Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects

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 ${\mathrm{La}}_{1.475}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ by measuring the O $K_{\alpha }$ PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra.

Figure 1

(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 $M_{\alpha ,\beta }$, Cu $L_{\alpha ,\beta }$, La $M_{\alpha ,\beta }$, Nd $M_{\zeta }$, La $M_{\zeta }$, and O $K_{\alpha }$ 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 $\alpha$ 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 $\beta$ relative to the surface.

Figure 2

The TEY (dashed black), O $K$ edge IPFY (red), and TFY (dotted blue) as a function of photon energy through the (a) Cu $L$ and (b) Nd $M$ edges. The spectra are scaled and offset to match above and below the absorption edges. The IPFY and TEY spectra are in good agreement while the TFY spectra are distorted due to self-absorption effects and NXES contributions to the TFY. (c) The Cu $L$ IPFY measured for two different geometries. (d) The IPFY spectra offset to match at a point in the pre-edge region.