Tunnel ionization

Tunnel ionization is a process in which electrons in an atom (or a molecule) pass through the potential barrier and escape from the atom (or molecule). In an intense electric field, the potential barrier of an atom (molecule) is distorted drastically. Therefore, the length of the barrier that electrons have to pass decreases and electrons can escape from the atom (molecule) easily.

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As an electric field of light is an alternating electric field, the direction of the electric field reverses after the half period of the field. Because electrons have a charge, electrons escaping by tunnel ionization come and go to the atom (molecule) in every half period. In this process, some electrons recombine with the nucleus (nuclei). Because the electrons have gained a large quantity of kinetic energy by acceleration from the electric field, surplus energy is released as light. The energy of this light is so high that this method is an effective way of generating ultraviolet light.

When the recombination does not occur, further ionization proceeds by collision between high-energy electrons and a parent atom (molecule). Consequently, a multivalent ion is created and this ion is collapsed by Coulomb repulsion. This is called Coulomb explosion.

References and background

1. Tunnel Ionization probability for H-like atoms considering potential well was initially proposed by Keldysh (Sov. Phys. JETP, 1965).
2. Theory extended to ionization from excited electronic states by Peremolov et. al (Sov. Phys. JETP, 1966).
3. Modified for complex atoms and ions by Ammosov et al. (ADK, JETP, 1986).
4. Tunneling Ionization of atoms was first observed experimentally by Chin et al. (J. Phys. B, 1985).
5. Extensive comparison of ADK predictions have been done by many workers like Augst et al. (J. Opt. Soc. Am, 1991) etc.
6. Theoretical and Experimental works regarding ionization related to advanced particle accelerators, fast-ignitor scheme of ICF are noteworthy.

The theoretical work of ADK is so far the most valid and highly appreciated theory.

Physical Process

Tunneling Ionization is a QM phenomenon; a non-zero probability event for observing a particle escaping from the deformed Coulomb potential barrier, obviously this phenomenon is forbidden by classical laws.

As Coulomb potential barrier is lowered, its width decreases. Increases probability of electron tunneling penetrating the barrier. The ADK probability formula gives the expression for the probability of tunnel ionization.