Capturing transient core-to-core resonances in Kr in intense extreme-ultraviolet laser fields by electron-ion coincidence spectroscopy

Letter

Capturing transient core-to-core resonances in Kr in intense extreme-ultraviolet laser fields by electron-ion coincidence spectroscopy

Mizuho Fushitani, Makoto Yamada, Hikaru Fujise, Shigeki Owada, Tadashi Togashi, Kyo Nakajima, Makina Yabashi, Akitaka Matsuda, Yasumasa Hikosaka, and Akiyoshi Hishikawa

Phys. Rev. A 107, L021101 – Published 23 February 2023

Abstract

Resonances between core-hole states have been long hypothesized to understand nonlinear responses of atoms and molecules exposed to intense x-ray free-electron laser (XFEL) pulses, but evaded clear identification in previous studies due to their transient nature. Here we report clear evidence of core-to-core resonances by state-of-the-art electron-ion coincidence spectroscopy on Kr atoms. The observed $3 p − 3 d$ resonance manifested the significance in multiple ionization to highly charged states against the ultrafast core-hole decay. The present study, elucidating the role of the core-to-core resonance in electronic responses of matter to high-frequency laser fields, will advance our understanding of nonlinear spectroscopy and ultrafast imaging by ultrashort intense EUV and x-ray pulses.

Received 12 October 2022
Accepted 9 February 2023

DOI:https://doi.org/10.1103/PhysRevA.107.L021101

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Autoionization & Auger processes Multiphoton or tunneling ionization & excitation Single- and few-photon ionization & excitation

Free-electron lasers

Inner-shell processes Photoionization Ultraviolet photoelectron spectroscopy

Atomic, Molecular & Optical

Authors & Affiliations

Mizuho Fushitani ^1,2,*, Makoto Yamada¹, Hikaru Fujise¹, Shigeki Owada ^2,3, Tadashi Togashi ^2,3, Kyo Nakajima ^2,3, Makina Yabashi^2,3, Akitaka Matsuda ^1,2, Yasumasa Hikosaka ^4,2,†, and Akiyoshi Hishikawa ^1,2,5,‡

¹Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan
²RIKEN, SPring-8 Center, Sayo, Hyogo 679-5148, Japan
³Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan
⁴Institute of Liberal Arts and Sciences, University of Toyama, Toyama 930-0194, Japan
⁵Research Center for Materials Science, Nagoya University, Nagoya, Aichi 464-8602, Japan

^*fusitani@chem.nagoya-u.ac.jp
^†hikosaka@las.u-toyama.ac.jp
^‡hishi@chem.nagoya-u.ac.jp

