Constituents of Matter / 3 / 3.1 / 3.1.3

3.1.3 Observing A Quantum Object: The Davisson-Germer Experiment

In April 1925 an accident occurred in Clinton Davisson’s laboratory at the Western Electric Company. We let him describe what happened:

At the time we were continuing an investigation of the distribution-in-angle of electrons scattered by a target of ordinary, polycrystalline nickel. During the course of this work a liquid-air bottle exploded at a time when the target was at high temperature; the experimental tube was broken, and the target heavily oxidized by the inrushing air. The oxide was eventually reduced and a layer of the target removed by vaporization, but only after prolonged heating at various high temperatures in hydrogen and in vacuum. When the experiments were continued it was found that the distribution-in-angle of the scattered electrons had completely changed. This marked alteration in scattering pattern was traced to a re-crystallization of the target that occurred during the prolonged heating. Before the accident we had been bombarding many small crystals, but in the tests subsequent to the accident we were bombarding only a few large ones. The actual number was of the order of ten.

C. Davisson and H.L. Germer, Diffraction of Electrons by a Crystal of Nickel

The essential parts of the apparatus employed by Davisson were an electron gun, the target, and a double Faraday box designed to collect the scattered electrons. The electrons were emitted thermally from a tungsten ribbon and projected from the gun into a field-free enclosure containing the target and the collector by means of a potential difference. The target was a block of nickel cut from a bar in which crystal growth has been induced by straining and annealing. The collector could be moved with respect to the target, and the target could be rotated about the direction of the incident beam of electron, so that it was possible to measure the intensity of scattering in any direction in front of the target. Davisson was using this experimental set-up to investigate the atomic structure of nickel much like Rutherford before him used his experiments with $\alpha$ particles to study the atomic structure of gold. Because his electrons were much slower than Rutherford’s $\alpha$ particles, Davisson expected they would explore only the outer regions of the atom rather than the nuclear region inside the atom. Consequently, the angle distribution of the back-scattering electrons in all experiments prior to the accident was relatively smooth. After the accident, however, when the target changed from a polycrystalline structure to one containing only a few single crystals, the angle distribution changed dramatically. As reported by Davisson and Germer when describing the scattering intensity as a function of the bombarding potential for various colatitude angles extended from the surface of the target:

From the curves, we see, for example, that the colatitude curves for bombarding potentials near 55 volts are characterized by exceptional intensities at colatitude angles near 50 degrees.

When displaying the same data as a function of the colatitude angle for various values of the accelerating potential, the authors found similarly unexpected curves:

We see a slight hump at 60 degrees in the colatitude curve for 40 volts, and observe that as the bombarding potential is increased this hump develops into a strong spur which reaches a maximum development at 54 volts in colatitude 50 degrees, then decreases in intensity and finally disappears at about 66 volts in colatitude 40 degrees.

These curves are similar to the curves obtained by von Laue in his 1912 x-ray scattering experiments off crystals. Associating them with the von Laue beams that would have emerged from the crystal within the range of observation if the incident beam were a beam of heterogeneous x-rays, the authors calculated the equivalent wavelength of the electron beam from the diffraction data and found acceptable agreement with:

where $h$ is Planck’s constant, $m$ the mass of the electron, and $v$ its speed. Under normal conditions, this result would have been simply incomprehensible. As it was, however, Louis de Broglie had already suggested in his doctoral dissertation published in 1924 that electrons could be associated to a wave of exactly the wavelength found by Davisson and Germer. Consequently, their paper quickly came to be viewed as the experimental verification of de Broglie’s hypothesis.

What makes this accident significant for us however is the fact that it transformed what was originally an investigation of the target by means of electrons into an investigation of the electrons themselves. The experiment designed to explore nickel by means of electrons became an observation of electrons by means of nickel, an observation which revealed them to behave as if they were waves. The former was intended to explore just how useful J.J. Thomson’s explanation of electric discharge phenomena in vacuum, Rutherford’s explanation of $\alpha$ particle scattering, and Bohr’s explanation of atomic spectra, would be in explaining the structure of nickel; the latter, became a measurement designed to answer the question about what an electron was. While questions concerning electrons, atoms, and photons were asked before, it was not until now that they were asked through a direct observation of them. Viewed like that, the Davisson-Germer experiment’s answer to the ontological question is a very strange one: the electron is neither a particle whose behavior is captured by mechanics nor a wave whose behavior is captured by optics. It is not a particle because it diffracts when passing through a crystal, and it is not a wave because the wavelength which is consistent with the diffraction pattern remembers the mass and velocity of a particle. It is not something which we have ever encountered in our experience nor described in our classical theory of that experience.