De Broglie Waves, Bohr's Quantization, and Electron Scattering in Physics

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1
 
PHYS 3313 – Section 001
Lecture #16
 
Monday, Mar. 24, 2014
Dr. 
Jae
hoon 
Yu
 
De Broglie Waves
Bohr’s Quantization Conditions
Electron Scattering
Wave Packets and Packet Envelops
Superposition of Waves
Electron Double Slit Experiment
Wave-Particle Duality
 
 
 
Monday, Mar. 24, 2014
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Monday, Mar. 24, 2014
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
2
 
Announcements
 
Research paper template has been uploaded onto
the class web page link to research
Special colloquium on April 2, triple extra credit
Colloquium this Wednesday at 4pm in SH101
 
De Broglie Waves
 
Prince Louis V. de Broglie suggested that mass particles
should have wave properties similar to electromagnetic
radiation 
  
 
Thus the wavelength of a matter wave is called the de
Broglie wavelength:
 
 
This can be considered as the probing beam length scale
Since for a photon, 
E = pc 
and 
E = hf
, the 
energy can be
written as
 
 
 
 
 
Monday, Mar. 24, 2014
 
3
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
What is 
the formula for De Broglie Wavelength?
(a) for a tennis ball, m=0.057kg.
 
 
 
(b) for electron with 50eV KE, since KE is small, we can use non-relativistic
expression of electron momentum!
 
 
 
What are the wavelengths of you running at the speed of 2m/s?  What about your
car of 2 metric tons at 100mph?  How about the proton with 14TeV kinetic energy?
What is the momentum of the photon from a green laser?
 
 
Calculate the De Broglie wavelength of (a) a tennis ball of mass 57g traveling 25m/s
(about 56mph) and (b) an electron with kinetic energy 50eV.
 
Ex 5.2: De Broglie Wavelength
 
Monday, Mar. 24, 2014
 
4
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Bohr’
s Quantization Condition
 
One of Bohr’
s assumptions concerning his hydrogen atom
model was that the angular momentum of the electron-nucleus
system in a stationary state is an integral multiple of 
h
/2
π
.
The electron is a standing wave in an orbit around the proton.
This standing wave will have nodes and be an integral number
of wavelengths.
 
 
The angular momentum becomes:
 
 
 
Monday, Mar. 24, 2014
 
5
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Electron Scattering
 
 
Davisson and Germer experimentally observed that electrons were diffracted
much like x rays in nickel crystals. 
   
  
 
George P. Thomson (1892–1975), son of J. J. Thomson,
reported seeing the effects of electron diffraction in
transmission experiments. The first target was celluloid,
and soon after that gold, aluminum, and platinum were
used. The randomly oriented polycrystalline sample of
SnO
2
 produces rings as shown in the figure at right.
 
Monday, Mar. 24, 2014
 
6
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Photons, which we thought were waves, act particle
like (eg Photoelectric effect or Compton Scattering)
Electrons, which we thought were particles, act
particle like (eg electron scattering)
De Broglie: All matter has intrinsic wavelength.
Wave length inversely proportional to momentum
The more massive… the smaller the wavelength… the
harder to observe the wavelike properties
So while photons appear mostly wavelike, electrons
(next lightest particle!) appear mostly particle like.
How can we reconcile the wave/particle views?
 
Monday, Mar. 24, 2014
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
7
 
De Broglie matter waves suggest a further description.
The displacement of a wave is
 
This is a solution to the wave equation
 
 
Define the wave number 
k
 and the angular frequency 
as:
 
The wave function is now:
 
Wave Motion
 
Monday, Mar. 24, 2014
 
8
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Wave Properties
 
The phase velocity is the velocity of a point on the
wave that has a given phase (for example, the crest)
and is given by
 
A phase constant 
 shifts the wave:
     
   .
 
Monday, Mar. 24, 2014
 
9
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
(When 
=
/2)
 
Principle of Superposition
 
When two or more waves traverse the same region, they act
independently of each other.
Combining two waves yields:
 
 
The combined wave oscillates within an envelope that
denotes the maximum displacement of the combined
waves.
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Monday, Mar. 24, 2014
 
10
10
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Fourier Series
 
Adding 2 waves isn’t localized in space… but
adding lots of waves can be.
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Summing an infinite number of waves yields the
Fourier integral:
 
 
 
Monday, Mar. 24, 2014
 
11
11
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Wave Packet Envelope
 
The superposition of two waves yields a wave number and angular
frequency of the wave packet envelope.
 
 
The range of wave numbers and angular frequencies that produce the
wave packet have the following relations:
 
 
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The localization of the wave packet over a small region to describe a
particle requires a large range of wave numbers. Conversely, a small
range of wave numbers cannot produce a wave packet localized within
a small distance.
 
