1.7 A practical introduction to x-ray absorption spectroscopy  (Page 3/5)

Transmission and fluorescence modes

X-ray Absorption measurements can be performed in several modes: transmission, fluorescence and electron yield; where the two first are the most common. The choice of the most appropriate mode to use in one experiment is a crucial decision.

The transmission mode is the most used because it only implies the measure of the X-ray flux before and after the beam passes the sample. Therefore, the adsorption coefficient is defined as [link] . Transmission experiments are standard for hard X-rays, because the use of soft X-rays implies the use the samples thinner than 1 μm . Also, this mode should be used for concentrated samples. The sample should have the right thickness and be uniform and free of pinholes.

The fluorescence mode measures the incident flux I ₀ and the fluorescence X-rays I _f that are emitted following the X-ray absorption event. Usually the fluorescent detector is placed at 90° to the incident beam in the horizontal plane, with the sample at an angles, commonly 45°, with respect to the beam, because in that position there is not interference generated because of the initial X-ray flux ( I ₀ ). The use of fluorescence mode is preferred for thicker samples or lower concentrations, even ppm concentrations or lower. For a highly concentrated sample, the fluorescence X-rays are reabsorbed by the absorber atoms in the sample, causing an attenuation of the fluorescence signal, it effect is named as self-absorption and is one of the most important concerns in the use of this mode.

Sample preparation for xas

Sample requirements

Uniformity

The samples should have a uniform distribution of the absorber atom, and have the correct absorption for the measurement. The X-ray beam typically probes a millimeter-size portion of the sample. This volume should be representative of the entire sample.

Thickness.

For transmission mode samples, the thickness of the sample is really important. It supposes to be a sample with a given thickness, t , where the total adsorption of the atoms is less than 2.5 adsorption lengths, µ E t ≈ 2.5; and the partial absorption due to the absorber atoms is around one absorption length ∆ µ E t ≈ 1, which corresponds to the step edge.

The thickness to give ∆ µ E t = 1 is as [link] . where ρ is the compound density, n is the elemental stoichiometry, M is the atomic mass, σ E is the adsorption cross-section in barns/atom (1 barn = 10 ^-24 cm ² ) tabulated in McMaster tables, and E + and E - are the just above and below the energy edge. This calculation can be accomplished using the free download software HEPHAESTUS.

Total x-ray adsorption.

For non-concentrate samples, the total X-ray adsorption of the sample is the most important. It should be related to the area concentration of the sample (ρ t , in g/cm ² ). The area concentration of the sample multiplied by the difference of the mass adsorption coefficient ( ∆µ E /ρ ) give the edge step, where a desired value to obtain a good measure is a edge step equal to one, (∆µ E /ρ)ρ t ≈ 1.

The difference of the mass adsorption coefficient is given by [link] , where ( µ E /ρ) i is the mass adsorption coefficient just above (E+) and below (E-) of the edge energy and f i is the mass fraction of the element i . Multiplying the area concentration, ρ t, for the cross-sectional area of the sample holder, amount of sample needed is known.