Abstract
An accurate description of the interaction of intense hard X-ray pulses with heavy atoms, which is crucial for many applications of free-electron lasers, represents a hitherto unresolved challenge for theory because of the enormous number of electronic configurations and relativistic effects, which need to be taken into account. Here we report results on multiple ionization of xenon atoms by ultra-intense (about 1019 W/cm2) femtosecond X-ray pulses at photon energies from 5.5 to 8.3 keV and present a theoretical model capable of reproducing the experimental data in the entire energy range. Our analysis shows that the interplay of resonant and relativistic effects results in strongly structured charge state distributions, which reflect resonant positions of relativistically shifted electronic levels of highly charged ions created during the X-ray pulse. The theoretical approach described here provides a basis for accurate modeling of radiation damage in hard X-ray imaging experiments on targets with high-Z constituents.
Highlights
An accurate description of the interaction of intense hard X-ray pulses with heavy atoms, which is crucial for many applications of free-electron lasers, represents a hitherto unresolved challenge for theory because of the enormous number of electronic configurations and relativistic effects, which need to be taken into account
It was observed that resonant excitations of inner-shell electrons into densely-spaced Rydberg states and unoccupied valence orbitals can dramatically enhance the ionization in the soft X-ray regime via a process dubbed resonance-enabled or resonance-enhanced X-ray multiple ionization (REXMI)[11,12]
Based on the good agreement of the charge state distributions (CSDs) obtained from experiment and theory including relativistic and resonance effects, we are able to quantitatively describe and predict the degree of ionization for heavy atoms after the interaction with intense hard X-rays as a function of fluence
Summary
An accurate description of the interaction of intense hard X-ray pulses with heavy atoms, which is crucial for many applications of free-electron lasers, represents a hitherto unresolved challenge for theory because of the enormous number of electronic configurations and relativistic effects, which need to be taken into account. A quantitative description of the resonant enhancement, which is essential, for example, for the proposed high-intensity phasing schemes[7,8], requires a relativistic treatment[17] This is extremely challenging since the combination of deep inner-shell ionization, relativistic effects, and resonant excitations creates an enormous computational complexity, especially at high X-ray intensities, where more than 20 high-energy photons can be absorbed by a single atom. We present experimental data on the multiple ionization of Xe atoms by ultra-intense XFEL pulses at several hard X-ray wavelengths and, by comparison with theory, demonstrate that relativistic and resonant effects play a crucial role in this regime They significantly enhance ionization and give rise to surprisingly structured charge state distributions (CSDs), which reflect resonance positions of relativistically shifted electronic levels of highly charged ions reached during the X-ray pulse as a consequence of the X-ray-driven ionization dynamics.
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