  The ionization of an atom by a high-frequency intense laser pulse, where the
energy of a single-photon is sufficient to ionize the system, is investigated
from first principles. It is shown that as a consequence of an AC Stark effect
in the continuum, the energy of the photoelectron follows the envelope of the
laser pulse. This is demonstrated to result in strong dynamic interference of
the photoelectrons of the same kinetic energy emitted at different times.
Numerically exact computations on the hydrogen atom demonstrate that the
dynamic interference spectacularly modifies the photoionization process and is
prominently manifested in the photoelectron spectrum by the appearance of a
distinct multi-peak pattern. The general theory is shown to be well
approximated by explicit analytical expressions which allow for a transparent
understanding of the discovered phenomena and for making predictions on the
dependence of the measured spectrum on the properties of the pulse.
