Multiphoton and tunneling ionization probability of atoms and molecules in an intense laser field

We theoretically studied ionization of atoms exposed to an intense laser field by using three different methods, i.e., the numerical solution of the single-active-electron approximation based time-dependent Schrödinger equation (SAE-TDSE), the Perelomov–Popov–Terent'ev (PPT) model, and the Ammosov–Delone–Krainov (ADK) model. The ionization of several linear molecules in a strong laser field is also investigated with the molecular ADK (MO-ADK) and the molecular PPT (MO-PPT) model. We show that the ionization probability from the PPT and the MO-PPT model agrees well with the corresponding SAE-TDSE result in both the multiphoton and tunneling ionization regimes. By considering the volume effect of the laser field, the ionization signal obtained from the PPT and the MO-PPT model fits well the experimental data in the whole range of the multiphoton and tunneling ionization regimes. However, both the ADK and MO-ADK models seriously underestimate the ionization probabilities (or signals) in the multiphoton regime.