Ion Emission

the emission of positive and negative ions into a vacuum or gaseous medium by the surface of a solid or liquid (emitter). To leave the surface, an ion must have a sufficient energy to overcome the forces restraining it. The ion may acquire such energy as a result of heating (thermionic emission) or bombardment of the emitter, which in this case is called the target, by an ion beam (ion-ion emission), electrons (electron-ion emission), or photons (photodesorption). In all cases ion emission may take place as emission both of particles of the emitter itself and of impurities, which are inevitable in real materials.

Thermionic emission. Thermionic emission takes place as the result of evaporation as ions of particles of the emitter or other particles in the emitter in the form of impurities or incident upon its surface from without. In the latter case, and sometimes in general, thermionic emission is called surface ionization. The degree of ionization α, which is equal to the ratio of the number of ions n_i to the number of neutral particles n₀ of the same composition that evaporate from the surface of the emitter in a given period of time, is the quantitative characteristic of thermionic emission. Here the following relationship is satisfied:

where Q₀ and Q_i are the heats of evaporation of the particles in the neutral and ionic states, k is the Boltzmann constant, T is the absolute temperature of the emitter, and A is the ratio of the statistical weights of the particles in the ionic and neutral states. The quantities Q_i, and Q₀, are related to the work function ϕ of the emitter and the ionization energy V of the particles (for positive ions) or the electron affinity energy S (for negative ions) by the equations

(2) Q₀ - Q_i = ϕ - VQ₀ - Q_i = S - ϕ

It follows from equations (1) and (2) that the greater the quantity ϕ during the emission of positive ions and the smaller ϕ during the emission of negative ions, the greater will be the degree of ionization α. When ϕ < V and ϕ > S, the quantity a—and consequently the ion current—increases with increasing T(Figure 1). The ion current density j during thermionic emission depends not only on the quantity α but also on the rate of evaporation of the particles from the surface.

Thermionic emission is used to produce ion beams in ion sources, to indicate weak molecular beams (for example, in quantum frequency standards), and for ionic implantation of admixtures into semiconductors. In physicochemical research thermionic emission is used to determine the ionization energy and electron affinity energy of atoms, molecules, and radicals; the heats of evaporation and desorption of ions and neutral particles; and the dissociation energy of molecules.

Figure 1. Dependence of the logarithm of ion current density on the emitter temperature T upon evaporation of tungsten (W) and rhenium (Re) in the form of positive and negative ions

If an emitter is located in an electric field that accelerates evaporating ions, the heat of evaporation of the ions O_i, decreases as the field intensity E near the surface of the emitter increases (the Schottky effect for ions); when T = const., this is accompanied by an increase in a, according to equation (1).

In strong fields (E ⋍ 10⁸ volts per cm [V/cm]), ion emission takes place with high probability (a ≫ 1) at room temperature and lower. In this case ion emission is called field emission (autoionic emission or field evaporation). Fields of ~10⁸V/cm are created, for example, at the surface of fine points with a radius of curvature of 100–1,000 angstroms. In such electric fields not only singly charged but also doubly charged ions may be emitted. Field-ion emission may be considered as the evaporation of ions through the potential barrier, which is lowered by the field. The ion current grows as the field E increases, and in weaker fields ions primarily from impurities escape.

Field-ion emission is used to prepare samples in field-ion and field-emission microscopes. To produce a sharp image by means of an ion projector an atomically smooth surface of the sample must be created. Field-ion emission smooths out the surface of a point, since the electric field is stronger near the edges and sharp projections, leading to preferential evaporation of ions from these points.

Ion-ion emission (secondary ion emission). Ion-ion emission takes place upon irradiation of a surface by a beam of ions (primary ions). In this case the emission (expulsion) of secondary ions and neutral particles is observed. Primary ions reflected from the surface (which sometimes have changed their sign) and ions of the target material and impurities are present in the beam of emerging ions. Ion-ion emission is characterized by the coefficient of emission K, which is equal to the ratio of the flux of secondary ions n_s of a given type to the flux n_p of primary ions bombarding the surface. Usually A” is a fraction of a percent for singly charged ions. The value of K depends on the target material, its temperature, the type of primary ions and their kinetic energy, the angle of incidence to the surface, and the composition and pressure of the gas surrounding the target (Figure 2). The spatial distribution of secondary ions is determined by the energy and angle of incidence of the primary ions. The average energy of secondary ions usually does not exceed 10 electron volts. However, when the angle of incidence of fast ions to the target is gentle the average energy may be much higher. Ion-ion emission is used to study adsorption and catalysis, to investigate the properties of a surface, and for other purposes.

Electron-ion emission. Upon impact against a surface an electron expends part of its kinetic energy to break the bond between a particle of the emitter and the surface. Here the particle may leave the surface in the form of an ion. Electron-ion emission is used in studying the state of adsorbed particles.

Photodesorption. The absorption of a photon of light may lead to the decay of a target molecule into ions or to the ionization of an atom or molecule. Some of the ions that arise in the process may leave the surface.

If the emitter is irradiated by an intensive light flux (a laser beam with a pulse power of ~ 10⁸–10⁹ W/cm²), then the emission from the target material of ions with charges of various multiplicity and even totally lacking electrons (for example, Co²⁷⁺) is observed. In this case the highly ionized plasma that forms within the emitter upon vaporization of the substance is the ion source.

Figure 2. Dependence of the coefficient of ion-ion emission Kfor various secondary ions (H⁻, H⁺, O⁺, and Mo⁺) on the velocity v(cm/ sec) of primary ions [H⁺(1), Ne⁺(2), Ar⁺(3), and Kr⁺(4)] upon bombardment of a molybdenum target