Physics · Ch 7 — Dual Nature of Radiation and Matter
Davisson – Germer experiment
Davisson – Germer experiment
De Broglie's hypothesis of matter waves, though a bold theoretical proposal, needed direct experimental confirmation, and this came in 1927 from Clinton Davisson and Lester Germer, who demonstrated that a beam of electrons is diffracted -- an unmistakably wave-like behaviour -- when it strikes a crystalline solid. A crystal's regularly spaced atomic planes can act as a three-dimensional diffraction grating for any wave whose wavelength is comparable to the spacing between those planes, and de Broglie wavelengths of electrons (and X-ray wavelengths too) happen to fall in exactly that range, around m -- far too short for an ordinary diffraction grating (built for visible light) to resolve, but a perfect match for the atomic spacing inside a crystal.
In the Davisson-Germer apparatus, a filament is heated by a low-tension battery to emit electrons by thermionic emission; these electrons are then accelerated through a known potential difference by a high-tension battery, collimated into a narrow beam by two thin aluminium diaphragms, and directed onto a single crystal of nickel. The nickel atoms scatter the incident electron beam in many directions, and a rotatable electron detector measures the intensity of the scattered beam as a function of the scattering angle between the incident and scattered directions.
At an accelerating voltage of 54 V, the measured intensity of the scattered electron beam, plotted against , shows a pronounced peak at -- exactly the signature of constructive interference between electron waves diffracted from successive atomic layers of the nickel crystal. Using the known interplanar spacing of nickel, the electron wavelength corresponding to this peak works out experimentally to 1.65 Å. This can be directly compared against the theoretical de Broglie prediction for a 54 V electron, using : Å -- in excellent agreement with the experimentally observed 1.65 Å. This close match is the direct, quantitative experimental verification of de Broglie's hypothesis that moving electrons possess a genuine wave character. …
What this figure shows. An electron gun assembly is shown with a filament F heated by a low-tension (L.T.) battery to emit electrons by thermionic emission, which are then accelerated by a high-tension (H.T.) battery toward an anode aluminium cylinder, passing through two thin aluminium diaphragms that collimate the beam into a narrow, well-defined incident beam striking a single nickel crystal target. The electrons scattered off the nickel crystal at various angles are shown reaching a rotatable electron detector, positioned so the scattering angle between the incident and scattered beams can be varied and the intensity of the scattered beam measured as a function of that angle -- the complete apparatus that …
What this figure shows. A polar-style graph is shown with the scattering angle marked around an arc from 0° through 30°, 50°, 60° up to 90°, and the intensity of the diffracted electron beam plotted as the radial distance from the centre at each angle, for a fixed accelerating voltage of V. The plotted curve shows a pronounced bump or peak reaching its maximum intensity at exactly , which is the key experimental result: this sharp maximum is the signature of constructive interference between electron waves diffracted off successive atomic layers of the nickel crystal, and from the crystal's known interplanar spacing this peak angle corresponds to an experimentally measured electron wavelength of 1.65 Å, in excellent agreement with the 1.67 Å value predicted dir …