Abstract
We study the evolution of ionization probability in a simple one-dimensional model atom with a delta-function potential for very short, intense laser pulses. The intensities are in the ranges where tunnelling and over-the-barrier ionization take place. A full numerical solution For the time-dependent Schrodinger equation is obtained and a detailed comparison is made between the results for rectangular and smooth pulses. The ultra-intense field limit is verified to follow the ionization expected from an oscillating Volkov wavepacket which is uniformly spreading. No signs of ionization suppression or bound state stabilization are seen.

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