text: The dynamics of the MPI can be described by finding the time evolution of the state of the atom which is described by the Schrödinger equation. The form of this equation in the electric field gauge, assuming the single active electron (SAE) approximation and using dipole approximation, is the following
  where  is the electric field of the laser and  is the static Coulomb potential of the atomic core at the position of the active electron. By finding the exact solution of equation (1) for a potential  ( the magnitude of the ionization potential of the atom), the probability current  is calculated. Then, the total MPI rate from short-range potential for linear polarization, , is found from
  where  is the frequency of the laser, which is assumed to be polarized in the direction of the  axis. The effect of the ionic potential, which behaves like  ( is the charge of atomic or ionic core) at a long distance from the nucleus, is calculated through first order correction on the semiclassical action. The result is that the effect of ionic potential is to increase the rate of MPI by a factor of
  Where  and  is the peak electric field of laser. Thus, the total rate of MPI from a state with quantum numbers  and  in a laser field for linear polarization is calculated to be
  where   is the Keldysh's adiabaticity parameter and . The coefficients ,  and    are given by
  The coefficient  is given by
  where
  The ADK model is the limit of the PPT model when  approaches zero (quasi-static limit). In this case, which is known as quasi-static tunnelling (QST), the ionization rate is given by
  In practice, the limit for the QST regime is . This is justified by the following consideration. Referring to the figure, the ease or difficulty of tunneling can be expressed as the ratio between the equivalent classical time it takes for the electron to tunnel out the potential barrier while the potential is bent down. This ratio is indeed , since the potential is bent down during half a cycle of the field oscillation and the ratio can be expressed as 
  where  is the tunneling time (classical time of flight of an electron through a potential barrier, and  is the period of laser field oscillation.