Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects A. J. Achkar $^{1}$, T. Z. Regier$^{2}$, H. Wadati$^{3}$, Y.-J. Kim$^{4}$, H. Zhang4 and D. G. Hawthorn 1 $^{1}$Department of Physics and Astronomy, University of Waterloo, Waterloo, N2L 3G1, Canada $^{2}$Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0X4, Canada $^{3}$Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada $^{4}$Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario M5S 1A7, Canada (Dated: March 21, 2011) We demonstrate a method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy-sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L$_{2}$$_{,}$$_{3}$ and Nd M$_{4}$$_{,}$$_{5}$ absorption edges of the high- T$_{c}$ cuprate La$_{1}$$_{.}$$_{475}$Nd$_{0}$$_{.}$$_{4}$Sr$_{0}$$_{.}$$_{125}$CuO$_{4}$ by measuring the O K$_{α}$ PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra. PACS numbers: 78.70.Dm,78.70.En,61.05.cj,74.72.Gh sitivity given by the penetration depth of the incident and emitted photons (of order 1000 ˚ A in the soft x-ray region). FY is commonly measured as either the total fluorescence yield (TFY), which sums over all or a broad range of photon energies, or as the partial fluorescence yield (PFY), which uses an energy-sensitive detector to select fluorescence from a range of emitted photon ener- gies. PFY has been used as a probe which is sensitive to dilute concentrations of an element by focusing on the resonant emission from that element. 6 Since the development of brilliant sources of syn- chrotron radiation, x-ray absorption spectroscopy (XAS) has become a widely used probe of the electronic and spatial structure of materials in various fields of the nat- ural sciences. Multiple beamlines dedicated to XAS have been developed at virtually every synchrotron facility in the world. XAS is a core-level spectroscopy where a pho- ton excites a tightly bound core electron above the Fermi energy into an unoccupied state. In the near-edge re- gion (XANES), the resulting spectra provide element- and orbital-specific information about the unoccupied states of a material, revealing valuable information, such as the valence, spin-state, orbital symmetry, crystal field interaction, and the hybridization of atoms with their neighbors. 1–3 Further above an x-ray absorption edge, os- cillations in the absorption coefficient (EXAFS) provide information about the local atomic structure. 4 Restricting the use of FY, however, are self-absorption effects 7,8 which suppress the peaks in the measured spec- tra and make FY nonlinear with respect to the absorption coefficient. In FY, self-absorption effects occur when the penetration depth of the x-rays varies strongly near an x-ray absorption edge. EY can also exhibit distortions, although they are typically less significant than the self- absorption effects in FY. In EY, a saturation effect oc- curs when the electron escape depth is comparable to the x-ray penetration depth 9 (at its extreme all incident photons will generate electrons that escape the sample, producing an energy-independent signal). In transmis- sion, distortions can occur due to pinholes in samples or another saturation effect which comes from the exponen- tial dependence of the transmission intensity on the ab- sorption coefficient, limiting the dynamic range of trans- mission and leading to thickness-dependent distortions of the measured spectra. 10 X-ray absorption spectroscopy is most often measured in one of three ways: in transmission, using the elec- tron yield (EY), 5 or using the fluorescence yield (FY). 6 Transmission is perhaps the most direct method of mea- suring absorption. However, it is not commonly used in the soft x-ray region since the rapid attenuation of x-rays in materials limits transmission measurements to ultra-thin samples ( ∼ 1000 ˚ A), which is impractical for most studies. More common are EY and FY measure- ments. These techniques operate on the principle that the absorption cross-section is proportional to the num- ber of core-holes created. These core holes are filled by an electron having a lower binding energy, thereby emitting photons or electrons as decay products. By measuring the decay products one can achieve a measure of the lin- ear absorption coefficient. Of these two measures, EY is more surface sensitive, with the depth sensitivity given by the escape depth of electrons [20–200 ˚ A(Refs. 1-3)], whereas FY is more bulk sensitive, with the depth sen- In this paper, we introduce a method to measure the absorption coefficient that is bulk sensitive and does not suffer from saturation or self-absorption effects. The ap- proach uses an energy-sensitive detector to measure the PFY. Unlike previous PFY measurements, 6,7 however, we measure the normal (i.e., nonresonant) x-ray emis- sion spectrum (NXES) from a different element than the absorption edge we are probing with incident pho- arXiv:1001.1925v2 [cond-mat.str-el] 18 Mar 2011