Ionization of negative ions and atoms by electric pulses: zigzag dependence on pulse duration

The ionization of atoms and negative ions by an electric pulse of Gaussian time shape is studied. At 'small' (in the sense of application of perturbation theory) amplitude of the Gaussian pulse, a zigzag behaviour of the ionization probability as a function of the pulse duration is revealed. The zigzag behaviour is due to the competition between perturbation and tunnelling mechanism. Such behaviour is absent for the excitation probability. The maximum at short pulse duration is reproduced by the first-order perturbation theory and the position of the maximum is proportional to the inverse of the transition frequency.