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Soft X-Ray Emission and Meta-Stable Population Dynamics in Sm-Like Pb and Neighboring Ions

Dingbao SONG¹⁾, Hayato OHASHI²⁾, Hiroyuki A. SAKAUE³⁾, Nobuyuki NAKAMURA⁴⁾, Naoki KIMURA³⁾, Daiji KATO^1,3)

1)

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

2)

Institute of Liberal Arts and Sciences, University of Toyama, Toyama, Toyama 930-8555, Japan

3)

National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, Gifu 509-5292, Japan

4)

Institute for Laser Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan

(Received 14 October 2025 / Accepted 25 October 2025 / Published 24 December 2025)

Abstract

In recent years, the development of soft X-ray (SXR) light sources has progressed rapidly, particularly for applications within the water window region (23–44 Å), where high-resolution, high-contrast imaging of biological cells and macromolecules is possible. In this study, we analyze water window emission lines of samarium-like (Sm-like) Pb²⁰⁺ ions observed using a compact electron beam ion trap at an electron beam energy of 565 eV. The analysis is carried out using a collisional-radiative model based on atomic data from HULLAC. Three distinct meta-stable states of Pb²⁰⁺, each with a fractional population exceeding 1%, are identified. To explore the conditions under which meta-stable states are formed or suppressed across the isoelectronic sequence, we performed systematic atomic structure and transition rate calculations for Sm-like ions from Hg to Po (Z = 80–84). These results provide new insights into the role of energy level crossings and decay pathways in the formation of long-lived states, offering implications for optimizing soft X-ray emission in plasma-based light sources.

Copyright (c) 2025 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

highly charged ion, soft X-ray, meta-stable state, water window, lead, bismuth

DOI: 10.1585/pfr.20.1401057

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