chem 125 lecture 6 9/18/06 projected material this material is for the exclusive use of chem 125...
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Chem 125 Lecture 69/18/06
Projected material
This material is for the exclusive use of Chem 125 students at Yale and may not
be copied or distributed further.
It is not readily understood without reference to notes from the lecture.
BenjaminFranklin1706-1790
PhilosophicalTransactions
of theRoyal Society
1774
Portrait by Paulze Lavoisier 1783http://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.
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." …
Benj. Franklin to Wm. Brownrigg (1773)
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.
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. I then went to the windward side where they began to form; and there the oil, though not more than a teaspoonful, produced an instant calm over a space several yards square which spread amazingly and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass.
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;
1 tsp ≈ 5 cm3
0.5 acre ≈ 2000 m2 = 2 x 107 cm2
layer thickness ≈ 5 cm3 / 2 x 107 cm2
= 2.5 x 10-7 cm = 2.5 nm = 25 Å
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
Cantilever ChoiceOnly one of these five cantilevers is used in any one experiment. They differ in stiffness and ability to twist.
The smallest scaledivision is 20 m
Hair
Scanning Tunneling Microscopy (1999)
Geo. FlynnD. Yablon
(Columbia Univ)
Br
O
Br(CH2)11COOHon graphite
SNOM Scanning Near-FieldOptical MicroscopeScanning Near-FieldOptical Microscope
Glass Fiber
Aluminum Coating
100 nm Aperture
Lens
Emitted Light
Detector
Sample (scanned)
Light
Scanning Probe Microscopies(AFM, STM, SNOM)are really powerful.
Sharp points can resolve
individual moleculesand even atomsbut not bonds
MicrographiaRobert 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.”
ChargedParticle
How is Light a Wave?F
orce
at
Diff
eren
t P
ositi
ons
- O
neT
ime Up
Down
0
Position
Blurring Problem
Blurring Problem from Motion and Defects
Time Averaging
Space Averaging in Diffraction(Cooperative Scattering)
Advantage for SPM(Operates in Real Space)
In 1895 RöntgenDiscovers X-Rays
Shadow ofFrau Röntgen'sHand (1895)
In 1912 LaueInventsX-Ray
DiffractionCuSO4 Diffraction
(1912)
Wm. Lawrence Bragg(1890-1971)
Determined structure of ZnS from Laue'sX-ray diffraction
pattern (1912)
Youngest Nobel Laureate(1915)
What can X-ray diffraction show?
How does diffraction work?
Like all light, X-rays are waves.
Atoms?Molecules? Bonds?
Real Space "Reciprocal" Space
Material Diffraction Photo
Molecular Structure Fuzzy Pattern
Crystal Lattice Viewing Holes
Decreasing Spacing Increasing Spacing
Direct
Two Scattering Directions are Always Exactly in Phase
“scattering vector”
Specular perpendicular to scattering vector
All electrons on a planeperpendicular to
the scattering vectorscatter in-phase at
the specular angle !
Specular
10
Simplification from Electrons-on-a-Plane Trick
scattering vector 3
+2
+4
+1
324 1
12
3
Net in-phasescattering
TotalElectrons
Water
Oil“Thickness” ~ 200 nm
Path Difference = 400 nm
= 0.5
Strong400 nm
Scattering
No800 nm
Scattering
= 1
10
Simplification from Electrons-on-a-Plane Trick
scattering vector 3
+2
+4
+1
3
+2
-4
-1
0324 1
Suppose first path difference is half a wavelength.(Change or angle)
Net in-phasescattering
TotalElectrons
0.5
11.5
Filament
Light BulbFilament(helix)
X angle tellshelix pitch
Spot spacingtells scale
Spot spacingtells scale
Spots weakensuccessively
(from wirethickness)
(given &
screen dist)