  Ionization by relativistically intense laser pulses of finite duration is
considered in the framework of strong-field quantum electrodynamics. Our main
focus is on the formation of ionization supercontinua. More specifically, when
studying the energy distributions of photoelectrons ionized by circularly
polarized pulses, we observe the appearance of broad structures lacking the
interference patterns. These supercontinua extend over hundreds of driving
photon energies, thus corresponding to high-order nonlinear processes. The
corresponding polar-angle distributions show asymmetries which are attributed
to the radiation pressure experienced by photoelectrons. Moreover, our time
analysis shows that the electrons comprising the supercontinuum can form pulses
of short duration. While we present the fully numerical results, their
interpretation is based on the saddle-point approximation for the ionization
probability amplitude.
