Ionization of atoms in the tunnelling regime with experimental evidence using Hg atoms

Some 'Keldysh-like' theories are analysed leading to a more pragmatic definition of the tunnelling regime of ionization of atoms. Rather than using the more extreme definition of tunnelling (i.e. gamma <<1, F<<Fat), a more precise definition of the Keldysh gamma parameter ( gamma (0.5) and of the laser field F (F(FBSI) is proposed, where FBSI is the barrier suppression ionization field. New experimental results of the ionization of mercury atoms by an intense CO2 laser pulse are presented. This experiment, along with some previous experiments, have been analysed, and may be classified as tunnel ionization under this definition.