Fluorescence yield detection: Why it does not measure the X-ray absorption cross section

Abstract

It is shown that fluorescence yield (FY) detection does not measure a pure X-ray absorption spectrum if multiplet effects are important. The reason is that while the Auger decay is approximately constant, the fluorescence decay shows a strong dependence on the final state. With a coherent description of the X-ray absorption and X-ray emission processes, the observed differences between TEY and FY detected nickel 2 p edges can be reproduced accurately. In the, approximate, incoherent description the FY spectral shape identifies with the X-ray absorption spectrum multiplied with its state dependent fluorescence decay. Self-absorption effects are calculated and the implications for X-ray Magnetic Circular Dichroism (X-MCD) and generalisations to other systems are discussed.