Ionization is when an EM neutral atom gaing Em charge by losing or gaining an electron.
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 allows 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's 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 pulses. 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).
This wiki article says that tunneling ionization and MPI is essentially the same thing.
The progress of strong-field laser-atom physics has been caused and significantly motivated by the first experimental observations of above threshold ionization (ATI) [1-3] of atoms more than thirty years ago. ATI can be usually understood as a process of multiphoton absorption, when the atom (or ion or molecule) absorbs more photons from an electromagnetic (laser) field than the minimum number required to overcome the ionization threshold. Alternatively, for sufficiently intense fields and sufficiently low frequencies of the laser, the process may be understood as a tunneling through a suppressed barrier of the Coulomb potential [4,5]. Usually, the Keldysh parameter γ  is used to distinguish between multiphoton and tunneling ionization.
Now this says that the two are different things and the Keldysh parameter is used to distinguish between them.
Multiphoton ionization dominates when γ >1, and tunneling ionization prevails when γ <1.
Now what I do not understand is that one of them says that the two are the same thing, the other one says they are different.
- Is tunneling ionization the same as MPI or what is the real difference?