Tunnel ionization is ionization due to quantum tunneling.In classical ionization, an electron must have enough energy to make it over the potential barrier, but quantum tunneling allow the electron simply to go through the potential barrier instead of going all the way over it because of the wave nature of the electron. The probability of an electron tunneling through the barrier drops off exponentially with the width of the potential barrier. Therefore, an electron with a higher energy can make it further up the potential barrier, leaving a much thinner barrier to tunnel through and, thus, a greater chance to do so. In practice, tunnel ionization is observable when the atom or molecule is interacting with near-infrared strong laser pauses. This process can be understood as a process by which a bounded electron, through the absorption of more than one photon from the laser field, is ionized. This picture is generally known as multiphoton ionization (MPI).

. In this model the perturbation of the ground state by the laser field is neglected and the details o atomic structure in determining the ionization probability are not taken into account. The major difficulty with Keldysh's model was its neglect of the effects of Coulomb interaction on the final state of the electron. As it is observed from figure, the Coulomb field is not very small in magnitude compared to the potential of the laser at larger distances from the nucleus.