International Workshop on Atomic Interactions in Laser Fields - Abstracts

Suppression of photoionization in strong laser fields

A. Raczynski and J. Zaremba

Institute of Physics, Nicholas Copernicus University, Toruñ, Poland

A typical behaviour of atomic systems irradiated by strong and ultrastrong laser fields is that the ionization probability is relatively large and it increases when the field is increased or new channels are opened. However, there are situations in which the photoionization is suppressed, i.e. one observes a decrease of the overall ionization yield or at least of the ionization probability into some energy range, or the process is slowed down in some time intervals. The suppression may be caused by an increase of the field or by switching on another laser or by a special choice of the pulse parameters. This talk is a review of some of such situations, with stress put on a few recent results. In particular the following issues will be discussed:

• The resonant ionization may be suppressed if the upper state is coupled to a third state, in spite of opening of a new ionization channel. The process can be smoothly controlled by changing the time delay between the two pulses.
• The height of the above-threshold ionization (ATI) peaks in the photoelectron spectrum can be controlled in a two-colour experiment by changing the relative phase between the fundamental pulse and the harmonic one.
• The dynamics of the photoionization can be essentially changed if chirped pulses are used. In particular the process may be slowed down depending on the instantaneous value of the frequency. Fragments of the photoelectron spectrum can be selectively quenched.
• If the part of the atomic continuum to which photoionization occurs is coupled to another discrete state by a second laser, the photoionization may be suppressed in favour of a coherent population transfer to this state. The effectiveness of the transfer may be improved by using control lasers, which regulate the Stark shifts or modify the density of states in the continuum.
• In ultrastrong laser fields the stabilization may occur: the ionization probability (or the ionization rate) may be a locally decreasing function of the field intensity. The adiabatic stabilization may be interpreted as a population trapping in the so-called Kramers-Henneberger well. A generalization of the KH transformation allows one to give account not only of the well's oscillations but also its drift in the laboratory frame. The drift motion can be modelled using the Ehrenfest theorem. The stabilization is a combined effect of the complicated field dependence both of the drift motion and of the lifetime of the KH states.