Ionization above the coulomb barrier

The interaction of noble gases with laser pulses in the near-infrared and visible wavelength regime is dominated by above barrier ionization while tunnel ionization plays a subordinate role. We develop a theory of ionization in this parameter range. A condition for the applicability of the quasistatic approximation in the above barrier ionization regime is obtained. Ionization of arbitrary atoms by a laser pulse may then be calculated from static ionization rates. The static ionization rate of He is obtained by solving the full two-electron Schrodinger equation. Our analysis yields a verification of the single active electron approximation and indicates the possibility to create keV radiation by high harmonic generation in a He gas.