High Resolution Photoexcitation Measurements Exacerbate the Long-Standing Fe XVII Oscillator Strength Problem.

Steffen Kühn,René Steinbrügge,Keisuke Fujii,Sungnam Park,Moritz Hoesch,Maurice A Leutenegger,Gregory V Brown,Thomas Stöhlker,Jakob Stierhof,Sven Bernitt,Jörn Seltmann,Michael Rosner,Moto Togawa,Thomas Pfeifer,Charles Cheung,Christoph H Keitel,Ralf Ballhausen,Chintan Shah,Natalia S Oreshkina,М Chung ,M S Safronova ,V A Yerokhin ,S G Porsev ,J Wilms ,A Surzhykov ,F S Porter ,Z Harman ,N Hell ,J R Crespo López‐Urrutia ,M Bissinger ,M G Kozlov ,J C Berengut

doi:10.1103/physrevlett.124.225001

Abstract

For more than 40years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p-3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios f(3C)/f(3D) disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of f(3C)/f(3D)=3.09(8)(6) supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale abinitio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.

Highlights

For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p − 3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios fð3CÞ=fð3DÞ disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum
At the high peak brilliance of the Linac Coherent Light Source (LCLS) x-ray free-electron laser (XFEL), nonlinear excitation dynamics [32,33] or nonequilibrium time evolution [34] might have affected the result of Bernitt et al [25]
An effect of resonance-induced population transfer between Fe XVI and Fe XVII ions was postulated [35] since the Fe XVI line C (1⁄2ð2p5Þ1=23s3d5=2J1⁄43=2 → 1⁄22p63sJ1⁄41=2) appeared blended with the Fe XVII line 3D

Summary

Introduction

For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p − 3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios fð3CÞ=fð3DÞ disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p − 3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios fð3CÞ=fð3DÞ

Results

Conclusion