Tunnel Ionization - DC
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. Tunneling Ionization is a QM phenomenon; as 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 in the classical picture an electron does not have sufficient energy to escape.
When the atom is in an external DC field, the Coulomb potential barrier is lowered and the electron can tunnel through the barrier. In the case of an alternating electric field, the direction of the electric field reverses after the half period of the field. The ionized electron may come back to its parent ion. The electron may recombine with the nucleus (nuclei) and its kinetic energy is released as light (high harmonic generation). If the recombination does not occur, further ionization may proceed by collision between high-energy electrons and a parent atom (molecule). This process is known as non-sequential ionization.
DC tunneling ionization
Tunneling ionization from the ground state of a Hydrogen atom in an electrostatic (DC) field was solved schematically by Landau. This provides a simplified physical system that given it proper exponential dependence of the ionization rate on the applied external field. When E << E_a , the ionization rate for this system is given by the formula shown here.Related formulas
|w||ionization rate for this system (F*m-2)|