Included in atoms.inp
,
Proper sample preparation for an XAFS experiment requires knowledge of the absorption depth and edge step size of the material of interest. The statistics of data collection can be optimized by choosing the correct sample thickness. It is also necessary to avoid distortions to the data due to thickness and large particle size effects.
The density of the material is also reported. This number assumes that the bulk material will have the same density as the unit cell. It is included as an aid to sample preparation.
Typically, XAFS data is normalized to a single number representing the size of the edge step. While there are compelling reasons to use this simple normalization, it can introduce an important distortion to the amplitude of the chi(k) extracted from the absorption data. This distortion comes from energy response of the bare atom absorption of the central atom. This is poorly approximated away from the edge by a single number. Because this affects the amplitude of chi(k) and not the phase, it can be corrected by including a Debye-Waller factor and a fourth cumulant in the analysis of the data. These two ``McMaster corrections'' are intended to be additive corrections to any thermal or structural disorder included in the analysis of the XAFS.
The response of the I0 chamber varies with energy during an XAFS experiment. In a fluorescence experiment, the absorption signal is obtained by normalizing the IF signal by the I0 signal. There is no energy response in the IF signal since all atoms fluoresce at set energies. The energy response of I0 is ignored by this normalization. At low energies this can be a significant effect. Like the McMaster correction, this effect attenuates the amplitude of chi(k) and is is well approximated by an additional Debye-Waller factor and fourth cumulant.
atoms.inp
to determine the content of the I0 chamber by
pressure. It assumes that the remainder of the chamber is filled with
helium. It then uses McMaster's data to construct the energy response
of the chamber and regresses a polynomial to it in the manner
described above. sigma_I0ˆ2 and
sigma_I0ˆ4 are also written at the top of the output
file and intended as additive corrections in the analysis.
If the thickness of a sample is large compared to absorption length of the sample and the absorbing atom is sufficiently concentrated in the sample, then the amplitude of the chi(k) extracted from the data taken on it in fluorescence will be distorted by self-absorption effects in a way that is easily estimated. The absorption depth of the material might vary significantly through the absorption edge and the XAFS wiggles. The correction for this effect is well approximated as
1 + mu_abs / (mu_background+mu_fluor)
where mu_background is the absorption of the non-resonant
atoms in the material and mu_fluor is the total absorption
of the material at the fluorescent energy of the absorbing atom.
feff.inp
as the amplitude factor is intended to be a
multiplicative correction to the data or to the measured
S0ˆ2.