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Issue

Vol. 107, Iss. 2 — February 2023

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Images

Figure 1
(a) Schematic of transient core-to-core resonance at the $3 p^{- 1} - 3 d^{- 1}$ transition in the two-photon ionization of Kr, competing with ultrafast $3 d$ core-hole decay. (b) Energy level diagram of ${Kr}^{z +} (z$ = 0–4) [24, 26, 27, 28] relevant to the present study. Possible multiple ionization pathways are shown by arrows representing photoabsorption and electron emission (black: single-photon process; purple: multiphoton process). (c) Time-of-flight spectrum of Kr in intense EUV-FEL laser fields (121.5 eV, $1.2 \times 10^{14} W / {cm}^{2}$ ).
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Figure 2
Total electron spectra of Kr at $h ν = 121.5$ eV, recorded with a retarding voltage of $- 57$ V. The laser field intensities of $1.9 \times 10^{13} W / {cm}^{2}$ (gray) and 5.5 $\times 10^{14} W / {cm}^{2}$ (red). Sticks represent energies of photoelectrons ( $P_{1}$ ) to the $4 p^{- 1}, 4 s^{- 1}$ and satellite states of ${Kr}^{+}$ . The spectra are normalized at the $4 p$ photoelectron peak.
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Figure 3
Electron spectra recorded in coincidence with (a) ${Kr}^{+}$ , (b) ${Kr}^{2 +}$ , (c) ${Kr}^{3 +}$ , and ( $d) {Kr}^{4 +}$ ions, respectively, at $h ν$ = 120.5 eV ( $1.2 \times 10^{14} W / {cm}^{2}$ ). Thin sticks represent the electron energies expected for valence photoionization [ $P_{1}, P_{2}, P_{3}$ , and $P_{4}$ , in Fig. 1]. Thick stick spectra stand for the $3 p_{3 / 2}$ Auger peaks ( $A_{1}^{'}$ ) with the relative intensities [25, 26]. The dotted curve in (c) is the same one in (a), showing that there is a contribution in the ${Kr}^{3 +}$ yield from a sequential two-photon ionization pathway through the ${Kr}^{+}$ valence states in addition to a two-photon pathway via the ${Kr}^{+} 3 p_{3 / 2}^{- 1}$ state. (e)–(h) Same as (a)–(d), but for $h ν$ = 124.5 eV.
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Figure 4
(a) Auger electron yields from the ${Kr}^{+}$ core-hole states, $3 p_{3 / 2}^{- 1}$ (red) and $3 p_{1 / 2}^{- 1}$ (blue), decaying to ${Kr}^{2 +} (3 d^{- 1} 4 p^{- 1}$ ), as a function of the FEL photon energy. Expected transition energies [22], $h ν = 127.3$ eV ( $3 p_{1 / 2}^{- 1} - 3 d_{3 / 2}^{- 1}$ ), 120.7 eV ( $3 p_{3 / 2}^{- 1} - 3 d_{5 / 2}^{- 1}$ ), and 119.5 eV ( $3 p_{3 / 2}^{- 1} - 3 d_{3 / 2}^{- 1}$ ), are shown in the upper panel, together with the line strengths estimated from the statistical weights in the Russell-Saunders coupling scheme [39]. The stick spectra convoluted with the FEL width (1.1 eV) as well as the natural line widths of the $3 p_{3 / 2}^{- 1}$ (1.28 eV) and $3 p_{1 / 2}^{- 1}$ (1.65 eV) states [24] are also shown (solid lines) to compare with the experimental results. (b) FEL field-intensity dependence of the $4 p$ photoelectron yield (open square) and $3 p_{3 / 2}^{- 1}$ Auger electron yield to the $3 d^{- 1} 4 p^{- 1}$ state (solid circle). Results of the least-squares fitting to a power function $I^{k}$ are also shown.
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Figure 5
(a) Total yields of ${Kr}^{+}$ (gray), ${Kr}^{2 +}$ (black), ${Kr}^{3 +}$ (red), and ${Kr}^{4 +}$ (yellow) ions as a function of FEL intensity. Results of the simulation are obtained with $σ_{CC}$ = 1.6 Mb (solid lines). (b) Simulated relative yields in multiple ionization to ${Kr}^{3 +}$ from one-photon pathways (dotted line): (i) via ${Kr}^{+} (3 d^{- 1})$ with an electron emission sequence of $P_{1}^{'} - A_{d}$ and from two-photon pathway (solid line): (ii) via ${Kr}^{+} (4 l^{- 1})$ with $P_{1} - P_{2}^{'} - A_{2}$ and (iii) via ${Kr}^{2 +} (4 l^{- 1} 4 l^{' - 1})$ with $P_{1}^{'} - A_{1} - P_{3}$ [see Fig. 1]. (c) Same as (b), but for ${Kr}^{4 +}$ . Results of three two-photon pathways are shown (solid): (i) via ${Kr}^{+} (3 d^{- 1})$ with $P_{1}^{'} - A_{d} - P_{4}$ (blue), (ii) via ${Kr}^{+} (3 p^{- 1})$ with $P_{1}^{'} - A_{1}^{'} - A_{d}^{'}$ (red), and (iii) via ${Kr}^{+} (4 l^{- 1})$ with $P_{1} - P_{2}^{'} - A_{d}^{'}$ (gray). Three-photon pathways (dashed lines) are shown for those (i) via ${Kr}^{+} (4 l^{- 1})$ with $P_{1} - P_{2}^{'} - A_{2} - P_{4}$ , (ii) via ${Kr}^{2 +} (4 l^{- 1} 4 l^{' - 1})$ with $P_{1}^{'} - A_{1} - P_{3} - P_{4}$ , and (iii) via ${Kr}^{+} (3 p^{- 1})$ with $P_{1}^{'} - A_{1}^{'} - A_{2} - P_{4}$ .
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