Strong-field ionization of atoms and molecules: The two-term saddle   point method
  We derive an analytical formula for the ionization rate of neutral atoms and molecules in a strong monochromatic field. Our model is based on the strong-field approximation with transition amplitudes calculated by an extended saddle point method. We show that the present two-term saddle point method reproduces even complicated structures in angular resolved photo electron spectra. 
Thomas Kim Kjeldsen, Lars Bojer Madsen