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Giant x-ray multiphoton ionization of atoms and molecules
Sang-Kil Son, Koudai Toyota, Ludger Inhester, Yajiang Hao, Kota Hanasaki, Robin Santra, Benedikt Rudek, Artem Rudenko, and Daniel RollesMultiphoton Processes
(Gordon Research Conference, Bryant University, Rhode Island, US, June 24-29, 2018) [poster]
Interaction of atoms and molecules with intense x-ray free-electron laser (XFEL) pulses is characterized by sequential multiphoton multiple ionization. This sequential ionization model has been verified with a series of gas-phased atomic XFEL experiments. With recent advances of XFELs, we are able to reach extremely high peak intensity in the hard x-ray regime, which gives rise to new multiphoton phenomena that have not been observed with x-ray synchrotron radiation. In this contribution, I will discuss two recent studies of enhancement of x-ray multiphoton ionization: REXMI and CREXIM. First, REXMI stands for resonance-enabled or resonance-enhanced x-ray multiple ionization. Due to the broad energy bandwidth of XFEL pulses and multiple resonant excitations followed by Auger-like decays, the system can be further ionized, far beyond the prediction of the straightforward sequential multiphoton ionization model. In our recent experiment with heavy atoms, the charge-state distribution as an outcome of ionization dynamics shows not only a dramatic extension to high charge states but also a characteristic pattern in the extended ion yields, which can be explained only when both relativistic and resonance effects are taken into account in theory. Next, CREXIM represents charge-rearrangement-enhanced x-ray ionization of molecules. When a molecule consisting of heavy and light atoms is exposed to high-intensity XFEL pulses, the molecule can be further ionized, far beyond the prediction of the independent-atom model. We propose a mechanism behind molecular x-ray ionization enhancement as the repetition of single-photon absorption on heavy atoms accompanied with electron transfer from light atoms to heavy atoms via chemical bonding. I will present two case studies: Xe atom for REXMI and iodomethane molecule for CREXIM. We employ dedicated theoretical tools, XATOM and XMOLECULE, and we make a quantitative comparison with our recent experimental results. For both cases, we demonstrate that highly nonlinear behavior is exhibited and the degree of ionization is no longer proportional to photoionization cross section in the hard x-ray regime. These two ionization enhancement mechanisms can play an important role for the quantitative understanding of radiation damage at high x-ray intensity.
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