When the intensity of ionizing radiation increases, does ionization increase or
decrease? This is not a trifling question, when the radiation comes from an
x-ray free-electron laser (XFEL). If the higher intensity is attained by
shrinking the pulse duration, it had been established that the higher the
intensity, the less the ionization. This counterintuitive behavior, so-called
frustrated absorption or intensity-induced x-ray transparency, has been
considered one of the fundamental aspects in the XFEL–matter interaction.
It was believed that this paradigm of frustrated absorption is valid in
general; however, it has been experimentally verified only for systems
containing light elements at soft x rays. Here, we report that the paradigm of
frustrated absorption can break down at extremely high x-ray intensity.
With the help of a state-of-the-art theoretical tool, we examine the
pulse-duration dependence of the ionization dynamics of rare-gas atoms under
extreme irradiation conditions, which are accessible at current XFEL
facilities. When the fluence, defined as the number of x-ray photons per area,
is intermediate, ionization is suppressed for shorter pulses, which is
consistent with the frustrated-absorption phenomenon. However, when the fluence
is extremely high such that a certain limit can be overcome, ionization is
enhanced as the pulse duration is decreased, which is completely the opposite
of frustrated absorption.
Our finding of the breakdown of frustrated absorption establishes a new
paradigm for the interaction of matter with extremely intense x-ray pulses. Not
only does it represent a new manifestation of the complexity that can arise in
the interaction of a many-body system with high-intensity radiation, but we
also expect that this phenomenon will be of relevance to the design and the
interpretation of future XFEL experiments.
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Charge-state distribution of Xe at 1200 eV as a function of pulse duration.
See different behaviors of pulse-duration dependence for different fluences.
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