Chem 125 Lecture 5
9/12/08
Projected material
This material is for the exclusive use of
Chem 125 students at Yale and may not
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It is not readily understood without
reference to notes from the lecture.
Despite Earnshaw,
might there still be
shared-pair bonds
and lone pairs?
Scanning Probe Microscopies
(AFM, STM, SNOM)
are really powerful.
Sharp points can resolve
individual molecules
and even atoms
but not bonds
Lux
A lonely
architectural
curiosity on
Sterling
Chemistry
Laboratory
at Yale
University
(1923)
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.”
Strong
400 nm
Scattering
No
800 nm
Scattering
“Thickness” ~ 200 nm
Oil
Path Difference = 400 nm
=1l
= 0.5 l
Interference upon Scattering
Water
Chris Incarvito’s New Toys
User Operated - CCD Detector
~$200K
X-Ray
Tube
~$350K
Image Plate
"Seeing" Individual
Molecules, Atoms,
and Bonds?
Problem:
l
What IS light?
In What Way is Light a Wave?
Force on Charge at One Position
Up
Charged
Particle
0
Down
Time
Force at Different Positions - OneTime
In What Way is Light a Wave?
Up
Charged
Particle
0
Down
Position
Accelerated Electrons “Scatter” Light
direct beam
Why don’t protons or other nuclei scatter light?
Too heavy!
Interference of Ripples
Angular
Intensity
Distribution
at
great distance
depends on
Scatterer
Distribution
at
the origin
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.
"Seeing"
Molecules, Atoms, Bonds
Collectively
by X-Ray Crystallography
Blurring Problem from Motion and Defects
Blurring Problem
Time Averaging
Space Averaging in Diffraction
(Cooperative Scattering)
Advantage for SPM
(Operates in Real Space)
Shadow of
Frau Röentgen’s
hand (1896)
In 1895 Röntgen
Discovers X-Rays
In 1912 Laue
Invents
X-Ray
Diffraction
CuSO4 Diffraction
(1912)
Wm. Lawrence Bragg
(1890-1971)
Determined structure of
ZnS from Laue's
X-ray diffraction
pattern (1912)
(1915)
Courtesy Dr. Stephen Bragg
Youngest Nobel Laureate
B-DNA
R. Franklin
(1952)
Science, 11 August 2000
>100,000 atoms
+ hydrogens!
25 nm (250 Å)
What can X-ray diffraction show?
Molecules? Atoms? Bonds?
How does diffraction work?
Like all light, X-rays are waves.
Wave
Machines
Bragg Machine
http://www.eserc.stonybrook.edu/ProjectJava/Bragg/
in & out
same phase
Breaks?
by permission, Konstantin Lukin
Two Scattering Directions are Always Exactly in Phase
Specular
perpendicular to
scattering vector
Specular
All electrons on a plane
perpendicular
to
Direct
the scattering vector
scatter in-phase at
the specular angle !
Electrons-on-Evenly-Spaced-Planes Trick
3
4
2
1
Electrons-on-Evenly-Spaced-Planes Trick
Total
Electrons
Suppose l &
angle such that:
3
+4
1l
+2
2l
3l
3
4
2
1
+1
10
Net in-phase
scattering
Electrons-on-Evenly-Spaced-Planes Trick
Total
Electrons
3 3
+4 -4
0.5l
+2 +2
1l
1.5l
3
4
2
1
Suppose first path difference is half a wavelength,
because of change in l (or angle)
+1 -1
10 0
Net in-phase
scattering
spacing = 10.8 cm
…………………..
Q. What is the line spacing?
DIFFRACTION
MASK
(courtesy T. R. Welberry, Canberra)
10.6 m
View from
Ceiling
633 nm
End of Lecture 5
Sept 12, 2008