low ionization energy (such as alkali metals) with electrons occupying orbitals with high principal quantum number (i.e. far down the periodic table) ionize most easily under a DC field. Furthermore, for a Hydrogenic atom, the scaling of this characteristic ionization field goes as , where is the nuclear charge. This scaling arises because the ionization energy scales as and the orbital radius as . More accurate and general formulas for the tunneling from Hydrogen orbitals can also be obtained. As an empirical point of reference, the characteristic electric field for the ordinary Hydrogen atom is about (or ) and the characteristic frequency is . AC electric field The ionization rate of a hydrogen atom in an alternating electric field, like that of a laser, can be treated, in the appropriate limit, as the DC ionization rate averaged over a single period of the electric field's oscillation. Multiphoton and tunnel ionization of an atom or a molecule describe the same process by which a bounded electron, through the absorption of more than one photon from the laser field, is ionized. The difference between them is a matter of definition under different conditions. They can henceforth be called MPI (multiphoton ionization) whenever the distinction is not necessary. 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. When the intensity of the laser is strong, the lowest-order perturbation theory is not sufficient to describe the MPI process. In this case, the laser field on larger distances from the nucleus is more important than the Coulomb potential and the dynamic of the electron in the field should be properly taken into account. The first work in this category was published by Keldysh. He modeled the MPI process as a transition of the electron from the ground state of the atom to the Volkov states (the state of a free electron in the electromagnetic field). In this model, the perturbation of the ground state by the laser field is neglected and the details of 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 is observed from the figure, the Coulomb field is not very small in magnitude compared to the potential of