Ifeffit

Download

ifeffit@millenia.cars.aps.anl.gov

February 2005

13 participants
11 discussions

EXAFS sample prep
by Kelly, Shelly D. 02 Feb '05

02 Feb '05

Hi Ritika, I hope you do not mind, I'm sending this to the Ifeffit mailing list since others might be interested as well. This email message is about how to prepare an EXAFS sample. We are talking about measuring a Ti-edge from some sample in solution. But any edge and any sample in any matrix will also work just file. You want to set-up your sample so that you have a lot of Ti atoms in the path of the x-ray beam. This is because the EXAFS signal comes from the adsorption from the Ti atoms. Hence more Ti atoms give more adsorption events resulting in a smaller statistical error in the EXAFS spectra. BUT..... In transmission mode the EXAFS signal comes measuring the number of x-rays incident on the sample and transmitted through the sample. If all the x-rays are adsorbed by the Ti atoms in the sample then you do not have any transmitted x-ray signal. Hence you get really noise data because you have no statistically significantly transmitted x-ray signal. So you need your sample to be thin so that a lot of the x-rays pass through the sample such that you have something to measure. In fluorescence mode the EXAFS signal comes from the x-rays emitted by the Ti atoms as the core hole is re-filled. These emitted x-rays need to escape the sample without being re-adsorbed by the sample. Hence the sample needs to be dilute for fluorescence mode measurements. When you make a measurement in fluorescence mode it is always best to collect the transmitted x-ray signal as well. Then you can make a comparison to see if "self adsorption effects" are small. Transmission measurements typically have better signal to noise ratio than fluorescence measurements. So if you can, make your measurements in transmission mode. You can use Tkatoms or Artemis to calculate the best sample thickness and the best concentration to use for the measurements and which EXAFS mode will work best for your samples. >From the components of your sample you can calculate the thickness of the sample that will give 2.5 adsorption lengths. The transmitted x-ray intensity is given by the incident x-ray intensity multiplied by the exponential of the absorption coefficient times the sample thickness. I = I0 * exp(-mu(E) * x) Where I is the transmitted x-ray intensity I0 is the initial x-ray intensity Mu(E) is the absorption coefficient = weighted sum of absorption coefficient from each atom in the beam. X is the thickness of the sample. IF mu(E) * x = 2.5 then the percent of transmitted x-rays is about 10% of the incident x-ray intensity. This is just enough at NSLS where the incident x-ray intensity will be about 10^6 x-rays per second. This means that 10^4 x-rays per second will hit your transmitted Ion chamber. You want to make the transmitted ion chamber adsorb all (99%) of theses x-rays. Then your statistical error in the measurement will be 1/sqrt(N) or 1/sqrt(10^4) or 1%. You need to have at least this many x-rays to measure. At the beam line you measure the total absorption edge. The EXAFS signal, if you are lucky, makes up about 10% of the absorption edge. Hence you need to have a statistical error of no more than 1% to make a measurement. If you can make the sample thinner then you do even better. Now for the other problem: You want to get as many Ti atoms into the sample with a total sample thickness of 2.5 absorption lengths. To do this part you need to calculate the absorption of just the Ti atoms in the sample. I call this delta_mu(E). If you can get the delta_mu(E) * x to be between 0.1 and 1.0 with the total absorption of the sample below 2.5 then you can make the measurement in transmission mode. This means that between 10% and 70% of the absorption in the sample will be due to the Ti atoms. This is enough to get a good transmission signal. If you adsorption from the Ti atoms in the sample is between 1% and 10% then you will hope to make the measurement in fluorescence mode. Now for the practical part: Say you have a solution with 1% Ti-stuff in water by NUMBER of atoms. If you usually work by weight then you need to convert to number for this calculation. Take the feff.inp file for Ti-stuff. Look at the top of the file. You should find total_mu(E) (absorption of the entire sample at the Ti edge), and delta_mu(E) (absorption of Ti atoms at the Ti-edge). Then use Hephaestus to calculate the total_mu(E) for water at the Ti edge energy. To get the total absorption you need to calculate the weighted average for water and Ti-stuff. See how thick you can make your sample and satisfy the criteria for less than a total absorption length of 2.5. (99%_by_number_water_atoms * total_mu(E)_for_water + 1%_by_number_Ti-stuff * total_mu(E)_for_Ti-stuff ) * sample_thickness = 2.5 Once you have that thickness then you can see what the absorption for just the Ti atoms is. 1%_by_number_Ti-Stuff*delta_mu(E)_for_Ti * sample_thickness = absorption_length_for_Ti And then you can figure out if you have a fluorescence or transmission experiment. Absorption_length = 0.1 to 1.0 you have transmission. Absorption_length=0.01 to 0.1 you have fluorescence. Usually you have to go around and around with this sort of calculation to optimize the conditions for the measurements. Hope that Helps Shelly

1 0