Atoms in strong radiation fields are interesting objects for study, and
the research field that concerns itself with this study is a
comparatively young one. For a long period after the scovery of the
photoelectric effect. it was not possible to generate electro- magnetic
fields that did more than perturb the atom only slightly, and (first-or
er) perturbation theory could perfectly explain what was going on at
those low intensities. The development of the pulsed laser bas changed
this state of affairs in a rather dramatic way, and fields can be
applied that really have a large, or even dominant influence on atomic
structure. In the latter case, w speak of super-intense fields. Since
the interaction between atoms and electromagnetic waves is characterized
by many parameters other than the light intensity, such as frequency,
iQnization potential, orbit time, etc., it is actually quite difficult
to define what is exactly meant by the term 'super-intense'. Obviously
the term does not have an absolute meaning, and intensity should always
be viewed in relation to other properties of the system. An atom in a
radiation field can thus best be described in terms of various ratios of
the quantities involved. The nature of the system sometimes drastically
changes if the value of one of these parameters exceeds a certain
critical value, and the new regime could be called super-intense with
respect to that parameter.