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This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Observing pre-edge K-shell resonances in Kr, Xe, and XeF2

Publicated to:Physical review A: atomic, molecular and optical physics. 100 (2): - 2019-08-09 100(2), DOI: 10.1103/PhysRevA.100.022507

Authors: Southworth, S. H.; Dunford, R. W.; Ray, D.; Kanter, E. P.; Doumy, G.; March, A. M.; Ho, P. J.; Krassig, B.; Gao, Y.; Lehmann, C. S.; Picon, A.; Young, L.; Walko, D. A.; Cheng, L.;

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Abstract

The 1s -> np Rydberg transitions below the 1s ionization thresholds of Kr and Xe are obscured in x-ray absorption spectra due to core-hole lifetime broadening. However, the np spectator electrons associated with those resonances can affect the core-hole decay spectra. We report on ion charge-state distributions of Kr and Xe measured in coincidence with KL2,3 (K alpha(1,2)), KM2,3 (K beta(1,3)), and KN2,3 (K beta(2)) x-ray fluorescence as the incident x-ray energy is scanned through pre-edge resonances and ionization thresholds. The coincidence measurements select vacancy cascades that begin with a radiative transition that transfers 1s holes to the 2p, 3p, and 4p shells followed by emission of Auger electrons. We observe shifts of ion yields from higher to lower charge states that we attribute to np spectator electrons. For the special case of Kr1+ that is produced in coincidence with K beta(2) x rays, the ion yield decreases in the pre-edge region. This is attributed to production of neutral, metastable Kr 4p(-1) np states that reduce the Kr1+ yield. Model fits to the x-ray absorption spectra are presented to show the lifetime broadened pre-edge resonances and ionization thresholds. High-level relativistic coupled-cluster calculations that treat relativistic, electron correlation, and wave function relaxation effects on the same footing obtain agreement with the experimental 1s ionization energies of Kr and Xe to <2 eV. The Xe K-edge x-ray absorption spectrum and ion charge-state distributions of XeF2 were also recorded. Excitation of the 7 sigma(u) lowest unoccupied molecular orbital (LUMO) in XeF2 is observed in the pre-edge region. Our ab initio calculations find that the 6p(z) Rydberg state is strongly perturbed by the presence of the 7 sigma(u) LUMO. A fit to the measured LUMO, 6p(x), 6p(y), 6p(z), Rydberg states, and ionization threshold is guided by the relativistic coupled-cluster calculations. The F ligands modify the valence electron charge distribution and result in a similar to 2.3 eV chemical shift of the Xe 1s ionization energy relative to atomic Xe. Xe 1s(-1) core-hole decay in XeF2 results in ionization of the F ligands and energetic fragmentation into atomic ions. Ion charge-state spectra of XeF2 were recorded in coincidence with x-ray fluorescence for excitation on the LUMO resonance and above the Xe 1s ionization threshold. For the ion time of flight spectra recorded on the LUMO resonance, the Fq+ (q = 1-4) peaks are split into two peaks along the linear polarization direction of the incident x-ray beam. This effect is attributed to spatial alignment of XeF2 molecules by resonant x-ray absorption, and the peak splittings are used to measure the F ion fragmentation energies following K alpha(1,2), K beta(1,3), and K beta(2) x-ray fluorescence. We observe variations of the F ion charge state yields and fragmentation energies for the three fluorescence pathways that leave the molecule in different outer-shell hole states.

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