Davisson—Germer Experiment

Abstract

The Davisson-Germer experiment (1927) was the first measurement of the wavelengths of electrons. C. J. Davisson, who worked in the Bell Research Laboratories , received the Nobel Prize in Physics for the year 1937 together with George P. Thomson from the University of Aberdeen in Scotland, who independently also found experimental indications of electron diffraction. According to the Copen-hagen Interpretation of Quantum Mechanics, wave-particle duality leads to particles also exhibiting wave-like properties like extension in space and interference. Clinton J. Davisson (1881-1958) and Lester H. Germer (1896-1971) investigated the reflection of electron beams on the surface of nickel crystals. When the beam strikes the crystal, the nickel atoms in the crystal scatter the electrons in all directions. Their detector measured the intensity of the scattered electrons with respect to the incident electron beam. Their normal polycrystalline samples exhibited a very smooth angular distribution of scattered electrons. In early 1925, one of their samples was inadvertently recrystallized in a laboratory accident that changed its structure into nearly monocrystalline form. As a result, the angular distribution manifested sharp peaks at certain angles. As Davisson and Germer soon found out, other monocrystalline samples also exhibited such anomalous patterns, which differ with chemical constitution, angle of incidence and orientation of the sample. Only in late 1926 did they understand what was going on, when Davisson attended the meeting of the British Association for the Advancement of Science in Oxford. There Born spoke about de Broglie's matter-waves and Schrödinger's wave mechanics. Their later measurements completely confirmed the quantum mechanical predictions for electron wavelength λ as a function of momentum p: λ = h/p. But their initial experiments (unlike G.P. Thomson's) were conducted in the context of industrial materials research on filaments for vacuum tubes, not under any specific theoretical guidance. The phenomenon of electron diffraction is quite general and can be explained by the wave nature of atomic particles. Planes of atoms in the crystal (Bragg planes) are regularly spaced and can produce a constructive interference pattern, if the so-called Bragg condition (nλ = 2 d sin θ = D sin φ, where d is the spacing of atomic planes and D is the spacing of the atoms in the crystal) is satisfied. This condition basically states that the reflected beams from the planes of atoms in the crystal will give an intensity maximum, or interfere constructively, if the distance, which the wave travels between two successive planes (2 d sin θ), amounts to a whole number of wavelengths (nλ, n = 1, 2, 3. . .). This is illustrated in Fig. 1.