Study of multiple ionization processes of atoms with a femtosecond tera-watt laser

Recent progress in femtosecond (fs) tera-watt (TW) laser development has raised a new fundamental question in atomic and molecular physics, i.e., how atoms and molecules react against an intense electromagnetic field. As is well known, an atom exposed to photons of relatively low flux density is ionized only if the photon energy exceeds the ionization potential of the atom. On the other hand, when the photon energy is lower than the ionization threshold, the ionization probability becomes finite only for a relatively high photon flux, where a multi-photon ionization process starts to play an important role. For even higher photon fluxes, quasi-static field ionization (tunneling and above-barrier) becomes a more suitable picture to describe the ionization process.