Ionization: (3) Quantum ionization: (2) Non-sequential ionization: When the fact that the electric field of light is an alternating electric field is combined with tunnel ionization, the phenomenon of non-sequential ionization emerges. An electron that tunnels out from an atom or molecule may be sent right back in by the alternating field, at which point it can either recombine with the atom or molecule and release any excess energy or have the chance to further ionize the atom or molecule through high-energy collisions. This additional ionization is referred to as non-sequential ionization for two reasons: One, there is no order to how the second electron is removed, and, two, an atom or molecule with a +2 charge can be created straight from an atom or molecule with a neutral charge, so the integer charges are not sequential. Non-sequential ionization is often studied at lower laser-field intensities, since most ionization events are sequential when the ionization rate is high. The non-sequential ionization can be readily understood with one-dimensional models of atoms which until recently were the only to handle numerically. It happens when the angular momentum for both electrons remains so low that they effectively move in one dimensional space and may be true for linear polarization but is not for the circular. One may then look at two electrons as the two dimensional atom where the simultaneous ionization of both is just an ionization of one two-dimensional electron which results in jets of probability in 45^o direction on the two-electron plane emitted from the multiply charged nucleus or the square center. The sequential ionization on the other hand is represented by the emissions from the axes x and y when the two dimensional hyper-electron is first guided by the Coulomb potential channels from the hyper-nucleus and then ionized by the hyper-electric field in 45^o direction