Single and double ionization of helium by 1 GeV/nucleon ${\mathrm{U}}^{92+}$ impact was explored in a kinematically complete experiment. The relativistic ion generates a subattosecond ( ${10}^{-18\mathrm{}}\mathrm{s}$) superintense ( $I>\mathrm{}{10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) electromagnetic pulse, which is interpreted as a field of equivalent photons (Weizsäcker-Williams method). Cross sections, the emission characteristics of ions and electrons as well as momentum balances, are quantitatively discussed in terms of photoionization of the atom in this broadband, ultrashort virtual photon field.

• Received 14 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.3621