matter-waves and electron diffraction
DESCRIPTION
Matter-Waves and Electron Diffraction. “The most incomprehensible thing about the world is that it is comprehensible.” – Albert Einstein. Day 18, 3/31: Questions? Electron Diffraction Wave packets and uncertainty. Next Week: Schrodinger equation The Wave Function Exam 2 (Thursday). - PowerPoint PPT PresentationTRANSCRIPT
Matter-Waves and Electron Diffraction
“The most incomprehensible thing about the world is that it is comprehensible.”
– Albert Einstein
Day 18, 3/31: Questions? Electron DiffractionWave packets and uncertainty
Next Week:Schrodinger equation
The Wave FunctionExam 2 (Thursday)
2
Recently: 1. Single-photon experiments2. Complementarity and wave-particle duality3. Matter-Wave Interference
Today: 1. Electron diffraction and matter waves2. Wave packets and uncertainty
Exam II is next Thursday 3/31, in class.HW09 will be due Wednesday (3/30) by 5pm in my mailbox.HW09 solutions posted at 5pm on Wednesday
Double-Slit Experiment
• Pass a beam of electrons through a double-slit apparatus.
• Individual electrons are detected as points on the screen.
• Over time, a fringe pattern of dark and light bands appears.
Double-Slit Experiment with Single Electrons (1989)
A. Tonomura, J. Endo, T. Matsuda, T. Kawasaki and H. Ezawa, "Demonstration of Single-Electron Buildup of an Interference Pattern,“Amer. J. Phys. 57, 117 (1989).
http://www.hitachi.com/rd/research/em/doubleslit.html
cos( )E A kx t
Double-Slit Experiment with Light
Recall solution for Electric field (E)from the wave equation:
cos( )E A kx
exp( ) cos( ) sin( )ikx kx i kx
cos( ) Re[ cos( ) sin( )] Re[ exp( )]E A kx A kx iA kx A ikx
exp( )E A ikx
Let’s forget about the time-dependence for now:
Euler’s Formula says:
For convenience, write:
exp( )A ikx
1 2Total
1 1 1exp( )A ikx 2 2 2exp( )A ikx
1 1 2 2exp( ) exp( )A ikx A ikx
Double-Slit Experiment with Electrons
Before Slits
After Slits
&
1 1 2 2exp( ) exp( )A ikx A ikx
Double-Slit Experiment
2 *[ ] | ( ) ( ) ( )P x x x x
1 1 2 2 1 1 2 2exp( ) exp( ) exp( ) exp( )A ikx A ikx A ikx A ikx
1 1 1 1 2 2 2 2exp( ) exp( ) exp( ) exp( )A ikx A ikx A ikx A ikx
1 1 2 2 2 2 1 1exp( ) exp( ) exp( ) exp( )A ikx A ikx A ikx A ikx
2 21 2 1 2 2 1 2 1exp( ( )) exp( ( ))A A A A ik x x ik x x
2 21 2 1 2 2 12 cos ( )A A A A k x x
Double-Slit Experiment2 *[ ] | ( ) ( ) ( )P x x x x
2 2 21 2 1 2 2 1( ) 2 cos ( )x A A A A k x x
2 2 21 2 1 2 2 1
2( ) 2 cos ( )x A A A A x x
2 21 1 1 1 1 1( ) exp( ) exp( )x A ikx A ikx A
2 22 2 2 2 2 2( ) exp( ) exp( )x A ikx A ikx A
How do we connect this with what we observe?
Double-Slit Experiment2 2 2
1 2 1 2 2 12[ ] ( ) 2 cos ( )P x x A A A A x x
21 2
2 INTERFERENCE!
• In quantum mechanics, we add the individual amplitudes andsquare the sum to get the total probability
• In classical physics, we added the individual probabilities to getthe total probability.
2 2 212 1 2 1 2 1 2[ ] [ ] [ ]P x P x P x
21
22
21 2
2 21 2
Double-Slit Experiment
1
2D
sinx D D m
1 2
sin
L
H
D
1x2x
2 1x x x m 0,1,2,...m
Determining the spacing between the slits:
sinH L L
mD
m LHD
Spacing between bright fringes
L
H
D
1x2x
m LHD
Distance to first maximum for visible light:
Let , ,1m 500 nm 7
4
5 10 0.001 5 10
radD
45 10 mD
If , then 3 mL 3 mmH L
L
H
D
1x2x
Double-Slit Experiment2 *[ ] ( ) ( ) ( )P x x x x
2 2 21 2 1 2 2 1
2( ) 2 cos ( )x A A A A x x
For a specific λ, describes where we findthe interference maxima and minimaof the fringe pattern.
