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

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
Kinetic Picture
- electrons are inelastically scattered on a
surface
Picture from H.Lüth, Solid Surfaces, Interfaces
and Thin Films, Springer 2001
7
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 - 2.4105 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 _at_ ID gt 10pAdE 2meV _at_ ID
gt 70pA in straight through mode
Top picture from H.Lüth, Solid Surfaces,
Interfaces and Thin Films, Springer 2001, bottom
from www.specs.de
12
Limitations
13
New Concept
14
Theoretical Background
15
Pulse Formation
16
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
11 ?
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
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