Charge Exchange of Ions

the interaction of positive ions with neutral atoms, neutral molecules, or the surface of a solid, resulting in the exchange of electrons between the interacting particles. Analogous interactions of negative ions are also often referred to as charge exchanges.

The charge exchange of ions in gases and liquids follows the scheme A⁺ + B⁰ → A⁰ + B⁺, where the superscripts indicate the charges of the particles. If the charge exchange of ions is not accompanied by a change in the internal energy of the system of interacting particles, it is said to be resonant. Examples of resonant charge exchange include the exchange of an electron between an atomic ion and an atom of the same element and between a molecular ion and a molecule of the same substance.

The probability of charge exchange, which is expressed by the effective cross section, is determined by the nature of particles A and B and by the particles’ relative velocity; it depends on the parameter αΔ E/hv, where α is the dimension of the particle being ionized, ΔE is the change in internal energy, h is Planck’s constant, and v is the relative velocity of the particles. As v decreases, the charge exchange cross section of the ions decreases sharply in nonresonant charge exchange—when αΔE/hv ≫ 1—and increases monotonically in resonant charge exchange. A typical example is the charge exchange of protons in atomic and molecular hydrogen (see Figure 1). Charge exchange may play a significant role in the energy balance of hot plasma.

Figure 1. Effective cross sections σ of the charge exchange of protons in atomic hydrogen (resonant charge exchange) and molecular hydrogen (nonresonant charge exchange); v is the relative velocity of colliding particles

Resonant charge exchange that results in the formation of an excited—rather than ground-state—neutral atom or molecule is possible when an electron is captured in one of the free higher energy levels.

Charge exchange near the surface of a metal, which occurs when an ion captures an electron from the metal, proceeds similarly to ionic charge exchange in gases. The capture of two electrons with the formation of a negative ion is a special case of charge exchange; this occurs with positive atomic or molecular ions of electronegative gases.

The tunnel effect is an essential aspect of the mechanism of charge exchange. Various techniques that rely on charge exchange are used in plasma diagnosis.