Anomalous collision-free multiple ionization of atoms with intense picosecond ultraviolet radiation

Collisionless multiphoton absorption, resulting in multiple atomic ionization and exhibiting anomalously strong coupling, has been studied in the region spanning atomic number Z=2 (He) to Z=92 (U). The highest ion state identified is U10+, corresponding to absorption of 99 quanta (∼633 eV). Models of stepwise ionization using standard theoretical techniques are incapable of describing these results. A mode of interaction involving radiative coupling to a collective motion of an atomic shell is proposed. © 1983 The American Physical Society.