1. Chemistry 125: Lecture 5 X-Ray Diffraction
Because light is scattered by charged particles of small mass, the electron distribution in molecules can be determined by x-ray diffraction. The roles of molecular pattern and crystal lattice repetition can be illustrated by shining a visible laser through diffraction masks to generate scattering patterns reminiscent of those encountered in X-ray studies of ordered solids.
Synchronize when the speaker finishes saying, “…despite Earnshaw...” Synchrony can be adjusted by using the pause(||) and run(>) controls.
For copyright notice see final page of this file

2. Despite Earnshaw, might there still be shared-pair bonds and lone pairs?

3. Scanning Probe Microscopies (AFM, STM, SNOM)
are really powerful. Sharp points can resolve individual molecules and even atoms
but not bonds

4. Lux
In just the right light it is possible to perceive Boltwood’s foresight

5. A lonely architectural curiosity on Sterling Chemistry Laboratory at Yale University (1923)

6. Micrographia Robert Hooke (1665)
“But Nature is not to be limited by our narrow comprehension; future improvements of glasses may yet further enlighten our understanding, and ocular inspection may demonstrate that which as yet we may think too extravagant either to feign or suppose.”

7. Interference upon Scattering
Strong
400 nm
Scattering No
800 nm
Scattering
Path Difference = 400 nm
“Thickness” ~ 200 nm
Oil
= 1 l
= 0.5 l
Water
Interference upon Scattering

8. Chris Incarvito’s New Toys

9. User Operated - CCD Detector
~$200K
X-Ray
Tube

10. ~$350K
Image Plate
Curved
Image Plate

11. "Seeing" Individual Molecules, Atoms, and Bonds?
Problem:

12. Electromagnetic Waves
What IS light?
Electromagnetic Waves

13. In What Way is Light a Wave?
Force on Charge at One Position Up
Down
Charged
Particle
Time

14. In What Way is Light a Wave?
Force at Different Positions - OneTime Up
Down
Charged
Particle
Position

15. Accelerated Electrons “Scatter” Light
direct beam
Why don’t protons or other nuclei scatter light?
Too heavy!

16. Interference of Ripples
Angular
Intensity
Distribution at
great distance
depends on
Scatterer
the origin

17. But a lens for x-rays is hard to come by.
By refocussing, a lens can reassemble the information from the scattered wave into an image of the scatterers.
But a lens for x-rays is hard to come by.
Be sure to read the webpage on x-ray diffraction.

18. "Seeing" Molecules, Atoms, Bonds Collectively by X-Ray Crystallography

19. Blurring Problem Blurring Problem from Motion and Defects
Time Averaging
Space Averaging in Diffraction
(Cooperative Scattering)
Mask from T. R. Welberry, by permission
Advantage for SPM
(Operates in Real Space)

20. In 1895 Röntgen Discovers X-Rays
Shadow of
Frau Röentgen’s
hand (1896)
In 1895 Röntgen Discovers X-Rays
In 1912 Laue
Invents
X-Ray
Diffraction
CuSO4 Diffraction
(1912)

21. Wm. Lawrence Bragg (1890-1971) Determined structure of ZnS from Laue's
X-ray diffraction
pattern (1912)
Youngest Nobel Laureate
(1915)
Courtesy Dr. Stephen Bragg

22. B-DNA R. Franklin (1952)

23. Science, 11 August 2000

24. >100,000 atoms + hydrogens!
25 nm (250 Å)

25. What can X-ray diffraction show?
Molecules?
Atoms?
Bonds?
How does diffraction work?
Like all light, X-rays are waves.

26. Wave Machines

27. Bragg Machine Breaks? in & out same phase
in & out
same phase
Breaks?
by permission, Konstantin Lukin

28. Two Scattering Directions are Always Exactly in Phase
Specular
perpendicular to
scattering vector
“scattering vector”
Specular
All electrons on a plane
perpendicular to
the scattering vector
scatter in-phase at
the specular angle !
Direct

29. Electrons-on-Evenly-Spaced-Planes Trick
scattering vector 3 4 2 1

30. 3 +4 +2 +1 10 3 2 4 1 Electrons-on-Evenly-Spaced-Planes Trick
Total
Electrons
scattering vector 3
Suppose  &
angle such that: +4 1l +2 2l +1 3l 10 3 2 4 1
Net in-phase
scattering

31. Electrons-on-Evenly-Spaced-Planes Trick
Total
Electrons
scattering vector 3 3 +4 -4
0.5l 1l
1.5l +2 +2 +1 -1 10 3 2 4 1
Net in-phase
scattering
Suppose first path difference is half a wavelength,
because of change in  (or angle)

32. DIFFRACTION MASK (courtesy T. R. Welberry, Canberra)
spacing = 10.8 cm
…………………..
633 nm
View from
Ceiling
Q. What is the line spacing?
10.6 m
DIFFRACTION MASK (courtesy T. R. Welberry, Canberra)

33. End of Lecture 5 Sept 12, 2008
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J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0