I have to perform a high pressure X-ray absorption measurements on bismuth based sample at APS synchrotron beam line. I don't know how to load the sample inside the diamond anvil cell for XAS measurements.
X-ray absorption (XAS) measurement in diamond anvil cell (DAC) are similar to standard absorption measurements in the NIR-VIS-UV range. Direct (I_0) and sample transmitted (I_T) spectra are measured in the range of the K or L edge of the element to be studied. Obviously, as in optical measurements, the pressure transmitting medium must be transparent to the beam. Most pressure transmitting media used in the visible range can be used in XAS measurements (methanol-ethanol, silicone oil, etc).In this sense, a standard charge for optical range absorption measurements should serve for XAS measurements. Nevertheless, there are specific requirements to get reliable XANES and EXAFS spectra.
1) The sample must be VERY HOMOGENEOUS both in composition and thickness.
2) The sample size perpendicular to the beam must be much larger than the beam focus size, to guarantee that the transmitted spectrum does not contain any contribution from the focused beam tails do not cross the sample. Conditions 1 and 2 can be checked by verifying that the transmitted spectrum does not change when the cell is moved 5-10 microns across the beam.
3) The simple thickness must be such that the simple trasmittance at the X-ray edge is about 10%.
4) If your diamond anvils are standard single crystals the transmitted spectrum will always contain glitches arising from X-ray diffraction of the beam by the diamond anvils, especially if the absorption edge energies are larger than 10 keV (which is the case for bismuth K and L edges). For lower X-ray energies it is normally possible to find energy intervals without glitches by rotating the DAC while observing how the glitches move out or in the spectral range of XAS spectrum. For larger edge energies nanocrystalline diamond anvils must be used.
Reply to Alfredo Segura @ Dear sir, thank you very much for your answer.
Sir, I need one more clarification. Suppose if I want to load the sample for one absorption length (inside the DAC), then is there any relation between the sample volume and thickness of the sample chamber (gasket hole).
Dear Dr. Yesudhas,
Once you have determined the optimum sample thickness (from the composition and the spectral range to explore) and the minimum sample cross size (from the beam focus size), XAS experiments do not impose any specific conditions to the gasket hole size, beyond the general conditions common to any DAC experiment. Given the anvil culet size, the optimum gasket hole size is fixed by the pressure range you intend to explore, the hydrostaticity conditions you intend to maintain, the compressibility of your pressure transmitting medium...
Reply to Alfredo Segura. Thanks a lot sir.
Sir one of my doubts has not yet been clarified.
How to ensure the thickness and cross section inside the DAC. For example, if I want to load the sample with thickness and cross section say 30 micron by 30 micron respectively. How do I can precisely load this much thickness and cross section inside the DAC (for absorption measurements). Can I load the above size roughly.
Dear Dr. Yesudhas,
I am afraid I do not quite understand the sense of your doubt concerning the sample size and thickness. You will need a microscope in order to measure the sample size and thickness. The cross size is not critical (provided it is larger than the total X-Ray beam focus size). The thickness is far more critical. If the sample is in powder form, you will have to prepare a pellet and verify that it is homogeneous in composition and thickness. I would not advice you to try any XAS experiment with a non compacted powder because the results will be unreliable.