Chapter(2(*( Methodology(
2.3 XHray+Sources+
2.4.4 Data+Collection+
XAS data is usually collected in one of two main experimental set-ups, transmission or fluorescence mode. On some beamlines it is possible to collect data in both transmission and fluorescence mode simultaneously. This can be advantageous, but is not available at all facilities; as such the techniques are all presented seperately here. Which set-up is used is dependant upon the concentration and nature of the material being studied.6 Presented here is a summary of the XAS techniques and the experimental set-ups.
2.4.4.1 Transmission+XAFS+Experiments+
Transmission mode is the most prevalent method for collecting XAS data. The transmission set up involves two detectors, one before and one after the sample with the change in the intensity being related to the sample (Figure 2-12).
Figure 2-12 - Diagram of typical XAS set-up for transmission.
For succesful transmission measurements there must be a significant change in the edge step, which is essentially the height of the edge jump (Equation 2-6); where µ is the absorption coefficient, t is the sample thickness, I0 and It are the initial intensity and the intensity of the X-ray beam and the intensity after passing through the sample respectively. The edge jump should ideally be a change of between 0.5-1.5 in µ(E).6 This is to minimise the signal to noise (S/N) ratio ensuring better quality data is collected. The S/N ratio can be improved by
2.4.4.2 Fluorescence+XAFS+Experiments+
In cases where it is not possible to collect transmission data, XAS spectra may also be collected from the fluorescence photons and electrons. The absorption coefficient for fluorescence is different to transmission (Equation 2-7);
! ! =!!
!! Equation 2-7
In this mode the fluorescence detector is positioned at 90o to the incident beam to minimise scatter and the sample is placed at a 45o angle to the incident beam to ensure maximum amount of fluorescence electrons are detected. Fluorescence signals are much lower than transmission chamber readings, and as a result there is proportionally more noise. However in cases where there is a dilute/low concentration sample or a sample that is too highly absorbing it can be more beneficial to record the XAS data in fluorescence mode, due to it having a better signal to noise ratio than the corresponding transmission data.
Figure 2-13 – Diagram of a typical XAS set-up for Fluorescence experiments.
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2.4.4.3 Dispersive+XAFS+Experiments+
In order to study time-dependant phenomena, short timescales are required. Despite the advances in data acquisition for transition mode EXAFS or QuXAFS, the speed of data collection is limited by the speed of the monochromator and the data acquisition process; as such the fast QuXAFS measurements take place over time-scales of seconds to minutes.
However some phenomena occur over much shorter timescales, and as such are undetectable by standard XAS techniques. One technique that can be used to probe such effects is Dispersive XAFS or DXAFS.
DXAFS is usually performed in transition mode using polychromatic radiation. The sample is irradiated by a number of beams, which are then detected using a position sensitive detector.
This allows for the collection of a XAS pattern without any mechanical movement from the monochromator, which means that provided the flux is high enough and the data acquisition is fast, the study of time-dependant phenomena is possible.
No DXAFS experiments were performed over the course of the thesis work, however it is important to understand the techniques available and the benefits in order to be certain that the right analytical technique is chosen to answer the experimental question.
Figure 2-14 - Diagram of Dispersive XAFS experimental setup.
Synchrotron!
2.4.4.4 Electron+Yield+XAFS+Experiments+
Electron Yield (EY) - XAFS measurements can be used to study the surfaces of materials.
Again no EY-XAFS measurements were taken during this thesis work; this section is added in order to show the various techniques available. The detection method is similar to fluorescence measurements, in that it indirectly measures the absorption by detecting the decay products as the core-hole is refilled. The electrons detected are mainly the auger electrons ejected from near the surface of the material. The surface sensitive nature of the technique makes this method suitable for studying near surface phenomenon; it can also be useful for avoiding self-absorption effects that can occur when measuring in fluorescence.
The set-up for electron yield measurements is different compared to fluorescence and transmission as the sample is essentially inside the detector (Figure 2-15). For total electron-yield measurements the sample is kept under ultra-high vacuum, and the sample is irradiated by soft X-rays. For higher-energy measurements, known as conversion electron detection, the electrons that are emitted by the sample collide with a gas within the detector such as helium gas. This produces secondary electrons that can be collected as in standard detector ion chambers.
Figure 2-15 - Diagram of a typical XAS set-up for Electron Yield XAS experiments
2.4.4.5 InHsitu+cells+
The work undertaken in this thesis project involves a lot of in situ characterisation under a variety of conditions such as heating at high temperatures, under reducing atmosphere, steaming etc. Additionally whilst most experiments will involve solid samples, a variety of different phases may also be considered such as solids, solutions and gels. These cannot be mounted in simple sample holders but require specially designed cells.
I0
A variety of different cells have been designed and used by the Sankar research group over the course of many years and these can all be utilised for a wide number of in situ experiments under different conditions (Figure 2-16).
Figure 2-16 - Example of one of the in situ cells designed and used by the Sankar research group.
This is a furnace cell designed to mount 13mm pellets and heat them to temperatures of near 1000oC. This cell was used for the experiments in chapter 6.