XAFSmass

XAFSmass
A program for calculations of x-ray absorption for Windows

Mar 2003

pay also your attention to my programs for EXAFS and XANES

Author: K.V.Klementiev
Address: HASYLAB / DESY
Notkestrasse 85,
D-22607 Hamburg, Germany
E-mail Address: nospam@desy.de (BUT replace "nospam" by "Konstantin.Klementiev" ; this is done to short-cut scanners for spam mail)
Since May 2006 I have been working in ALBA/CELLS in Barcelona. If someone tried to reach me by the old address at DESY, please forgive my silence: the address is not active any longer. You can ask your questions here.
Theoretical references used:

http://www-cxro.lbl.gov/optical_constants/
B.L. Henke, E.M. Gullikson, and J.C. Davis, X-ray interactions: photoabsorption, scattering, transmission, and reflection at E=50-30000 eV, Z=1-92, Atomic Data and Nuclear Data Tables Vol.54 (no.2), 181-342 (July 1993)
http://www-phys.llnl.gov/Research/scattering/index.html
Lynn Kissel, B. Zhou, S. C. Roy, S. K. Sen Gupta and R. H. Pratt, Validity of Form-Factor, Modified-Form-Factor and Anomalous-Scattering-Factor Approximations in Elastic Scattering Calculations Acta Crystallographica A51, 271-288 (1995).
R. H. Pratt, Lynn Kissel and P. M. Bergstrom, Jr., New Relativistic S-Matrix Results for Scattering - Beyond the Usual Anomalous Factors/ Beyond Impulse Approximation, in Resonant Anomalous X-Ray Scattering, edited by G. Materlik, C. J. Sparks and K. Fischer (North-Holland: Amsterdam, 1994).
P. P. Kane, Lynn Kissel, R. H. Pratt and S. C. Roy, Elastic Scattering of Gamma-Rays and X-Rays by Atoms, Physics Reports 140, 75-159 (1986).
Lynn Kissel and R. H. Pratt, Rayleigh Scattering - Elastic Photon Scattering by Bound Electrons, in Atomic Inner-Shell Physics, edited by Bernd Crasemann (Plenum Publishing: New York, 1985).
http://www.bmsc.washington.edu/scatter/periodic-table.html
ftp://ftpa.aps.anl.gov/pub/cross-section_codes/
S. Brennan and P.L. Cowan, A suite of programs for calculating x-ray absorption, reflection and diffraction performance for a variety of materials at arbitrary wavelengths Rev. Sci. Instrum. 63, 850-853 (1992)
http://physics.nist.gov/PhysRefData/FFast/Text/cover.html
http://physics.nist.gov/PhysRefData/FFast/html/form.html
C. T. Chantler, Theoretical Form Factor, Attenuation, and Scattering Tabulation for Z = 1 - 92 from E = 1 - 10 eV to E = 0.4 - 1.0 MeV J. Phys. Chem. Ref. Data 24, 71-643 (1995)
http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html
J.H. Hubbell, Photon Cross Sections, Attenuation Coefficients and Energy Absorption Coefficients from 10 keV to 100 GeV, Natl. Stand. Ref. Data Ser. 29 (1969). J.H. Hubbell, Photon Mass Attenuation and Mass Energy-Absorption Coefficients for H,C,N,O,Ar and Seven Mixtures from 0.1 keV to 20 MeV, Radiat. Res. 70, 58-81 (1977).
http://ixs.csrri.iit.edu/database/programs/mcmaster.html
http://cars9.uchicago.edu/~newville/mcbook/
W. H. McMaster, N. Kerr Del Grande, J. H. Mallett, and J. H. Hubbell Compilation of X-Ray Cross Sections Lawrence Livermore National Laboratory Report UCRL-50174 Section II Revision I (1969) available from National Technical Information Services L-3, U.S. Dept. of Commerce

Usage:

Calculation of mass and absorption step for powder samples

calculation of mass and absorption step for powder samples

A typical application is the calculation of the mass for a powder sample. The optimal optical sample thickness μd depends on the absorption levels selected for the ionization chambers (see below). Typically, μd is between 2 and 3 (e.g. for a 17.5% absorption level for the 1st chamber and a 50% level for the 2nd chamber, the optimal thickness is 2.41). However, if you get the absorption step more that 1.5, it is recommended to reduce the sample mass to avoid potential thickness effect due to possible inhomogeneity in the wafer. If your sample is diluted and you get a very low absorption step, do not try to make the wafer thicker hoping that you will get better spectra – you will not: The optimal thickness gets the best signal-to-noise ratio (it is in this sense it is optimal). You can only try to measure your absorption spectra with another registration technique: in fluorescence or electron yield modes.

Calculation of thickness and absorption step for samples with known density

Here you can calculate the thickness of the sample with known density (usually, a foil). Commercial foils are highly homogeneous in thickness, so that you may ignore large step jumps and pay attention to the total μd only.

calculation of thickness and absorption step for samples with known density

Calculation of gas pressure for ionization chambers

calculation of gas pressure for ionization chambers

For nitrogen, do not forget the 2: N_2, not just N.
Start with the 2nd ionization chamber (IC). If a reference foil is placed between the 2nd and the 3rd IC, the fraction of x-rays absorbed by the 2nd IC is usually set to 50%. If the reference foil is not needed, one can select total absorption (100%). For these two cases the optimal absorption of the 1st IC at a certain μd is found from the following figures showing the levels of signal-to-noise ratio:
signal-to-noise ratio for 50%-absorbing IC2

signal-to-noise ratio for 50%-absorbing IC2

signal-to-noise ratio for 100%-absorbing IC2

Calculation of an unknown elemental concentration

Case 1: You know the composition of the matrix.
You need an absorption spectrum taken without the sample (empty spectrum) but with the same state of the ionization chambers. You then subtract it from the spectrum of the sample (e.g. in VIPER) and get a real (i.e. not shifted vertically) absorption coefficient. Determine the value of μd above the edge (μ_Td), the edge jump (Δμd) and its uncertainty (δμd). Specify the chemical formula with x.

calculation of an unknown elemental concentration

Case 2: You know the sample mass and area.
Determine the edge jump (Δμd). For the pure element find such a value for μ_Td that the absorption step in the pull-down list was equal to your experimental Δμd. This will give you the mass of the element of interest. Just divide it by the total mass to get the weight percentage.

Calculation of two unknown elemental concentrations

Here you also need empty spectra (for each of the two edges) to find the unshifted values for absorption coefficient above both edges.

calculation of two unknown elemental concentrations

Finding the scattering factors f''

If you need to know the scattering factor f'' at different energies and/or its jump at an edge (Δf''), XAFSmass provides a graphical tool for this.

For example, you may need these values to determine the composition of a binary compound if you have the experimental edge heights at two edges.
The absorption step Δμd at the absorption edge of energy E is proportional to Δf''ν/E, where ν is the amount of (resonantly) absorbing atoms in mole. Hence, the atomic ratio of two elements in the same sample is ν_A/ν_B = (Δμd)_A/(Δμd)_B ·[Δf''_A/Δf''_B · E_A/E_B]. For binary compounds A_xB_1-x the concentration x is calculated then as x = (ν_A/ν_B)/[1+(ν_A/ν_B )].

Citation: You are requested to cite XAFSmass as: "K. V. Klementiev, XAFSmass, freeware: www.desy.de/~klmn/xafsmass.html"

pay also your attention to my programs for EXAFS and XANES