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.