Monday, Mar. 24, 2014
 
12
12
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
A Gaussian wave packet describes the envelope of a
pulse wave.
 
 
 
 
 
 
 
 
The group velocity is
 
Gaussian Function
 
 
Monday, Mar. 24, 2014
 
13
13
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Dispersion
 
Considering the group velocity of a de Broglie wave packet
yields:
 
The relationship between the phase velocity and the group
velocity is
 
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Monday, Mar. 24, 2014
 
14
14
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Waves or Particles?
 
Young’s double-slit diffraction
experiment demonstrates the wave
property of light.
However, dimming the light results
in single flashes on the screen
representative of particles.
 
Monday, Mar. 24, 2014
 
15
15
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Electron Double-Slit Experiment
 
C. Jönsson of Tübingen,
Germany, succeeded in 1961
in showing double-slit
interference effects for
electrons by constructing very
narrow slits and using
relatively large distances
between the slits and the
observation screen.
This experiment demonstrated
that precisely the same
behavior occurs for both light
(waves) and electrons
(particles).
 
Monday, Mar. 24, 2014
 
16
16
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Which slit?
 
To determine which slit the electron went through: We set up a light
shining on the double slit and use a powerful microscope to look at
the region. After the electron passes through one of the slits, light
bounces off the electron; we observe the reflected light, so we know
which slit the electron came through.
Use a subscript 
ph
 to denote variables for light (photon). Therefore
the momentum of the photon is
 
 
The momentum of the electrons will be on the order of
  
         .
 
The difficulty is that the momentum of the photons used to determine
which slit the electron went through is sufficiently great to strongly
modify the momentum of the electron itself, thus changing the
direction of the electron! The attempt to identify which slit the electron
is passing through will in itself change the interference pattern.
 
Monday, Mar. 24, 2014
 
17
17
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Wave particle duality solution
 
The solution to the wave particle duality of an event
is given by the following principle.
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Monday, Mar. 24, 2014
 
18
18
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Uncertainty Principle
 
It is impossible to measure simultaneously, with no
uncertainty, the precise values of 
k
 and 
x
 for the same
particle. The wave number 
k
 may be rewritten as
 
 
For the case of a Gaussian wave packet we have
 
 
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Monday, Mar. 24, 2014
 
19
19
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Energy Uncertainty
 
If we are uncertain as to the exact position of a particle, for
example an electron somewhere inside an atom, the particle
can’
t have zero kinetic energy.
 
The energy uncertainty of a Gaussian wave packet is
 
 
combined with the angular frequency relation
 
Energy-Time Uncertainty Principle:
  
  .
 
 
Monday, Mar. 24, 2014
 
20
20
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Probability, Wave Functions, and the Copenhagen
Interpretation
 
The wave function determines the likelihood (or probability)
of finding a particle at a particular position in space at a
given time.
 
 
The total probability of finding the electron is 1. Forcing this
condition on the wave function is called normalization.
 
Monday, Mar. 24, 2014
 
21
21
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
The Copenhagen Interpretation
 
Bohr’s interpretation of the wave function consisted
of 3 principles:
The uncertainty principle of Heisenberg
The complementarity principle of Bohr
The statistical interpretation of Born, based on probabilities
determined by the wave function
Together these three concepts form a logical
interpretation of the physical meaning of quantum
theory. According to the Copenhagen interpretation,
physics depends on the outcomes of measurement.
 
Monday, Mar. 24, 2014
 
22
22
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
 
Particle in a Box
 
A particle of mass 
m
 is trapped in a one-dimensional box of width
 
l
.
The particle is treated as a wave.
The box puts boundary conditions on the wave. The wave function must be zero at the walls
of the box and on the outside.
In order for the probability to vanish at the walls, we must have an integral number of half
wavelengths in the box.
 
 
 
The energy of the particle is
    
 .
 
The possible wavelengths are quantized which yields the energy:
 
 
 
The possible energies of the particle are quantized.
 
Monday, Mar. 24, 2014
 
23
23
 
PHYS 3313-001, Spring 2014
Dr. Jaehoon Yu
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Discover the fascinating concepts of De Broglie waves, Bohr's quantization conditions, and electron scattering in physics. Delve into the wave-particle duality, electron double-slit experiments, and the groundbreaking observations by Davisson and Germer. Uncover the implications of mass particles having wave properties and the intriguing behavior of electrons in diffraction experiments. Dive into the world of quantum physics with this intriguing lecture series.