2( )x
What is the wavelength of an electron?
As a doctoral student (1923), Louis de Broglie proposedthat electrons might also behave like waves.
• Light, often thought of as a wave, had been shown to have particle-like properties.
• For photons, we know how to relate momentum to wavelength:
Matter Waves
hcE hf
E p c
hp
From the photoelectric effect:
From special relativity:
Combined:
de Broglie wavelength:
hp
Confirmed experimentally byDavisson and Germer (1924).
Matter Waves
de Broglie wavelength: hp
In order to observe electron interference, it would be best to perform a double-slit experiment with:
A) Lower energy electron beam.B) Higher energy electron beam.C) It doesn’t make any difference.
Matter Waves
m LHD
de Broglie wavelength: hp
In order to observe electron interference, it would be best to perform a double-slit experiment with:
A) Lower energy electron beam.B) Higher energy electron beam.C) It doesn’t make any difference.
Matter Waves
m LHD
Lowering the energy will increase the wavelength of the electron.
Can typically make electron beams with energies from 25 – 1000 eV.
de Broglie wavelength: hp
For an electron beam of 25 eV, we expect Θ (the angle betweenthe center and the first maximum) to be:
A) Θ << 1B) Θ < 1C) Θ > 1D) Θ >> 1
45 10 mD
D
Use
and remember:
&
Matter Waves
2
2pEm
m LHD
346.6 10 J sh
de Broglie wavelength: hp
For an electron beam of 25 eV:
Matter Waves
34
131 19 2
(6.6 10 J s)
[2 (9.1 10 kg) (25 1.6 10 / )]
x
x eV x J eV
2
22pE p mEm
m LHD
7 6(3 )(4.9 10 ) 1.5 10 1.5H L m m m
de Broglie wavelength: hp
346.6 10 J sh For an electron beam of 25 eV:
for 45 10 mD
3410
24
6.6 10 2.4 10 2.7 10
h mp
74.9 10D
Too small to easily see!
Matter Waves
m LHD
de Broglie wavelength: hp
For an electron beam of 25 eV, how can we make the diffraction pattern more visible?
A) Make D much smaller.B) Decrease energy of electron beam.C) Make D much bigger.D) A & BE) B & C
Matter Waves
m LHD
de Broglie wavelength: hp 346.6 10 J sh
For an electron beam of 25 eV, how can we make the diffraction pattern more visible?
A) Make D much smaller.B) Decrease energy of electron beam.C) Make D much bigger.D) A & BE) B & C
25 eV is a lower bound on the energy of a decent electron beam.
Decreasing the distance between the slits will increase Θ.
Matter Waves
Slits are 83 nm wide andspaced 420 nm apart
S. Frabboni, G. C. Gazzadi, and G. Pozzi, Nanofabrication and the realization of Feynman’s two-slit experiment, Applied Physics Letters 93, 073108 (2008).
LHD
If we were to use protons instead of electrons,but traveling at the same speed, what happens to the distance to the first
maximum, H?A) IncreasesB) Stays the sameC) Decreases
L
H
D
1x2x
2
2pEm
hp
Which slit did this electron go through?
A) LeftB) RightC) BothD) NeitherE) Either left or right, we just can’t know which one.
Each electron passes through both slits, interferes with itself, then becomes localized when detected.
• For a low-intensity beam, can have as little as one electron in the apparatus at a time.• Each electron must interfere with itself• Each electron is “aware” of both paths.• Asking which slit the electron went through disrupts the fringe pattern.
Screen
Describe photon’s EM wave spread out in space.
High Amplitude
Low Amplitude
Light Waves
2probability of photon detection (amplitude of EM wave)
Light Waves
x
E-field x-section at screen
Photons
ρ(x) =probability density
2probability of photon detection (amplitude of EM wave)
32
Electron double slit experiment. Display=Magnitude of wave function
Large Magnitude (|Ψ|)= probability of detecting electron here is high
Small Magnitude (|Ψ|)= probability of detecting electron here is low
Follow same prescription for matter waves:
• Describe massive particles with wave functions:
• Interpret (wave amplitude)2 as a probability density:
( )x
2 *( ) ( ) ( ) ( )x x x x
What are these waves?EM Waves (light/photons)
• Amplitude = electric field• tells you the probability
of detecting a photon.• Maxwell’s Equations:
• Solutions are sine/cosine waves:
2EE
2
2
22
2 1tE
cxE
( , ) sin( )E x t A kx t
( , ) cos( )E x t A kx t
Matter Waves (electrons/etc)
• Amplitude = matter field• tells you the probability
of detecting a particle.• Schrödinger Equation:
• Solutions are complex sine/cosine waves:
2
ti
xm
2
22
2
( , ) expx t A i kx t cos( ) sin( )A kx t i kx t
Matter Waves
xL-L
• Describe a particle with a wave function.
