Code for the Analysis of ReflEXAFS data (CARD)

Presentation

CARD is a new code for the analysis of ReflEXAFS data. The code is the result of a collaboration of the LISA beamline, the Physics Department of the Universita' di Tor Vergata and the ESRF Data Analysis Unit. The conceptual bases of the procedure can be found in the paper F. Benzi et al. Rev. Sci. Instrum 79, 103902 (2008) and in this pdf presentation by F. Benzi. The code runs under GNU/LINUX (tested under Debian-Ubuntu) and is written in python (python 2.6 release). Its presently most advanced version (CARD_04) possesses a complete Graphical User Interface (GUI) for the data treatment and uses the PPM code for the reflectivity calculation and fit. The picture below shows a typical screenshot of the program.

You can contact Francesco d'Acapito for details on this project.

Working Principle

It is well known that ReflEXAFS data present oscillations that are not simply related to those of the absoprtion coefficient μ . A detailed discussion of this aspect can be found in Heald et al. PRB 38 (1988), 1016 paper. CARD creates modified theoretical EXAFS paths in order to realize the fit of the reflectivity function R(k) in k space. To do so, the following experimental data are needed:

The reflectivity at fixed energy E and variable angle φ, R_E(φ). A good practice is to take it at two different energies above and below the edge and well far from it.
The reflectivity at fixed angle φ and variable energy E, R_φ(E). This is the real Reflexafs dataset and depending on the value of φ respect to the critical angle φ_c the penetration in the material can vary considerably.

Then CARD enters into play. It carries out the following operations:

It creates a model for the layer structure of the sample by fitting the R_E(φ) data. In this way the roughness σ , the electron density δ and the thickness t of the layers of the sample are determined.

With these data it generates the theoretical refraction index of our sample in terms of δ(E) and β(E). These functions are used to generate the R_φ(E) at the experimental data collection angle φ.

The theoretical paths generated with an external code (the present versions all work with Feff but it could be adapted to other codes) are summed to the β(E) function and an oscillating reflectivity R_φ(E) is obtained.

from this function a modified theoretical exafs path is obtained by background subtraction and reduction to amplitude and phase form.

The modified theoretical paths are successively used with IFEFFIT for the quantitative data analysis.

Installation

The latest version with a full Graphic User Interface is CARD_04 and it can be dowloaded here CARD_04. Copy it your personal directory, unzip and un-tar it. Have a close look to the README file and proceed with the installation.

A complete and detailed hands-on example can be found in the DEMO directory. Here you will find some experimental files as well as a detailed guide on how use CARD.

Acknowledgements

We thank here all the people who have contributed to the developmento of CARD:

Mauro Rovezzi (ESRF), Elia Chinchio (ESRF), Claudio Ferrero (ESRF), Alessandro Mirone (ESRF), Ivan Davoli (Univ. Tor Vergata Roma), Paolo Ghigna (UNiv. Pavia), Sonia Pin (SLS).

Last Revised 13 Jan 2012, Francesco d'Acapito.