Chem 125 Lecture 69/18/06

Projected material

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It is not readily understood without reference to notes from the lecture.

flatto

flat

62 mm

convex to

flat

Newton’s Rings

~1 mmgap

at rim

1 mm

Simpler Measurementof Smaller Distance

1774

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.

Franklin's ExperimentLondon, 1762

Clapham Common

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

Are there Electron Pairs?

Scanning Probe Microscopyfor Feeling Individual

Molecules, Atoms, Bonds?

The Challenge of Resolution

http://webs.wichita.edu/facsme/bonails.jpg

Scanning Tunneling Microscopy (1981)

Heinrich Rohrer

Gerd Binnig

Nobel Prize (1986)

Atomic Force Microscopy

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 Electron Micrographsof

AFM Cantilever & Tip

~20 nm wide at bottom

Cantilever/Chip HolderTips

Cantilever/Chip Holder

Sensitive to< 1 molecule

change inheight !

COOH tip

Scanning Tunneling Microscopy (1999)

Geo. FlynnD. Yablon

(Columbia Univ)

Br

O

Br(CH2)11COOHon graphite

Quantum corral

STMImageof Featomson Cu

SNOM Scanning Near-FieldOptical MicroscopeScanning Near-FieldOptical Microscope

Glass Fiber

Aluminum Coating

100 nm Aperture

Lens

Emitted Light

Detector

Sample (scanned)

Light

SNOM image of nanofabricated material

www.orc.soton.ac.uk

mscale

red

wav

elen

gth

Scanning Probe Microscopies(AFM, STM, SNOM)are really powerful.

Sharp points can resolve

individual moleculesand even atomsbut not bonds

Lux

A lonelyarchitecturalcuriosity on

SterlingChemistryLaboratory

at YaleUniversity

(1923)

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.”

"Seeing" IndividualMolecules, Atoms,

and Bonds?

Problem:

What IS light?

How is Light a Wave?F

orce

on

Cha

rge

at O

ne P

ositi

on

Up

Down

0

Time

ChargedParticle

ChargedParticle

How is Light a Wave?F

orce

at

Diff

eren

t P

ositi

ons

- O

neT

ime Up

Down

0

Position

Interference of Ripples

"Seeing" IndividualMolecules, Atoms, Bonds

Collectivelyby X-Ray Crystallography

End

Be sure you have read the webpage on x-ray diffraction.

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)

B-DNAR. Franklin

(1952)

Science, 11 August 2000

25 nm (250 Å)

>100,000 atoms+ hydrogens!

What can X-ray diffraction show?

How does diffraction work?

Like all light, X-rays are waves.

Atoms?Molecules? Bonds?

WaveMachines

Bragg Machinehttp://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

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

Simplification from Electrons-on-a-Plane Trick

scattering vector

324 1

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

Benzene Snowflake

Slide with Random

(but Oriented)"Benzenes"

Benzene Snowflake

Isolated“Benzenes”

Benzene Snowflake

1

√3√3

[1]Reciprocal

Isolated“Benzenes”

Closely-spacedplanes give

high angles.

Benzene Snowflake

Isolated“Benzenes”

2D Latticeof

“Benzenes”

Filament

Light BulbFilament(helix)

Filament

Light BulbFilament(helix)

X angle tellshelix pitch

Spot spacingtells scale

Spot spacingtells scale

Spots weakensuccessively

(from wirethickness)

(given &

screen dist)

HELIXw

S

Svw

SCuriousIntensitySequence

B-DNAR. Franklin

(1952)

EvenDouble Helix

Wouldcancel"Odd

reflections"

wS

Svw

SCuriousIntensitySequence

B-DNAR. Franklin

(1952)

OffsetDouble Helix

BASE STACKING

B-DNAR. Franklin

(1952)

wS

Svw

S

MAJOR& MINORGROOVES

HELIX DIAMETER

“Confus’d Pulses of Light”

mica