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LAB Electron Diffraction ELECTRON DIFFRACTION by nikeborome


									                    ELECTRON DIFFRACTION
                                               Ken Cheney



We will explore the de Broglie wavelength of particles by diffracting
electrons through a carbon film and observing the result on a CRT type



     = Lambda = the wavelength of the particle or wave.
    h = Plank’s Constant
    f = frequency
    v = mu = Frequency
    n = Integer
    a = Perpendicular distance between planes of atoms
    p = Momentum
    i = The angle between the incoming wave and the normal to the plane
     = The angle between the plane itself and the incoming wave
     = The angle of deflection of waves, from the incoming direction to the
    outgoing direction = 2
    L = The distance between the scattering carbon film and the screen
    D = The diameter of the circles on the screen
    m = The mass of the electron
    Va = The voltage that accelerated the electron


In 1924 Louis de Broglie suggested that particles might have wave
properties. The wavelength he proposed was given by:
                                  = h/p

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Recall that light was first shown to be a wave by Young who showed that
light interfered with itself, defiantly a property of waves not particles.

This suggests trying to demonstrate the wave properties of matter by
producing an interference pattern.

         George Thomson was jointly awarded the Nobel Prize for Physics in 1937 for his
         work in Aberdeen in discovering the wave-like properties of the electron. The
         prize was shared with Clinton Joseph Davisson who had made the same discovery
         independently. Whereas his father had seen the electron as a particle (and won his
         Nobel Prize in the process), Thomson demonstrated that it could be diffracted like
         a wave, a discovery proving the principle of wave-particle duality which had first
         been posited by Louis-Victor de Broglie in the 1920s as what is often dubbed the
         de Broglie hypothesis.

         Actually Davisson and Germer found this effect for reflection (initially by
         accident) while Thompson did a transmission experiment.

This wave like behavior of particles also led to an inability to say exactly
where a particle was or would be, one could only calculate (with fantastic
accuracy) the probabilities of finding a particle her or there. This
uncertainty made many excellent physicists (such as Einstein) very unhappy!


See “BRAGG SCATTERING WITH MICROWAVES” for the derivation of
the equation that gives the direction of maximum scattering from parallel
planes of atoms:

           cos(i)=2a/n            or usually in this case                sin() = 2a/n

Where i is the optimum angle between the normal to the plane and the
incoming wave, a is the separation of the reflecting planes, n is an integer, 
lambda is the wavelength of the wave or particle, and  is the angle between
the plane and the incoming wave.

There are many possible separations between parallel planes in different
directions. For example in a cubic crystal some planes are through the faces
of the cubes and other planes are diagonally across the cube. We will be
interested in the ration of these two interplaner distances so show that for
these two choices the ratio is one to the square root of two.

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We will also be interested in hexagonal shapes as sketched below. Calculate
the ratios of the interplaner distances for the planes shown and any others
you can devise.

           B                                                                         B
C                   C
                                                D                                              D

             B                                                                       B

C                    C


Graphite forms in sheets one atom thick. Within the sheets the atoms are
arranged in hexagons with nearest neighbors along the side of the hexagon
1.415 Angstroms apart.

The sheets are arranged in parallel layers 3.35 Angstroms apart.

The sheets are offset so each carbon atom is directly below and above the
center of hexagons in the sheets below and above.

For a fine tutorial on this see :

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We want to check:

         1. Are electrons (or some charged particles ) producing the circles on
            the screen? Try a magnet!
         2. If it is electrons is there evidence that their wavelength depends on
            their momentum as predicted by de Broglie?
         3. Can we calculate the interatomic spacing from our observations,
            and is the result reasonable?
         4. Do we have evidence of the shape of the crystal?


We want to use the de Broglie wavelength of electrons to predict the relation
between the circles the electrons trace on a CRT type screen and the
accelerating voltage given the electrons. This analysis should yield the
interatomic spacing.

According to de Broglie
        = h/p
       Kinetic Energy = ½ mv2 = p/2m
       The energy given the electrons by the accelerating voltage = qVa
       Combining these:
        = h / 2mqVa                                                 Eq. 1
What we observe
       The accelerated electrons go through a carbon film, travel a distance
L, to a CRT type screen and produce circles of diameter D, they have been
deflected through an angle :
       sin() = D/2L                                                Eq. 2

                                                                
  Plane of atoms
The angle of deflection is twice the angle between the incoming wave and
the plane (specular reflection) so alpha equals two theta. For small angles :
         sin( )  sin(2 ) 2sin( )                           Continuation of in    Eq. 3
And from Bragg
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    sin() = n/2a                                                                   Eq. 4
Combining equations 1-4 and arranging to look linear:
         1/ Va  aRD
                1 2mq
               2 L nh
Notice that R is a constant so if we plot RD verses 1/ Va we should get a
straight line with slope a if de Broglie and Bragg are right!


This is FAR from the complete theory! We can just hope that this simplified
version will give some interesting approximations.

Once the rings have been seen we know that electrons do indeed have wave
properties, the details can be worked out later!


The tube that does the diffraction has an electron source like a CRT with a
heater (filament) heated by 6.3v AC, an anode that boils off the electrons, a
shield with feedback through a resistor to prevent too much current
(damaging the carbon film), a carbon fill where the diffraction occurs, and a
CRT type screen that shows where the electrons go.

The carbon is probably a random mix of graphite (ordered sheets of carbon)
mixed in surrounding amorphous carbon.

The random mix of graphite means that the electrons will strike graphite
planes at all possible angles. In our case only two angles will result in useful
Bragg angles but these angles occur in all directions around the line of the
incoming electrons, hence circular traces on the screen.

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                     ELECTRON DIFFRACTION TUBE

  Connectors                  Filamen                                       Carbon
                                              Cathode                       Film      Electrons

        6.3 v AC


                                                                                     Glass Tube
                               High Voltage Va


Get data and do calculations to check 1-4 in “Our Plan” above.

Try several voltages from 2500 to 5000 volts.

Plot as suggested above for each circle, two lines and two slopes and two
interatomic spacing.

Find the ratio of the interatomic spacing and see if the ratio matches the ratio
for any planes you can draw through a cubic or hexagonal crystal.

The spacing should be of the order of two angstroms, 2 10-10m.

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