2 agree well for the Cu L and Nd M edges, although some discrepancy is seen at the Nd M$_{4}$ edge. In contrast, the TFY is influenced by self-absorption, which visibly dis- torts the spectra. The IPFY measurement, however, is not distorted by self-absorption effects and provides an absorption spectrum comparable in shape to TEY while maintaining a probing depth similar to TFY. These experimental findings can be understood by ex- amining the expected x-ray emission intensity I ( E$_{i}$,E$_{f}$ ) as a function of the incident and emitted photon ener- gies, E$_{i}$ and E$_{f}$ respectively. I ( E$_{i}$,E$_{f}$ ) normalized to the incident photon intensity, I$_{0}$ ( E$_{i}$ ), is given by tons. The inverse of this NXES PFY, which we term inverse partial fluorescence yield (IPFY), is shown to be experimentally (at the Cu L and Nd M edges of La$_{1}$$_{.}$$_{475}$Nd$_{0}$$_{.}$$_{4}$Sr$_{0}$$_{.}$$_{125}$CuO$_{4}$) and theoretically proportional to the absorption cross-section and to be free of self- absorption or saturation effects. The principle of the technique is different than standard FY or EY. While FY and EY are measures of the number of core-hole ex- citations through an absorption edge, IPFY is effectively a measure of the x-ray attenuation length, akin to trans- mission measurements. This technique is anticipated to have wide-reaching applicability to XAS in a variety of materials using either hard or soft x-rays. A single crystal of La$_{1}$$_{.}$$_{475}$Nd$_{0}$$_{.}$$_{4}$Sr$_{0}$$_{.}$$_{125}$CuO$_{4}$ (LNSCO), grown by the traveling solvent floating zone method, was studied at the Canadian Light Source’s 11ID-1 SGM beamline. The LNSCO crystal was cleaved along the a − b plane in vacuum at a pressure of 5 · 10 − 8 Torr and maintained at this pressure during measurement. The total electron yield (TEY) was measured using drain current and the total fluorescence yield was measured with a channel plate detector. The energy resolved PFY was also measured using a silicon drift detector with an energy-resolution of ∼ 100 eV. The sample temperature was ∼ 200K for data in Figs. 1, 2(a) and 2(b) and room temperature for the data in Figs. 2(c) and 2(d). The sample geometry is indicated in Fig. 1(d). For the mea- surements in Figs. 1 and 2(a) and 2(b), photons were set normal to the sample surface ( α =90 $^{◦}$) and the chan- nel plate and silicon drift detectors were positioned at β ≃ 52 ◦ and 41 . 6 $^{◦}$, respectively. For all measurements, the polarization was linear horizontal and perpendicular to the sample’s c axis. In Fig. 1 we show the energy-resolved x-ray emission (normalized to the incident photon intensity) as the inci- dent photon energy is scanned through the Cu L and Nd M absorption edges. As a function of emitted photon en- ergy, x-ray emission peaks are observed corresponding to the following transitions Nd M$_{α,β}$ ≃ 990 eV (4 f → 3 d ), Cu L$_{α,β}$ ≃ 940 eV (3 d → 2 p ), La M$_{α,β}$ ≃ 830 eV (4 f → 3 d ), Nd M$_{ζ}$ ≃ 760 eV (4 p → 3 d ), La M$_{ζ}$ ≃ 630 eV (4 p → 3 d ) and O K$_{α}$ ≃ 524 eV (2 p → 1 s ). As expected, the Cu L and Nd M emission peaks as the incident energy passes through the Cu L and Nd M edges, respectively. In contrast, the O K emission dips at the Cu L and Nd M absorption edges, as shown in Fig. 1(c) by the inte- grated intensity of the O K emission vs. incident photon energy. The latter result can easily be understood: As the ab- sorption increases (penetration depth decreases) through the Cu L and Nd M edges, fewer oxygen atoms are ex- cited and subsequently fewer oxygen atoms emit at the O K edge. Moreover, it is apparent that the inverse of the O K emission, which we label IPFY, is proportional to the x-ray absorption and can be a useful measure of XAS. This is verified by comparing, in Fig. 2, the IPFY signal to the TEY and TFY, which agree with previous work on XAS of LSCO 11 and Nd. 12 The TEY and IPFY where C = η ( E$_{f}$ )Ω / 4 π is a constant given by the detec- tor efficiency η ( E$_{f}$ ) and the solid angle Ω of the detector. µ ( E$_{i}$ ) and µ ( E$_{f}$ ) are the total linear attenuation coeffi- cients of the material at energies E$_{i}$ and E$_{f}$ . α and β are the angle of incidence and angle of detection, respec- tively. The exponential terms in eqn. 1 account for the I ( E$_{i}$,E$_{f}$ ) I$_{0}$ ( E$_{i}$ ) = C ∑ X ∫ ∞ 0 ω$_{X}$ ( E$_{i}$,E$_{f}$ ) µ$_{X}$ ( E$_{i}$ ) sin α e − ( µ ( E i ) sin α + µ ( E f ) sin β $^{$^{)}$z}$dz, (1) FIG. 1. (Color online) (a) Normalized PFY of LNSCO as a function of E$_{i}$ and E$_{f}$ as the incident photon energy is scanned through the Cu L and Nd M edges. (b) The emission spectra for incident photon energies of 1003 eV (black), 934 eV (dotted red), and 925 eV in (dashed blue) taken in a 1-eV window. Emissions corresponding to the Nd M$_{α,β}$ , Cu L$_{α,β}$ , La M$_{α,β}$ , Nd M$_{ζ}$ , La M$_{ζ}$ , and O K$_{α}$ transitions are observed as a function of emitted photon energy. (c) The PFY at the O K edge integrated over a 50-eV window centered at 524 eV [the region spanned by the horizontal lines in (a)]. The O K edge emission dips at the Cu L and Nd M absorption edges where the absorption is maximal. (d) The experimental geometry: x-rays of energy E$_{i}$ are incident on the sample at an angle of α relative to the sample surface. Incident x-rays excite a core electron in an atom (highlighted in gold). This atom decays back to the ground state by emitting a photon of energy E$_{f}$ that is detected at angle β relative to the surface. 1250 1000 750 500 Emitted Photon Energy (eV) Incident Photon Energy (eV) 0.5 0.4 0.3 0.2 0.1 PFY (499-549 eV) 1020 1000 980 960 940 920 Cu L$_{3}$ Cu L$_{2}$  Nd  M$_{5}$  Nd  M$_{4}$ 1.5 1.0 0.5 0.0 Intensity (arb. units)  Nd  M Cu L  La   Nd  M  La   O  K E$_{i}$ =1003 eV (Nd  M$_{4}$ ) =934 eV (Cu  L$_{3}$ ) E$_{i}$ =925 eV  (b) (c) (d) 0.0 β z E$_{i}$ E$_{f}$ α 1.0 0.8 0.6 0.4 0.2 (a)