Physics · Ch 7 — Dual Nature of Radiation and Matter
Electron emission
Electron emission
Inside any metal, the electrons in the outermost (valence) shells of the atoms are only loosely bound to their nuclei, so even at ordinary room temperature a huge number of these electrons wander about freely inside the bulk of the metal, moving in random directions with a range of different kinetic energies. Despite this freedom of motion inside the metal, an individual free electron cannot simply walk out through the surface, because the positive nuclei near the surface pull it back with an attractive force. This attractive pull sets up what is called the surface barrier or potential barrier -- an energy hurdle that a free electron must cross before it can leave the metal altogether.
Most free electrons do not have enough kinetic energy on their own to clear this barrier. Electron emission is simply the process of supplying an electron with just enough extra energy, from some outside source, to let it cross the barrier and escape from the metal's surface. The work function, denoted and measured in electron-volts (eV), is defined as the minimum energy needed for an electron to escape from a given metal's surface. One electron-volt is the kinetic energy gained by an electron accelerated through a potential difference of exactly 1 V: . If, for instance, a free electron already carries 0.5 eV of kinetic energy inside the metal and the surface barrier requires 3 eV to clear, only the shortfall, eV, needs to be supplied from outside -- and this 2.5 eV is exactly the metal's work function. Work function varies from metal to metal (Table 7.1 lists typical values from 2.14 eV for caesium up to 5.65 eV for platinum), and a smaller work function makes a metal more effective for electron emission, since less extra energy is needed to free its electrons -- which is why caesium and the other alkali metals are favoured in photoelectric devices. …
What this figure shows. A two-panel schematic contrasting a metal block at room temperature, where a scatter of free electrons is shown moving randomly inside the bulk of the metal without enough energy to escape the surface, against the same metal block after it has been heated, where the free electrons near the surface are shown carrying visibly more thermal energy and beginning to break free of the surface. The pairing makes concrete the definition of thermionic emission: it is only once the electrons' thermal energy exceeds the surface barrier that they can be liberated, and heat …
What this figure shows. A thin metal filament is shown glowing under an applied current, with a stream of electrons drawn leaving its heated surface and moving outward into the surrounding vacuum. This is the practical device-level picture of thermionic emission introduced in Figure 7.1: a filament heated to incandescence inside an evacuated tube (as used in cathode ray tubes and X-ray tubes) continuously boils off free electrons from its surface, and this stream of thermally liberated electrons is exactly the source of the electron beam that such tubes go on to …
What this figure shows. A flat metal surface is drawn with a dense arrangement of positive charges on one side and free electrons just inside the surface on the other, with a very strong externally applied electric field pointing from the positive side toward the metal. The strong field is shown pulling the free electrons out through the surface directly, without any need for extra heating -- illustrating that in field emission the potential barrier is overcome purely by the mechanical pull of an intense external electric field, the operating principle exploited in devices s …
What this figure shows. A beam of electromagnetic radiation is shown striking the surface of a metal block, with free electrons drawn being knocked loose from the illuminated surface and emitted outward as a separate stream of emitted electrons. This is the picture of photoelectric emission: radiation of a suitable frequency transfers its energy directly to the free electrons at the metal surface, giving them enough energy to overcome the surface barrier -- the number of electrons emitted this way scales with how intense (bright) the incident radiation is, and this figure is the device-free, purely surfac …
What this figure shows. A beam of high-speed electrons is shown striking the surface of a metal target, and a separate, lower-energy stream of secondary electrons is drawn being knocked out of the free electrons at the metal's surface as a result of that impact, distinguishing the incoming high-speed primary beam from the outgoing secondary electrons. This illustrates secondary emission: kinetic energy is transferred from the fast primary electrons to the target's own free electrons on collision, which is enough to liberate some of them as secondary electrons -- the mechanism exploited in image intensifiers and photomultiplier tubes t …
| Metal | Symbol | Work function (eV) | Metal | Symbol | Work function (eV) |
|---|---|---|---|---|---|
| Cesium | Cs | 2.14 | Aluminium | Al | 4.28 |
| Potassium | K | 2.30 | Mercury | Hg | 4.49 |
| Sodium | Na | 2.75 | Copper | Cu | 4.65 |
| Calcium | Ca | 3.20 | Silver | Ag | 4.70 |