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, as the length of the barrier that electrons have to pass decreases, the electrons can escape from the atom's potential more easily. Tunneling Ionization is a quantum mechanical phenomenon, since in the classical picture an electron does not have sufficient energy to overcome the potential barrier of the atom. When the atom is in a DC external field, the Coulomb potential barrier is lowered and the electron has an increased, non-zero probability of tunnelling through the potential 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, using parabolic coordinates. This provides a simplified physical system that given it proper exponential dependence of the ionization rate on the applied external field. When , the ionization rate for this system is given by: Landau expressed this in atomic units where . In SI units the previous parameters can be expressed as: , . The ionization rate is the total probability current through the outer classical turning point. This is found using the WKB approximation to match the ground state hydrogen wavefunction though the suppressed coulomb potential barrier. A more physically meaningful form for the ionization rate above can be obtained by noting that the Bohr radius and Hydrogen atom ionization energy are given by , , where is the Rydberg energy. Then, the parameters and can be written as , . so that the total ionization rate can be rewritten . This form for the ionization rate emphasizes that the characteristic electric field needed for ionization is proportional to the ratio of the ionization energy to the characteristic size of the electron's orbital . Thus, atoms with