Abstract
In the near-threshold region the photodetachment of an electron from a negative ion is considered in a model where the interaction is switched on suddenly. Several near-threshold regimes for the decay are investigated, and the authors give a definition of the conditions of their applicability, the time dependence of the residual probability, the competition of the exponential and non-exponential decay, etc. The asymptotical residual probability found for an infinitely large pulse duration is shown to be finite below threshold and to be equal to zero above the threshold. The role of the Stark shift of the ground level is discussed.

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