Experimentally, the absorption cross section of a sample can be obtained in the transmission mode which means by measuring the incoming and transmitted light through a sample. However, transmission mode measurements require an energy dependent optimization of thickness and/or concentration for each sample. An indirect way to determine the absorption cross section is to detect the photons that are emitted after electrons of lower binding energy fill the created core hole, giving rise to the total fluorescence yield (TFY). To a limited extent, the TFY can be considered as proportional to the absorption cross section for dilute and thin samples and are therefore often used for recording X-ray absorption spectra. It is important to note that in general the TFY can be affected by the saturation effect which depends on the sample concentration.
Schematic views of the flow cell and the micro-jet taken from the review of Lange et al