Breakdown of frustrated absorption in x-ray sequential multiphoton ionization

Abstract

We investigate the frustrated absorption phenomenon of atomic systems driven by x-ray pulses of extremely high intensity. When an atom is exposed to intense x-ray pulses generated by x-ray free-electron lasers (XFELs), it undergoes complex ionization dynamics characterized by sequential multiphoton multiple ionization. Counterintuitively, as the pulse duration becomes shorter so that the intensity increases, the ionization becomes suppressed because of hollow-atom formation and the reduction of cross section. This is called frustrated absorption. However, as we report here, the paradigm of frustrated absorption can break down at extremely high intensity. By using a state-of-the-art theoretical tool, we examine the pulse-duration dependence of x-ray multiphoton ionization dynamics of heavy atoms, revealing that the reduced ionization for shorter pulses is due to the suppression of Auger decays, rather than the frustration of photoabsorption. Moreover, we predict a situation where ionization is, in fact, enhanced as the pulse duration is decreased and explain the mechanism why this happens. The present results demonstrate that the breakdown of frustrated absorption will emerge at the highest fluence currently available at XFEL facilities and will play an important role when terawatt-attosecond x-ray pulses come into realization.