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. MPI of molecules Contrary to the abundance of theoretical and experimental work on the MPI of rare gas atoms, the amount of research on the prediction of the rate of MPI of neutral molecules was scarce until recently. Walsh et al. have measured the MPI rate of some diatomic molecules interacting with a 10.6 μm CO2 laser. They found that these molecules are tunnel-ionized as if they were structureless atoms with an ionization potential equivalent to that of the molecular ground state. Talebpour et al. were able to quantitatively fit the ionization yield of diatomic molecules interacting with a Ti:sapphire laser pulse. The conclusion of the work was that the MPI rate of a diatomic molecule can be predicted from the PPT model by assuming that the electron tunnels through a barrier given by instead of barrier which