X-ray Emission Spectroscopy at X-ray Free Electron Lasers: Limits to Observation of the Classical Spectroscopic Response for Electronic Structure Analysis

Abstract

X-ray free electron lasers (XFELs) provide ultrashort intense X-ray pulses suitable to probe electron dynamics, but can also induce a multitude of nonlinear excitation processes. These affect spectroscopic measurements and interpretation, particularly for upcoming brighter XFELs.Here we elaborate on the limits to observing classical spectroscopy, where only one photon is absorbed per atom for a Mn 2+ in a light element (O, C, H) environment. X-ray emission spectroscopy (XES) with different incident photon energies, pulse intensities, and pulse durations is presented. A rate equation model based on sequential ionization and relaxation events is used to calculate populations of multiply ionized states during a single pulse and to explain the X-ray induced spectral lines shifts. This model provides easy estimation of spectral shifts, which is essential for experimental designs at XFELs, and illustrates that shorter X-ray pulses will not overcome sequential ionization but can reduce electron cascade effects. TOC GRAPHICS

Effects of x-ray free-electron laser pulse intensity on the Mn Kβ1,3 x-ray emission spectrum in photosystem II—A case study for metalloprotein crystals and solutions

“…In terms of quantifying the electronic structure effects of our measurements, when applying the rate equation model from Ref. 52 and adopting a 1s photoelectric cross section of ∼1.5 × 10 −20 cm 2 , we obtain an α value of 0.18 for R1. This parameter is formulated to encapsulate most of the pulse effects and allow for easy comparison between experiments.…”

Chemical Sensitivity of Kβ and Kα X-ray Emission from a Systematic Investigation of Iron Compounds

“…We thus assume in the present context that multiple ionization processes are sufficiently weak such that they do not influence our conclusions. While this may hold for XES measurements at synchrotron radiation sources, in the case of experiments at X-ray free electron lasers multiple ionized states might contribute considerably . Finally, the chemical sensitivity in K-emission may also arise from screening effects where the core hole potential is screened less efficiently for higher oxidation states.…”

“…While this may hold for XES measurements at synchrotron radiation sources, in the case of experiments at X-ray free electron lasers multiple ionized states might contribute considerably. 47 Finally, the chemical sensitivity in K-emission may also arise from screening effects where the core hole potential is screened less efficiently for higher oxidation states. This change in core hole screening affects 1s and 2p/3p electrons differently, resulting in an energy shift of the K-fluorescence lines.…”

Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy

“…It was demonstrated for serial femtosecond crystallography at Å resolution, that an X-ray probe pulse length on the order of tens of femtoseconds can ‘outrun’ the anticipated sample damage caused by an intense X-ray pulse containing 10 11 –10 12 photons focused down to a few μm 13 , 24 . Recent diffraction and spectroscopy data using micron-sized focused XFEL beams with intensity levels reaching 10 17 –10 18 W cm -2 have indicated that such intensities could trigger various damage mechanisms impacting experimental observables within the typical X-ray pulse durations of 10–30 fs 14 – 17 . In particular some efforts have been made to model and predict spectral shifts and tolerable photon densities for solution phase experiments 17 , and to describe the effect of solvent photoionization on the spectral shape 16 .…”

“…Recent diffraction and spectroscopy data using micron-sized focused XFEL beams with intensity levels reaching 10 17 –10 18 W cm -2 have indicated that such intensities could trigger various damage mechanisms impacting experimental observables within the typical X-ray pulse durations of 10–30 fs 14 – 17 . In particular some efforts have been made to model and predict spectral shifts and tolerable photon densities for solution phase experiments 17 , and to describe the effect of solvent photoionization on the spectral shape 16 . In this letter we report a systematic time-dependent study of the effects of high XFEL pulse intensity on the electronic structure of Fe compounds at various metal concentration probed by X-ray emission spectroscopy (XES).…”