  • De Broglie Waves
  • Bohrs Quantization
  • Electron Scattering
  • Wave-Particle Duality
  • Physics

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  1. PHYS 3313 Section 001 Lecture #16 Monday, Mar. 24, 2014 Dr. Jae Jaehoon Yu De Broglie Waves Bohr s Quantization Conditions Electron Scattering Wave Packets and Packet Envelops Superposition of Waves Electron Double Slit Experiment Wave-Particle Duality Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Yu 1 Dr. Jaehoon Yu

  2. Announcements Research paper template has been uploaded onto the class web page link to research Special colloquium on April 2, triple extra credit Colloquium this Wednesday at 4pm in SH101 Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 2

  3. De Broglie Waves Prince Louis V. de Broglie suggested that mass particles should have wave properties similar to electromagnetic radiation many experiments supported this! Thus the wavelength of a matter wave is called the de Broglie wavelength: l = h p This can be considered as the probing beam length scale Since for a photon, E = pc and E = hf, the energy can be written as E =hf = pc = pl f Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 3

  4. Ex 5.2: De Broglie Wavelength Calculate the De Broglie wavelength of (a) a tennis ball of mass 57g traveling 25m/s (about 56mph) and (b) an electron with kinetic energy 50eV. l =h What is the formula for De Broglie Wavelength? (a) for a tennis ball, m=0.057kg. l =h p= 0.057 25 p 6.63 10-34 = 4.7 10-34m (b) for electron with 50eV KE, since KE is small, we can use non-relativistic expression of electron momentum! 1240eV nm 2 0.511MeV 50eV hc l =h h = p= = = 0.17nm 2meK 2mec2K What are the wavelengths of you running at the speed of 2m/s? What about your car of 2 metric tons at 100mph? How about the proton with 14TeV kinetic energy? What is the momentum of the photon from a green laser? Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 4

  5. Bohrs Quantization Condition One of Bohr s assumptions concerning his hydrogen atom model was that the angular momentum of the electron-nucleus system in a stationary state is an integral multiple of h/2 . The electron is a standing wave in an orbit around the proton. This standing wave will have nodes and be an integral number of wavelengths. nl = nh 2pr = p The angular momentum becomes: nh 2p pp = nh L = rp = 2p=n Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 5

  6. Electron Scattering Davisson and Germer experimentally observed that electrons were diffracted much like x rays in nickel crystals. direct proof of De Broglie wave! l =Dsinf n George P. Thomson (1892 1975), son of J. J. Thomson, reported seeing the effects of electron diffraction in transmission experiments. The first target was celluloid, and soon after that gold, aluminum, and platinum were used. The randomly oriented polycrystalline sample of SnO2 produces rings as shown in the figure at right. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 6

  7. Photons, which we thought were waves, act particle like (eg Photoelectric effect or Compton Scattering) Electrons, which we thought were particles, act particle like (eg electron scattering) De Broglie: All matter has intrinsic wavelength. Wave length inversely proportional to momentum The more massive the smaller the wavelength the harder to observe the wavelike properties So while photons appear mostly wavelike, electrons (next lightest particle!) appear mostly particle like. How can we reconcile the wave/particle views? Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 7

  8. Wave Motion De Broglie matter waves suggest a further description. The displacement of a wave is 2p l Y x,t ( )= Asin ( ) x-vt This is a solution to the wave equation 2Y x2=1 2Y t2 v2 Define the wave number k and the angular frequency as: k 2p l Y x,t w =2p l = vT and T [ ] ( )= Asin kx-wt The wave function is now: Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 8

  9. Wave Properties The phase velocity is the velocity of a point on the wave that has a given phase (for example, the crest) and is given by vph=l T= l 2p 2p T =w k A phase constant shifts the wave: Y x,t ( )= Asin kx-wt +f [ = Acos kx-wt [ (When = /2) ] . ] Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 9

  10. Principle of Superposition When two or more waves traverse the same region, they act independently of each other. Combining two waves yields: ( )= Y1x,t Dk 2 x-Dw cos kavx-wavt ( )+Y2x,t ( )= 2Acos ( ) Y x,t t 2 The combined wave oscillates within an envelope that denotes the maximum displacement of the combined waves. When combining many waves with different amplitudes and frequencies, a pulse, or wave packet, can be formed, which can move at a group velocity: ugr=Dw Dk Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 10

  11. Fourier Series Adding 2 waves isn t localized in space but adding lots of waves can be. The sum of many waves that form a wave packet is called a Fourier series: ( Aisin kix-wit [ ] i )= Y x,t Summing an infinite number of waves yields the Fourier integral: ( [ ] )= A k ( )cos kx-wt Y x,t dk Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 11

