Resonance two-photon ionisation with ultrashort laser pulses

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Abstract
An expression is derived for the amount of two-photon ionisation produced by sub-nanosecond laser pulses under resonance conditions. On this time scale damping effects can be neglected and the ionisation probability becomes proportional to the cube of the pulse duration. A similar prediction for resonance multiphoton processes of higher order leads to a qualitative agreement with recent experimental observations. Finally, it is demonstrated that the same result can be obtained from a simple analysis of a two-step model.