Davisson Germer Experiment-Electron Diffraction, Setup of Davisson Germer Experiment (For CBSE, ICSE, IAS, NET, NRA 2022)

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Davisson Germer Experiment - Electron Diffraction

  • Initial atomic models proposed by scientists could only explain the particle nature of electrons but failed to explain the properties related to their wave nature.
  • In the year 1927, C. J. Davisson and L. H. Germer carried out an experiment, known as Davisson Germer experiment to explain the wave nature of electrons through electron diffraction.

Setup of Davisson Germer Experiment

  • An electron gun comprising of a tungsten filament F was coated with barium oxide and heated through a low voltage power supply.
  • While applying suitable potential difference from a high voltage power supply, the electron gun emits electrons which were again accelerated to a particular velocity.

The Experimental Setup T of Davisson Germer Experiment

Davisson Germer Experiment
  • Electron gun: It is a Tungsten filament that emits electrons via thermionic emission i.e.. , it emits electrons when heated to a particular temperature.
  • Electrostatic particle accelerator: Two opposite charged plates (positive and negative plate) are used to accelerate the electrons at a known potential.
  • Collimator: The accelerator is enclosed within a cylinder that has a narrow passage for the electrons along its axis. Its function is to render a narrow and straight (collimated) beam of electrons ready for acceleration.
  • Target: It is a Nickel crystal. The electron beam is fired normally on the Nickel crystal. The crystal is placed such that it can be rotated about a fixed axis.
  • Detector: It is used to capture the scattered electrons from the Ni crystal. The detector can be moved in a semicircular arc as shown in the diagram above.
  • In this cylinder perforated with fine holes along its axis, these emitted electrons were made to pass through it, thus producing a fine collimated beam.

Observations of Davisson Germer Experiment

From this experiment the below observations can be derived,

  • We obtained the variation of the intensity (I) of the scattered electrons by changing the angle of scattering, .
  • By changing the accelerating potential difference, the accelerated voltage was varied from 44 V to 68 V.
  • With the intensity (I) of the scattered electron for an accelerating voltage of 54 V at a scattering angle we could see a strong peak in the intensity.
  • This peak was the result of constructive interference of the electrons scattered from different layers of the regularly spaced atoms of the crystals.
  • With the help of electron diffraction, the wavelength of matter waves was calculated to be 0.165 nm.

Co-Relating Davisson Germer Experiment and De Broglie Relation

From the de Broglie equation, we have:

  • Where, m is the mass of an electron, e is the charge on an electron and h is the Plank՚s constant.
  • Therefore, for a given V, an electron will have a wavelength given by equation (1) .
  • The following equation gives Bragg՚s Law:

So, the value of d was already known from the X-ray diffraction experiments. Hence for various values of θ, we can find the wavelength of the waves producing a diffraction pattern from equation (2) .

Observations of the Davisson and Germer Experiment

The detector used can only detect the presence of an electron in the form of a particle.

Intensity of Scattering (X-Axis) and the Angle of Scattering for Given Values of Potential Difference

Intensity of Scattering
  • So, as a result, the detector receives the electrons in the form of an electronic current.
  • The intensity (strength) of this electronic current received by the detector and the scattering angle is studied.
  • This current as the electron intensity.

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