We study the ionization of extreme Stark states in sodium by THz electromagnetic half-cycle pulses. The results of our full-quantum calculations reveal the presence of an oscillatory pattern in the ionization spectra of extreme red ͑downhill͒ states that have been kicked away from the ion core ͑downhill with respect to the potential imposed by the Stark field͒. We find no oscillations in the spectra of extreme blue ͑uphill͒ states that have been kicked towards the ion core ͑also downhill with respect to the Stark potential͒. The oscillatory pattern in the red state ionization spectra is explained with a one-dimensional semiclassical model in terms of interferences between two classical paths. This model also predicts that the blue state ionization spectra should also show oscillatory behavior. The absence of the oscillations in the full calculations can be regarded as a breakdown of the one-dimensional model in representing the ionization dynamics of these states. We find that the one-dimensional model fails when the duration of the THz pulse is comparable to the classical Kepler orbit time.

We investigate the focusing, that is, the reversing of spreading of continuum wave packets created with chirped laser pulses in the weak-field limit. Focusing can be accomplished when a negative position−momentum correlation is created in the wave packet. Specializing to constant and linear (repulsive) potentials, we show analytically that in a constant potential wave packet spreading can be compensated with positively chirped laser pulses, whereas in the linear potential negatively chirped laser pulses are required, in the short-pulse limit. The analytical results for the linear potential are supported by numerical simulations. The results are discussed in the context of a classical model for wave packet focusing.

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