71, 3318 (1999).
  • 4. Why Study Fragmentation?
    • To learn what factors contribute to fragmentation patterns seen in SIMS spectra
    • To establish control over these factors in order to allow easier interpretation of spectra and assay of complex organic mixtures
    • To expand applications for imaging SIMS of organic molecules – i.e. screening combinatorial libraries
  • 5. Types of Fragmentation 1) Direct Desorption 2) Desorb then Fragment 3) Ion-Beam Induced
  • 6. The ARTOF-SIMS Machine
  • 7. ARTOF-SIMS Sample Manipulator a) Azimuthal Rotation: Sample rotates like the hands of a clock, around the center of the crystal. b) Polar Rotation: Sample rotates like a revolving door, around the axis perpendicular to the azimuthal axis.
  • 8. Ion Trajectories
  • 9. Simulation Parameters
  • 10. Simulation Results
  • 11. Simulation vs. Real
  • 12. Machine Automation
  • 13. Design Goals
    • Easy to use Graphical User Interface (GUI)
      • Most common controls on main window
      • Redundancy for heavily used features
    • High-speed data throughput
      • Optimized I/O is ~100 times faster
      • Device driver allows application level I/O
      • real-time Windows NT/2000
      • Interrupt driven / multithreaded code
    • High level of automation
      • “ Go button” ease of use
  • 14. Controllable Parameters
  • 15. CD 3 S-Cu (100): The Model System 1) Chemisorption to a four-fold hollow 3) No domains 2) No tilt angle due to short R group 4) Simplified mass spectra Imanishi, S. Takenaka, T. Yokoyama, Y. Kitajima and T.Ohta J. PHYS. IV FRANCE 7 (1997)
  • 16. FCC (100) Surface 1) Red atom is an ejecting atom 2) Blue atoms are blocking atoms 3) SIMS yield decreases with blocking
  • 17. Azimuthal Distribution of Cu(100) 50 0
  • 18. 1200 L CD 3 SH on Cu(100) High Mass
  • 19. 1200 L CD 3 SH on Cu (100) Low Mass
  • 20. Overall Fragmentation of CD 3 SH (total fragments signal) (total monolayer signal) F =
  • 21. Direct Desorption Fragmentation Low energy cascade that desorbs an ion which does not have enough energy to fragment.
  • 22. Direct Desorption Fragmentation
  • 23. Fragmentation After Desorption High energy cascade leads to desorbed species which have enough energy to further fragment.
  • 24. Fragmentation After Desorption
  • 25. Ion-Beam Induced Fragmentation The ion beam directly breaks a bond which causes desorption of the fragment into vacuum.
  • 26. Ion-beam Induced Fragmentation (fragment signal) (total monolayer signal) C =
  • 27. Two-Body Model Simulation
  • 28. Ion Trajectories at 35 º Incidence Angle
  • 29. Summary
    • Specialized TOF-SIMS machine built
      • Designed with computer to optimize parameters
      • Fully automated: computer manages the system
    • Fragmentation shows orientation effects
      • CuM + yield is higher with normal primary ions
      • Primary ion interactions create fragmentation
    • Three distinct fragmentation mechanisms
      • Direct desorption of CuM +
      • CD 3 + metastable decay of high energy M +
      • C + and D + fragments created by primary ions
  • 30. Acknowledgements
    • People
      • Nick Winograd
      • Barbara Garrison
      • Winograd and Garrison Groups
    • Money
      • ONR, NIH, NSF
  • 31. Further Evidence for Cu 2 SCD 3 + Ejection
    • No SCD 3 + ions seen in SIMS
    • Small amounts of CuSCD 3 + are seen
    • Cu 2 + itself is formed by recombination i.e. there are no ejected Cu dimers
    • Peak width of Cu 2 SCD 3 + is consistent with dimer formation not trimer
  • 32. Further Evidence for CD 3 + Formation XSCD 3 +  XS + CD 3 +
  • 33. Image Potential
  • 34. Ion Profile Above Extraction Plate