Editing Tunnel ionization (section)

=== 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.<ref>{{cite journal | last1=Walsh | first1=T D G | last2=Decker | first2=J E | last3=Chin | first3=S L | title=Tunnel ionization of simple molecules by an intense CO<sub>2</sub> laser | journal=Journal of Physics B: Atomic, Molecular and Optical Physics | publisher=IOP Publishing | volume=26 | issue=4 | date=1993-02-28 | issn=0953-4075 | doi=10.1088/0953-4075/26/4/002 | pages=L85–L90| s2cid=250888196 }}</ref> have measured the MPI rate of some diatomic molecules interacting with a {{val|10.6|u=um}} CO<sub>2</sub> 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. A. Talebpour et al.<ref>{{cite journal | last1=Talebpour | first1=A | last2=Larochelle | first2=S | last3=Chin | first3=S L | title=Suppressed tunnelling ionization of the molecule in an intense Ti:sapphire laser pulse | journal=Journal of Physics B: Atomic, Molecular and Optical Physics | publisher=IOP Publishing | volume=31 | issue=2 | date=1998-01-28 | issn=0953-4075 | doi=10.1088/0953-4075/31/2/003 | pages=L49–L58| s2cid=250791262 }}</ref><ref>{{cite journal | last1=Talebpour | first1=A. | last2=Yang | first2=J. | last3=Chin | first3=S.L. | title=Semi-empirical model for the rate of tunnel ionization of N<sub>2</sub> and O<sub>2</sub> molecule in an intense Ti:sapphire laser pulse | journal=Optics Communications | publisher=Elsevier BV | volume=163 | issue=1–3 | year=1999 | issn=0030-4018 | doi=10.1016/s0030-4018(99)00113-3 | pages=29–32}}</ref> 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 <math> {Z_\text{eff}} / {r} </math> instead of barrier <math> {1} / {r} </math> which is used in the calculation of the MPI rate of atoms. The importance of this finding is in its practicality; the only parameter needed for predicting the MPI rate of a diatomic molecule is a single parameter, {{tmath|1= Z_\text{eff} }}. Using the semi-empirical model for the MPI rate of unsaturated hydrocarbons is feasible.<ref>{{cite journal | last1=Talebpour | first1=A | last2=Larochelle | first2=S | last3=Chin | first3=S L | title=Multiphoton ionization of unsaturated hydrocarbons | journal=Journal of Physics B: Atomic, Molecular and Optical Physics | publisher=IOP Publishing | volume=31 | issue=12 | date=1998-06-28 | issn=0953-4075 | doi=10.1088/0953-4075/31/12/012 | pages=2769–2776| s2cid=250797867 }}</ref> This simplistic view ignores the ionization dependence on orientation of molecular axis with respect to polarization of the electric field of the laser, which is determined by the symmetries of the molecular orbitals. This dependence can be used to follow molecular dynamics using strong field multiphoton ionization.<ref>{{cite journal | last=Jaron-Becker | first=A. | title=Molecular Dynamics in Strong Laser Fields | journal=IEEE Journal of Selected Topics in Quantum Electronics | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=18 | issue=1 | year=2012 | issn=1077-260X | doi=10.1109/jstqe.2011.2108271 | pages=105–112| s2cid=16703524 }}</ref>