Inverse Partial Fluorescence Yield Spectroscopy

Abstract

X-ray absorption spectroscopy (XAS) is a powerful probe of electronic and spatial structure that has been at the heart of many advances in physics, biology, chemistry, engineering and the earth sciences. Unfortunately, the existing experimental techniques are subject to fundamental limitations that complicate the interpretation of x-ray absorption spectra in many important cases. These limitations have motivated an effort to develop an alternative measure of the absorption cross-section that is not subject to the same set of limitations. In this thesis, a technique known as inverse partial fluorescence yield (IPFY) is described which addresses this problem. IPFY differs from existing approaches in a significant way — by using an energy-discriminating photon detector, one gains access to fluorescence information from both resonant and non-resonant x-ray emission processes.

We will show that the non-resonant emission is fundamentally related to the total absorption cross-section of a material through an inverse relation. This will be proven by extension of the general theory of fluorescence yield for the case of a thick, homogeneous specimen. We will also demonstrate the utility of IPFY with measurements of NiO, NdGaO₃, LNSCO, and stainless steel 304 at soft and intermediate x-ray energies. These experiments will highlight some essential features of IPFY spectroscopy and demonstrate how it can be an invaluable tool when the other experimental techniques fail to provide reliable spectra. We will also demonstrate how one can exploit the geometry dependence of IPFY to quantitatively determine the composition of a sample and the total x-ray absorption coefficient. Additionally, we will consider the special cases of multilayers and powder specimens, where the theory of fluorescence yield requires approximations and is not as well-behaved as in thick, homogenous specimens.

Ultimately, these experiments and theoretical developments will be used to support the claim that IPFY is a bulk sensitive measure of the total x-ray absorption coefficient. Moreover, we will show that IPFY is not affected by saturation effects, is insensitive to surface contamination layers and provides reliable spectra even for strongly insulating materials. These properties make IPFY a suitable spectroscopic technique for studying XAS in a wide range of materials.