Electron emission

Late in the nineteenth century, experiments on electric discharge through gases at low pressure led to several key discoveries. At a pressure of about 0.001 mm of mercury a discharge passes between the electrodes, and the glass facing the cathode glows; for soda glass the glow is yellowish-green.

William Crookes found these cathode rays in 1870 and in 1879 proposed that they were streams of fast, negatively charged particles. J. J. Thomson confirmed this: using crossed electric and magnetic fields he measured their speed and their specific charge e/m.

These particles travelled at roughly a tenth to a fifth of light's speed. Their measured e/m, 1.76 × 10¹¹ C/kg, came out the same whatever metal made the cathode and whatever gas filled the tube, so the particles are universal. Roentgen discovered X-rays in 1895, and Thomson named the particles electrons in 1897.

Particles given off by metals under ultraviolet light, and by very hot metals, have the same e/m as cathode-ray particles: all of them are electrons. In 1913 Millikan's oil-drop experiment showed that charge always comes in whole multiples of 1.602 × 10⁻¹⁹ C, so charge is quantised; e and e/m together give the electron's mass.

Free electrons inside a metal cannot simply leave it. An electron that starts to leave makes the surface positive, and the pull of the ions draws it back.

The least energy an electron must be given to escape from the surface is the work function φ₀ of the metal. It depends on the metal and on the state of its surface, and it is usually quoted in electron volts: 1 eV, the energy an electron gains across a potential difference of 1 V, equals 1.602 × 10⁻¹⁹ J.

The energy can be supplied in three ways. Thermionic emission: heating gives the free electrons enough thermal energy to escape. Field emission: a very strong electric field, of the order of 10⁸ V m⁻¹, pulls electrons out, as in a spark plug. Photoelectric emission: light of suitable frequency ejects electrons, called photoelectrons.