Autoionisation (field ionisation), the process by which atoms and molecules of a gas are ionised in strong electric fields. A bound electron in an atom can be pictured as sitting in a potential well (Fig. 1,а). When an electric field of strength E is switched on, the potential energy eEx is added to the initial potential energy V0(x) of an electron at the point x, where e is the charge of the electron. The potential well thereby becomes asymmetric — on one side of it a potential barrier of finite width x1x2 is formed (Fig. 1, б), through which the electron can “leak”, that is, the tunnel effect takes place and ionisation from the lower level of the atom becomes possible. The probability W(V, E) that the electron tunnels through the potential barrier is given by the formula:
where V(x)=V0(x)+eEx and E— the potential and the total energy of the electron respectively, m is its mass. The tunnelling probability W(V, E) increases sharply as the area of the barrier above the line x1x2 decreases. This happens when the field strength E is raised, or when the energy E of the electron in the atom is raised in some other way (for example, when electrons tunnel from excited levels). Thus the probability that a hydrogen atom autoionises from the ground state reaches an appreciable value only at E≈108 V/cm, whereas from excited states it does so already at E=106 V/cm. The first autoionisation to be detected experimentally was precisely that of excited atoms: in the emission spectrum of hydrogen atoms placed in an external electric field of strength ≈106 V/cm, a fall was found in the intensity of the lines associated with quantum transitions of electrons from the highest excited states to the ground state. The phenomenon was explained by the fact that autoionisation of excited atoms becomes a more probable process than their radiat, transition to the ground state, and the glow of these lines dies away.
Autoionisation near the surface of a metal has been studied most fully, since it is used in the field-ion microscope to obtain a magnified image of the surface (see Ion projector). The probability of autoionisation at the surface of a metal proves to be considerably greater than in free space at the same field strength, which is due to the action of the “image” forces that lower the potential barrier (see Schottky effect). Autoionisation is possible, however, only when the distance of the atom from the surface exceeds a certain critical distance xкр. The reason is that at ordinary temperatures the electron can tunnel into the metal only if its ground energy level in the atom has been raised by the electric field at least to the Fermi level in the metal (see Fermi energy) (Fig. 2).
If the atom comes closer to the surface than x<xкр also leads to a sharp decrease in W. Autoionisation therefore takes place in practice within a certain zone near xкр. In the working regime of the field-ion microscope the half-width of this zone is 0.2—0.4 A. Autoionisation is also used in building ion sources for mass spectrometers. The merit of such sources is that they have no heated electrodes and that dissociation of the molecules under analysis can be avoided in them. Besides, such ion sources make it possible to observe specific chemical reactions that occur only in strong electric fields.