High Energy Resolution Off-Resonant Spectroscopy for X-Ray Absorption Spectra Free of Self-Absorption Effects

W. Błachucki, J. Szlachetko, J. Hoszowska, J.-Cl. Dousse, Y. Kayser, M. Nachtegaal, and J. Sá

Phys. Rev. Lett. 112, 173003 – Published 30 April 2014

Abstract

X-ray emission spectra recorded in the off-resonant regime carry information on the density of unoccupied states. It is known that by employing the Kramers-Heisenberg formalism, the high energy resolution off-resonant spectroscopy (HEROS) is equivalent to the x-ray absorption spectroscopy (XAS) technique and provides the same electronic state information. Moreover, in the present Letter we demonstrate that the shape of HEROS spectra is not modified by self-absorption effects. Therefore, in contrast to the fluorescence-based XAS techniques, the recorded shape of the spectra is independent of the sample concentration or thickness. The HEROS may thus be used as an experimental technique when precise information about specific absorption features and their strengths is crucial for chemical speciation or theoretical evaluation.

Received 13 December 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.173003

Authors & Affiliations

W. Błachucki^1,*, J. Szlachetko^2,3, J. Hoszowska¹, J.-Cl. Dousse¹, Y. Kayser^1,†, M. Nachtegaal², and J. Sá²

¹Department of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland
²Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland
³Institute of Physics, Jan Kochanowski University, 25-406 Kielce, Poland

^*wojciech.blachucki@unifr.ch
^†Present address: Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland.

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 112, Iss. 17 — 2 May 2014

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Schematic of the RIXS process for an incident photon energy $ℏ ω_{1}$ involving excitation of an electron from the initial state of energy $E_{i}$ to an unoccupied state $E$ above the Fermi level, and electron deexcitation from the final state of energy $E_{f}$ accompanied by the emission of a photon with the energy $ℏ ω_{2}$ . For incident photon energies greater or equal to the absorption edge energy, the x-ray scattering can be considered as a two-step process and $ℏ ω_{2} = | E_{i} | - | E_{f} |$ (a). Otherwise, the excitation and deexcitation occur coherently and a one-step picture applies, where $ℏ ω_{2} (E) = ℏ ω_{1} - | E_{f} | - E < | E_{i} | - | E_{f} |$ (b).
Reuse & Permissions
Figure 2
Ta $L_{3}$ absorption edge (9.881 keV) scanned with an x-ray beam of energy varying in the range 9.850–9.915 keV. The data were recorded for samples of different thicknesses both in transmission (a) and fluorescence mode in high energy resolution (b) and TFY (c). The spectra depicted in (b) and (c) were normalized to coincide at 9.905 keV. High energy resolution XAS spectra were obtained by integrating the XES data within a 2 eV energy window centered at the Ta $L α_{1}$ ( $L_{3} M_{5}$ ) fluorescence line (8.146 keV), while TFY XAS spectra result from integrating the XES data within the whole Ta $L α_{1, 2}$ ( $L_{3} M_{5, 4}$ ) domain. (d) RIXS plane recorded for the $10 μ m$ thick sample showing the integration area (dotted lines) for the high energy resolution XAS. Note that the integration energy interval is smaller than the initial state lifetime broadening of 4.68 eV.
Reuse & Permissions
Figure 3
Off-resonant XES spectra recorded for different sample thicknesses at an incident x-ray beam energy of 9.855 keV (a) and the reconstructed HEROS spectra corresponding to the Ta $L_{3}$ absorption edge (b). The reconstruction was done by means of Eq. (1). The XES spectra were normalized to coincide at the emission energy of 8.096 keV in (a) and the HEROS spectra at the excitation energy of 9.905 keV in (b). The scatter of the data at the top of the two peaks for different sample thicknesses is shown enlarged in the insets.
Reuse & Permissions
Figure 4
Calculations showing the effects of the target thickness on the Ta $L_{3}$ edge in the conventional fluorescence mode XAS (a) and in the HEROS (b), for $ϕ = θ = 45 °$ . In the XAS case the calculations were performed for a fixed x-ray emission energy corresponding to the Ta $L α_{1}$ line (8.146 keV). In the HEROS case, the fluorescence yield ratios were first calculated. The calculations were performed for the beam energy used in the HEROS measurements (26 eV below the Ta $L_{3}$ edge). Then the energy scale of the x-ray emission was converted to the excitation energy scale using Eq. (1). For both XAS and HEROS, the yield ratios were calculated with respect to the $0.5 μ m$ thick sample (i.e., $d_{2} = 0.5 μ m$ ).
Reuse & Permissions

Physical Review Letters