Transcript Lecture Slides 9/18/06

Chem 125 Lecture 6
9/18/06
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Newton’s Rings
62 mm
1 mm
~1 mm
gap
at rim
flat
convex
to
flat
Simpler Measurement
of Smaller Distance
1774
Philosophical
Transactions
of the
Royal Society
1774
Benjamin
Franklin
1706-1790
Portrait by Paulze Lavoisier 1783
http://moro.imss.fi.it/lavoisier
Benj. Franklin to Wm. Brownrigg (1773)
…I had, when a youth, read and smiled
at Pliny's account of a practice among the
seamen of his time, to still the waves in a
storm by pouring oil into the sea; as well as
the use made of oil by the divers... I think
that it has been of late too much the mode
to slight the learning of the ancients. The
learned, too, are apt to slight too much the
knowledge of the vulgar.
Benj. Franklin to Wm. Brownrigg (1773)
In 1757, being at sea in a fleet of ninety-six sail
bound against Louisbourg, I observed the wakes of
two of the ships to be remarkably smooth, while all
the others were ruffled by the wind, which blew fresh.
Being puzzled with the differing appearance, I at last
pointed it out to our captain and asked him the
meaning of it. "The cooks," said he, "have I suppose
been just emptying their greasy water through the
scuppers, which has greased the sides of those ships a
little." …
recollecting what I had formerly read in Pliny, I
resolved to make some experiment of the effect of
oil on water when I should have the opportunity.
Clapham
Common
Franklin's Experiment
London, 1762
Benj. Franklin to Wm. Brownrigg (1773)
At length being at Clapham, where there is on the common
a large pond which I observed one day to be very rough with
the wind, I fetched out a cruet of oil and dropped a little of it
on the water. I saw it spread itself with surprising swiftness
upon the surface; but the effect of smoothing the waves was
not produced;
for I had applied it
first on the leeward side of the pond where the waves were
greatest; and the wind drove my oil back upon the shore.
3
1
tsp
≈
5
cm
I then went to the windward side where they began to form;
and there the oil, though3not more7 than2 a teaspoonful,
layer thickness ≈ 5 cm / 2 x 10 cm
produced an instant
calm over a space several yards square
-7
= 2.5
x 10amazingly
cm = 2.5and
nmextended
= 25 Å itself gradually till it
which
spread
reached the lee side, making all that quarter of the pond,
perhaps half an acre, as 0.5
smooth
a looking
acreas≈ 2000
m2 =glass.
2 x 107 cm2
Benj. Franklin to Wm. Brownrigg (1773)
When put on water it spreads instantly
many feet round, becoming so thin as to
produce the prismatic colours for a
considerable space, and beyond them so
much thinner as to be invisible except in
its effect of smoothing the waves at a
much greater distance. It seems as if a
mutual repulsion between its particles
took place as soon as it touched the water
Are there Electron Pairs?
Scanning Probe Microscopy
for Feeling Individual
Molecules,
Atoms,
Bonds?
The Challenge of Resolution
http://webs.wichita.edu/facsme/bonails.jpg
Scanning Tunneling
Microscopy (1981)
Gerd Binnig
Heinrich Rohrer
Nobel Prize (1986)
Atomic Force Microscopy
Cantilever Choice
Only one of these five cantilevers is used
in any one experiment. They differ in
stiffness and ability to twist.
Hair
The smallest scale
division is 20 mm
Scanning Electron Micrographs
of
AFM Cantilever
&
Tip
~20 nm
wide at bottom
Tips
Cantilever/Chip Holder
Cantilever/Chip Holder
Sensitive to
< 1 molecule
change in
height !
COOH tip
Scanning Tunneling Microscopy (1999)
Br(CH2)11COOH
on graphite
O
Br
Geo. Flynn
D. Yablon
(Columbia Univ)
Quantum corral
STM
Image
of Fe
atoms
on Cu
SNOM
Light
Scanning Near-Field
Optical Microscope
Glass Fiber
Aluminum Coating
100 nm Aperture
Sample (scanned)
Emitted Light
Lens
Detector
SNOM image of nanofabricated material
red wavelength
mm
scale
www.orc.soton.ac.uk
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 out understanding, and ocular
inspection may demonstrate that which as
yet we may think too extravagant either to
feign or suppose.”
"Seeing" Individual
Molecules, Atoms,
and Bonds?
Problem:
l
What IS light?
How is Light a Wave?
Force on Charge at One Position
Up
Charged
Particle
0
Down
Time
Force at Different Positions - OneTime
How is Light a Wave?
Up
Charged
Particle
0
Down
Position
Interference of Ripples
"Seeing" Individual
Molecules, Atoms, Bonds
Collectively
by X-Ray Crystallography
End
Be sure you have read the
webpage on x-ray diffraction.
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 Röntgen's
Hand (1895)
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)
Youngest Nobel Laureate
(1915)
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/
Breaks?
Real Space "Reciprocal" Space
Material
Diffraction Photo
Molecular Structure
Fuzzy Pattern
Crystal Lattice
Viewing Holes
Decreasing Spacing
Increasing Spacing
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 !
Simplification from Electrons-on-a-Plane Trick
3
4
2
1
Simplification from Electrons-on-a-Plane Trick
Total
Electrons
3
+4
1l
+2
2l
3l
3
4
2
1
+1
10
Net in-phase
scattering
Strong
400 nm
Scattering
No
800 nm
Scattering
“Thickness” ~ 200 nm
Oil
Path Difference = 400 nm
=1l
= 0.5 l
Water
Simplification from Electrons-on-a-Plane 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.
(Change l or angle)
+1 -1
10 0
Net in-phase
scattering
Benzene Snowflake
Slide with
Random
(but Oriented)
"Benzenes"
Benzene Snowflake
Isolated
“Benzenes”
Benzene Snowflake
Reciprocal
[1]
√3
1
Isolated
“Benzenes”
√3
Closely-spaced
planes give
high angles.
Benzene Snowflake
Isolated
“Benzenes”
2D Lattice
of
“Benzenes”
Filament
Light Bulb
Filament
(helix)
Filament
Light Bulb
Filament
(helix)
X angle tells
helix pitch
Spot spacing
tells scale
(given l &
screen dist)
Spots weaken
successively
(from wire
thickness)
B-DNA
R. Franklin
(1952)
Curious
Intensity
Sequence
S
vw
S
S
w
HELIX
Even
Double
Helix
Would
cancel
"Odd
reflections"
B-DNA
R. Franklin
(1952)
Curious
Intensity
Sequence
S
vw
S
S
w
Offset
Double
Helix
B-DNA
R. Franklin
(1952)
MAJOR Svw
& MINOR S S
w
GROOVES
HELIX DIAMETER
BASE
STACKING
“Confus’d Pulses of Light”
mica