Data Interchange

A single XAS spectrum is a very useful unit of currency for a number of reasons:

• We send data to and receive data from our collaborators
• We publish individual spectra in journal articles
• We extract XAS spectra from large, complex, multi-spectral data sets
• We write web-based and desktop applications (for instance, a standards database) that traffic in single spectra
• We write data analysis software that needs to reliably import data that comes from many sources.

Conventional XAS

1. In a conventional XAS experiment, we measure a sample somewhere between 2 and 10,000 scans, possibly requiring dead-time or other corrections
2. Some data processing happens to correct, calibrate, and/or align the data
3. Those scans are then merged into a single spectrum that becomes our unit of currency

A data interchange standard is about how we express the merged spectrum (i.e. the blue one).

Fancy experiments

Here are a few example of elaborate experiments that end up producing XAS spectra as μ(E) or χ(k).

Imaging experiments

In an imaging experiments the heterogeneity of our samples is measured. XAS can be measured on particular spots.

Tappero et al, New Phytologist 175:4, 641-654, (2007) doi:10.1111/j.1469-8137.2007.02134.x

Difraction anomalous fine structure (DAFS)

An anomalous scattering experiments yields energy-dependent scattering intensities. These are converted into χ(k) spectra and interpreted as position selective EXAFS.

Ravel et al. PRB 60, 778-785 (1999) doi:10.1103/PhysRevB.60.778

Non-resonant inelastic scattering

A NIXS experiment can used to measure a XANES spectrum in an X-ray energy loss channel.

Bergmann, et al. Chem. Phys. Lett. 369 184 (2003) doi:10.1016/S0009-2614(02)02003-1

Problems with beamline formats

• They require additional processing in order to display μ(E), including
  • Conversion to energy
  • Dead-time or other corrections
• Ambiguous metadata, for instance
  • How is the beamline identified?
  • What consitutes a user comment?
  • What describes the condition of the source or the beamline?
• XAS data analysis software and other plotting software may have difficulty importing and interpreting the data
• This data is probably not appropriate for submission to a journal as supplemental material

Goals of a data interchange format

1. Establish a common language for transferring data between XAS experimenters, data analysis packages, web applications, journals and anything else that needs to process XAS data.

2. Increase the relevance and longevity of experimental data by reducing the amount of data archeology future interpretations of that data will require.

3. Enhance the user experience by promoting inter-operability among data acquisition systems, data analysis packages, and other applications.

4. Provide a mechanism for extracting and preserving a single XAS or XAS-like data set from a related experiment or from a complex data structure.