At the XAS station X-ray absorption spectroscopic experiments in different measurement modes and with various sample environments will be possible.
Experimental Methods and Equipment
Transmission mode XAS
In this mode the X-ray photons passing through the sample are directly detected. For each photon energy E, selected via Bragg axis rotation of the DCM, the absorption coefficient µ is calculated from the logarithmic ratio of the incoming intensity I0 and the intensity It transmitted through the sample:
µ~ln [ I0 (E) / It (E) ]
I0 and It are measured with ionization chambers, and a third ionization chamber is used to measure the intensity transmitted through a reference sample (e.g., metal foil for energy calibration of the DCM). The gases and the pressure in the ionization chambers are optimized for the energy range of interest.
The transmission mode is the simplest and the most accurate method for studying thin samples (with thickness roughly about 1–2.5 absorption lengths) with a high concentration of the absorber element (>3-5%). The thickness of the sample should be uniform on the scale of an absorption length. Also, the sample should be as uniform as possible over the width of the beam, without “pinholes”.
Fluorescence mode XAS
In this mode the intensity If of a selected X-ray emission line of the element of interest is measured by a fluorescence detector while scanning the incoming X-ray energy with the DCM. To minimize elastic scattering the detector is mounted perpendicularly to the incoming beam. Absorption spectra are then obtained by dividing If by the incoming intensity I0 :
µ~ If (E) / I0 (E)
Since the concentration sensitivity in fluorescence mode is around 10−4% this measurement mode is chosen for diluted systems and biological samples, preferably with absorber concentrations below 3–5%.
Measurements in Total Electron Yield and Conversion Electron Yield mode.
Standard XAS measurements in transmission or fluorescence mode are bulk sensitive due to the high penetration depth of X-rays. Surface sensitivity can be achieved by detecting the electrons emitted from the surface of the sample or the electron current, i.e., Total Electron Yield (TEY). The probing depth can be changed and controlled by measuring Conversion Electron Yield (CEY), which is carried out in a gas environment, and the measured signal is due to electrons resulting from the ionization events initiated by high-energy electrons emitted from the sample. Transmission mode XAS measurements require uniform and sufficiently thin samples to transmit X-rays, whereas for TEY there are no limitations with respect to thickness and uniformity. Additionally, in TEY mode the signal intensity is higher because all electrons emitted from the sample, including photoelectrons, Auger electrons and secondary electrons are collected. However, TEY detection mode is limited to the conductive samples. Surface sensitive measurements using XAS in TEY/CEY mode are very important in nanotechnology (multilayers, nanoparticles, nanowires, etc.) as well as for projects with industrial partners in the field of coating and adhesive technology.
During commissioning and the first weeks of user operation, XAS measurements will be carried out under ambient conditions in transmission mode. Afterwards, equipment for various in situ and operando experiments will be implemented. Furthermore, combinations of XAS with complementary techniques (e.g., FTIR and XRD) are planned.
Please contact with beamline team in order to clarify the realization status of these upgrades.
In the process of commissioning the beamline team will develop and upload instructions for users how to prepare samples, taking into account their different states (powders, thin films, solid metals, solutions and liquids) and concentration of the element of interest.
- 3d metal foils (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se)
- 4d metal foils (Zr, Nb, Mo, Ru, Pd, Ag, Sn, Sb)
- 5d metal foils (Ta, Pt, Au, Pb)
On this page the beamline team will create a database of the XANES/EXAFS spectra measured during the commissioning of the beamline.
Recommended literature for new passionates
- XAFS for Everyone, Scott Calvin;
- X-ray absorption fine structure determination of pH-dependent U-bacterial cell wall interactions, Shelly Kelly;
- Fundamentals of XAFS, Matthew Newville;
- Industrial use of synchrotron radiation: love at second sight, Josef Hormes.