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Abstract of [Son21d]

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Resonance-enhanced multiphoton ionization in the x-ray regime

Sang-Kil Son, Aaron C. LaForge, Debadarshini Mishra, Markus Ilchen, Stephen Duncanson, Eemeli Eronen, Edwin Kukk, Stanislaw Wirok-Stoletow, Daria Kolbasova, Peter Walter, Rebecca Boll, Alberto De Fanis, Michael Meyer, Yevheniy Ovcharenko, Daniel E. Rivas, Philipp Schmidt, Sergey Usenko, Robin Santra, and Nora Berrah

Workshop on Science of “From Matter to Materials and Life”
(virtual, November 22-24, 2021) [poster]

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Multiphoton ionization is one of the fundamental nonlinear processes when matter interacts with intense laser fields. In particular, REMPI has been a widely-used spectroscopic technique due to high sensitivity and selectivity. Extending REMPI to the x-ray regime, however, requires entirely different physical processes and interpretation, due to accompanying ultrafast Auger decay processes. We present a first observation of REMPI in the short wavelength regime using ultraintense x rays from the European XFEL. We observe nonlinear ionization to create Ar17+, where photon energies are insufficient to directly ionize a 1s electron. With the aid of state-of-the-art theoretical modeling, we attribute the ionization to a two-color REMPI-like process where the second harmonic creates a 1s→2p transition and the fundamental pulse subsequently ionizes the system. This resonant phenomenon occurs not only for Ar16+, but through multiple lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI.

Tags: XATOM, XREMPI, Ar, experiment, SQS, European XFEL, CFEL, DESY

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