Resonance-Enhanced Multiphoton Ionization in the X-Ray Regime

Aaron C. LaForge, Sang-Kil Son (손상길), 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

Phys. Rev. Lett. 127, 213202 – Published 19 November 2021

Abstract

Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a $1 s$ electron, a different mechanism is required to obtain ionization to ${Ar}^{17 +}$ . We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a $1 s \to 2 p$ transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for ${Ar}^{16 +}$ , but also through lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile.

Received 15 February 2021
Accepted 14 October 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.213202

Physics Subject Headings (PhySH)

Autoionization & Auger processes Electronic structure of atoms & molecules Electronic transitions Multiphoton or tunneling ionization & excitation Nuclear & electron resonance Ultrafast phenomena

Atomic, Molecular & Optical

Authors & Affiliations

Aaron C. LaForge ^1,*, Sang-Kil Son (손상길)^2,3,†, Debadarshini Mishra ¹, Markus Ilchen^4,5, Stephen Duncanson ¹, Eemeli Eronen ⁶, Edwin Kukk⁶, Stanislaw Wirok-Stoletow ^2,7, Daria Kolbasova ^2,7, Peter Walter⁸, Rebecca Boll⁴, Alberto De Fanis⁴, Michael Meyer⁴, Yevheniy Ovcharenko ⁴, Daniel E. Rivas ⁴, Philipp Schmidt⁴, Sergey Usenko ⁴, Robin Santra ^2,3,7, and Nora Berrah¹

¹Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA
²Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
³The Hamburg Centre for Ultrafast Imaging, 22761 Hamburg, Germany
⁴European XFEL, 22869 Schenefeld, Germany
⁵Institut für Physik und CINSaT, Universität Kassel, 34132 Kassel, Germany
⁶Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁷Department of Physics, Universität Hamburg, 22607 Hamburg, Germany
⁸Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA