Cathode Luminescence

luminescence that occurs when a phosphor is excited by an electron beam; a type of radioluminescence. The original term designating a beam of electrons was “cathode ray”; hence the term “cathode luminescence.” The phenomenon occurs in gases, molecular crystals, and organic and crystal phosphors, but only crystal phosphors are resistant to the electron beam and provide a sufficiently bright glow. These are used as cathode phosphors.

To excite cathode luminescence the energy of the exciting electrons needs to be only about 1.5 times the ionization potential of the crystal phosphor. However, when such low-velocity electrons are used, steady luminescence cannot be achieved: the electrons very quickly charge the surface of the phosphor negatively, so that exciting electrons slow down and lose energy when they are repelled from it. If the electron energies are high, secondary electron emission occurs at the surface of the phosphor and the charge on the phosphor is carried off by the secondary electrons. Consequently, the electron beams used in practice have energies of 100 electron volts (eV) to 25 keV, and in some cases—such as lasers—up to 1 MeV.

As the high-energy electrons interact with the atoms of the phosphor’s lattice, they ionize them, thus creating a second generation of electrons that, in turn, ionize other atoms. This process continues until the energy of the knock-on electrons is sufficient for ionization. The electrons are decelerated in a thin layer of the phosphor (less than 10^-4 cm thick), so that the density of excitation is very high. The electron holes and electrons created as a result of the ionization migrate along the lattice and may be captured by centers of luminescence. Cathode luminescence occurs when the electrons and electron holes recombine at the centers of luminescence. The centers of cathode luminescence are the same as for photoexcitation, and consequently the spectra are similar. The efficiency of cathode luminescence is 1–10 percent; most of the energy of the electron beam is converted into heat.

The phenomenon of cathode luminescence is used extensively in technology, particularly vacuum electronics. It is cathode luminescence that causes the screens to glow in black-and-white and color television receivers, all oscillographs, and image converters. The phenomenon is the basis for AsGa, CdS, and ZnS lasers that are excited by an electron beam.