  12. Wave Packet Envelope The superposition of two waves yields a wave number and angular frequency of the wave packet envelope. Dk 2 x-Dw 2 The range of wave numbers and angular frequencies that produce the wave packet have the following relations: DkDx = 2p DwDt = 2p A Gaussian wave packet has similar relations: DkDx =1 DwDt =1 2 2 The localization of the wave packet over a small region to describe a particle requires a large range of wave numbers. Conversely, a small range of wave numbers cannot produce a wave packet localized within a small distance. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 12

  13. Gaussian Function A Gaussian wave packet describes the envelope of a pulse wave. Y x,0 ( ( ) )= Y x ( )= Ae-Dk2x2cos k0x ugr=dw The group velocity is dk Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 13

  14. Dispersion Considering the group velocity of a de Broglie wave packet yields: ugr=dE dp=pc2 E The relationship between the phase velocity and the group velocity is ugr=dw dk= dk )=vph+kdvph ( d vphk dk Hence the group velocity may be greater or less than the phase velocity. A medium is called nondispersive when the phase velocity is the same for all frequencies and equal to the group velocity. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 14

  15. Waves or Particles? Young s double-slit diffraction experiment demonstrates the wave property of light. However, dimming the light results in single flashes on the screen representative of particles. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 15

  16. Electron Double-Slit Experiment C. J nsson of T bingen, Germany, succeeded in 1961 in showing double-slit interference effects for electrons by constructing very narrow slits and using relatively large distances between the slits and the observation screen. This experiment demonstrated that precisely the same behavior occurs for both light (waves) and electrons (particles). Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 16

  17. Which slit? To determine which slit the electron went through: We set up a light shining on the double slit and use a powerful microscope to look at the region. After the electron passes through one of the slits, light bounces off the electron; we observe the reflected light, so we know which slit the electron came through. Use a subscript ph to denote variables for light (photon). Therefore the momentum of the photon is h >h Pph= lph d Pe=h ~h The momentum of the electrons will be on the order of . le d The difficulty is that the momentum of the photons used to determine which slit the electron went through is sufficiently great to strongly modify the momentum of the electron itself, thus changing the direction of the electron! The attempt to identify which slit the electron is passing through will in itself change the interference pattern. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 17

  18. Wave particle duality solution The solution to the wave particle duality of an event is given by the following principle. Bohr s principle of complementarity: It is not possible to describe physical observables simultaneously in terms of both particles and waves. Physical observables are the quantities such as position, velocity, momentum, and energy that can be experimentally measured. In any given instance we must use either the particle description or the wave description. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 18

  19. Uncertainty Principle It is impossible to measure simultaneously, with no uncertainty, the precise values of k and x for the same particle. The wave number k may be rewritten as k =2p l =2p h p= p2p =p h For the case of a Gaussian wave packet we have DkDx =DpDx =1 2 Thus for a single particle we have Heisenberg s uncertainty principle: DpxDx 2 Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 19

  20. Energy Uncertainty If we are uncertain as to the exact position of a particle, for example an electron somewhere inside an atom, the particle can t have zero kinetic energy. ( ) 2 Dp 2m 2 2 Kmin=pmin 2m 2ml2 The energy uncertainty of a Gaussian wave packet is DE = hDf = hDw = Dw 2p combined with the angular frequency relation DwDt =DEDt =1 2 Energy-Time Uncertainty Principle: . DEDt 2 Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 20

  21. Probability, Wave Functions, and the Copenhagen Interpretation The wave function determines the likelihood (or probability) of finding a particle at a particular position in space at a given time. P y ( )dy = Y y,t ( ) 2dy The total probability of finding the electron is 1. Forcing this condition on the wave function is called normalization. P y ( )dy - - + + Y y,t ( ) 2dy = =1 Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 21

  22. The Copenhagen Interpretation Bohr s interpretation of the wave function consisted of 3 principles: The uncertainty principle of Heisenberg The complementarity principle of Bohr The statistical interpretation of Born, based on probabilities determined by the wave function Together these three concepts form a logical interpretation of the physical meaning of quantum theory. According to the Copenhagen interpretation, physics depends on the outcomes of measurement. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 22

  23. Particle in a Box A particle of mass m is trapped in a one-dimensional box of widthl. The particle is treated as a wave. The box puts boundary conditions on the wave. The wave function must be zero at the walls of the box and on the outside. In order for the probability to vanish at the walls, we must have an integral number of half wavelengths in the box. The energy of the particle is . The possible wavelengths are quantized which yields the energy: The possible energies of the particle are quantized. Monday, Mar. 24, 2014 PHYS 3313-001, Spring 2014 Dr. Jaehoon Yu 23

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