First-principles calculations for the tunnel ionization rate of atoms and molecules

We present first-principles calculations for the tunnel ionization rate of some atoms and molecules in a static intense electric field. The Gamow state is calculated to describe the ionization process in the Kohn-Sham formalism with the self-interaction correction. The tunnel ionization rate is obtained from the imaginary part of the Gamow state eigenvalue. The ionization rates of rare-gas atoms Ar and Xe and diatomic molecules N₂, O₂, and F₂ are investigated. The calculations describe well the observed behavior of the tunnel ionization. The results also show good correspondence with the Ammosov-Delone-Krainov model for rare-gas atoms. We find that the properties of the highest occupied orbital have significant effects on the ionization rate. In particular, our calculation reproduces the suppression of the ionization rate of O₂ molecule in comparison with that of Xe atom. We also find that the ionization rates of O₂ and F₂ molecules are very sensitive to the relative angle between the electric field and the molecular axis, reflecting properties of the highest occupied orbital.