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Present Results in Time of Flight - High Resolution Electron Energy Loss Spectrometry

Present Results in Time of Flight - High Resolution Electron Energy Loss Spectrometry. by Christian Peineke October 29, 2002. Outline. Introduction. Conclusion. TOF-HREELS. Theoretical Background. Results. Construction. Introduction.

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Present Results in Time of Flight - High Resolution Electron Energy Loss Spectrometry

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  1. Present Results in Time of Flight - High Resolution Electron Energy Loss Spectrometry by Christian Peineke October 29, 2002

  2. Outline Introduction Conclusion TOF-HREELS TheoreticalBackground Results Construction

  3. Introduction Vibrational and electronic excitations of materials } Spectroscopy

  4. Introduction Optical Spectroscopy: • spectral range from FIR to X-Ray with many sources • detection difficult for low energies • very high spectral resolution • difficult to obtain both quantities é' and é" • no q dependent measurements or only for very small q

  5. Introduction Particle Spectroscopy: • very large spectral range with one source • detection easy for all energies of charged particles • high resolution possible • both quantities é' and é" are measured • nearly all q are accessible

  6. k = incoming electron ks = elast. scattered electron k' = inelast. scattered electron q = momentum transfer Kinetic Picture - electrons are inelastically scattered on a surface Picture from H.Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001

  7. Differential scattering cross section: Dynamic Picture Dielectric interaction of a charged particle with a surface Picture from H.Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001

  8. Applications - measurement of bulk, surface and adsorbate excitations in a broad energy range (10 meV - 30 eV, 100 cm-1 - 2.4·105 cm-1) - measurement of dispersion relations Picture from H.Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001

  9. Example The q dependent measurement: The big advantage of EELS Picture from Oshima et al. Phys. Rev. B 36, 7510 (1987)

  10. Dispersion Relations Pictures from Oshima et al. Phys. Rev. B 36, 7510 (1987)

  11. Common Equipment Resolution: dE = 1meV @ ID > 10pAdE = 2meV @ ID > 70pA in straight through mode Top picture from H.Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001, bottom from www.specs.de

  12. - maximum monochromatic current limited - maximum energy resolution limited - serial detection system àlong measurement times Limitations

  13. Sample Delay Function Generator Start Stop TAC MSP New Concept

  14. Theoretical Background

  15. calculated from electrostatics: Pulse Formation

  16. vmax vf vs vmin td Δt2 Δt1a Δt1b Energy resolution of the monochromator: vmax vf vs vmin td Çt2 Çt1a Çt1b Energy Resolution s d1 t s d1 t

  17. Analysis Plots showing dependency of energy resolution on several parameters

  18. Construction

  19. Housing

  20. Shielding

  21. Details

  22. Construction Gun and deflection plates Magnetic shielding

  23. Old Detector Standard flat anode mount (El-Mul Technologies) Pictrure from www.tectra.de

  24. New Detector Impedance matched detector

  25. Results 1+1 = ?

  26. Short Pulses Pulses obtained in straight through mode

  27. Monochromatization

  28. Loss Measurements Measured in reflection without monochromator:

  29. Loss Measurements

  30. Monochromated Losses

  31. Summary Results from straight through experiments: • Pulses in the length of nanoseconds can be generated • The pulse length can be reduced by the second pulse (~50%) • Elastically scattered electrons can be detected and identified • They show the correct behavior

  32. Summary Not completely solved: • UFE } Unidentified Flying Electrons • Pulse length reduction not proven in scattering • Monochromatization not proven

  33. Summary From April 2002: To do: - improve magnetic shielding - test system on magnetic parts - specimen holder Add ons: - transfer system - nice electronics - sell it

  34. Short term problems Long term problems Solution Gun not at optimum known, ready to implement Detector not at optimum known, easy to implement Magnetic shielding known, expensive to implement Specimen holder known, depending on other solutions Status

  35. Outlook No physical, but engineering problems!

  36. Acknowledgement • Prof. Dr. M. Dressel for making this thesis possible and reporting • Prof. Dr. T. Pfau for co-reporting • Dr. B. Gompf, for thoroughly advising the thesis and all the help • Mrs. G. Untereiner, for helping with many details • Mr. M. Scheffler, for contributing the reflectance factor measurements • Mr. T. Brandt, for plenty of help and a very good time in the lab

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