• Wave does not describe the path of the particle.
• Wave function contains information about the probability to find a particle at x, y, z & t.
( , )x t
( , 0)x t ( , , , )x y z t( , , , )r t
In general:
Simplified:
&( , )x t ( )x2 *( ) ( ) ( ) ( )x x x x
Matter Waves
xL-L
L-L x
( , 0)x t
2( , 0) ( , 0)x t x t
Probability of finding particlein the interval dx is ( )x dx
Wavefunction
Probability Density
2[ ] ( ) ( ) 1P x x dx x dx
Normalized wave functions
36
Below is a wave function for a neutron. At what value of xis the neutron most likely to be found?
A) XA B) XB C) XC D) There is no one most likely
place
Matter Waves
L
a b c ddx
x
( )x ( ) from to xx x L x LL
0 elsewhere
How do the probabilities of finding the electron near (within dx) of a,b,c, and d compare?
A) d > c > b > aB) a = b = c = dC) d > b > a > cD) a > d > b > c
An electron is described by the following wave function:
An electron is described by the following wave function:
L
a bdx
x
( ) from to xx x L x LL
0 elsewhere
( )x
How do the probabilities of finding the electron near (within dx) of a,b,c, and d compare?
A) d > c > b > aB) a = b = c = dC) d > b > a > cD) a > d > b > c
c d
( )x
2 22
2( ) ( ) xx xL
Plane WavesPlane waves (sines, cosines, complex exponentials)
extend forever in space:
1 1 1( , ) expx t i k x t
4 4 4( , ) expx t i k x t
3 3 3( , ) expx t i k x t
2 2 2( , ) expx t i k x t
Different k’s correspond to different energies, since2 2 2 2
22
12 2 2 2
p h kE mvm m m
If and are solutions to a wave equation,
then so is1( , )x t 2 ( , )x t
1 2( , ) ( , ) ( , )x t x t x t
Superposition (linear combination) of two waves
We can construct a “wave packet” by combining many
plane waves of different energies (different k’s).
Superposition
fourier_en.jnlp
41
Superposition ( , ) expn n n
n
x t A i k x t
Plane Waves vs. Wave Packets ( , ) expx t A i kx t
( , ) expn n nn
x t A i k x t
Which one looks more like a particle?
Plane Waves vs. Wave Packets ( , ) expx t A i kx t
( , ) expn n nn
x t A i k x t
For which type of wave are the position (x) and momentum (p) most well-defined?
A) x most well-defined for plane wave, p most well-defined for wave packet.B) p most well-defined for plane wave, x most well-defined for wave packet.C) p most well-defined for plane wave, x equally well-defined for both.D) x most well-defined for wave packet, p equally well-defined for both.E) p and x are equally well-defined for both.
Uncertainty Principle
Δx
Δxsmall Δp – only one wavelength
Δxmedium Δp – wave packet made of several waves
large Δp – wave packet made of lots of waves
Uncertainty Principle
In math:
In words:The position and momentum of a particlecannot both be determined with complete precision. The more precisely one is determined, the less precisely the other is determined.
What do (uncertainty in position) and(uncertainty in momentum) mean?
2x p
xp
• Photons are scattered by localized particles.
• Due to the lens’ resolving power:
• Due to the size of the lens:
Uncertainty Principle
2xhx p
x
2xhp
A RealistInterpretation:
•Measurements are performed on an ensemble of similarly prepared systems.• Distributions of position and momentum values are obtained.• Uncertainties in position and momentum are defined in terms of the standard deviation.
Uncertainty Principle
A StatisticalInterpretation:
2
1
1 N
x ii
xN
2x p
• Wave packets are constructed from a series of plane waves.• The more spatially localized the wave packet, the less uncertainty
in position.• With less uncertainty in position comes a greater uncertainty in momentum.
Uncertainty Principle
A WaveInterpretation:
Matter Waves (Summary)• Electrons and other particles have wave properties
(interference)• When not being observed, electrons are spread out in space
(delocalized waves)• When being observed, electrons are found in one place
(localized particles)• Particles are described by wave functions:
(probabilistic, not deterministic)• Physically, what we measure is
(probability density for finding a particle in a particular place at a particular time)
• Simultaneous measurements of x & p are constrained by the
Uncertainty Principle:
( , )x t
2( , ) ( , )x t x t
2x p