On Friday 08 October 2010 09:30:14 am Jatinkumar Rana wrote:

In EXAFS, the oscillations are due to the intereference between the outgoing photo electron from absorbing atom and incoming backscattered photoelectron from the scatterer. Depending upon, how much they are out of phase w.r.t each other we get oscillations in EXAFS curve. These oscillations are nothing but the variation in the absobtion coefficient of sample as the energy of incident photon is varied.

I wonder, how, the intereference between the photo electrons is related to absorbtion coefficient of sample ??

because, It = I0 * exp (-ut)

As always, it is important to consider the physical meaning of the terms we use. \mu is the absorption coefficient, i.e. the thing that goes up at the edge and up and down throughout the EXAFS. Another way to say this is that \mu is some kind of measure of the amplitude of the unoccupied (recall that electrons are fermions -- if an electron already occupies a particular statem, the photoelectron cannot transition into that state) portion of the density of states projected onto the final state angular momentum. That's a mouthful. In short, for a K edge, we measure the unoccupied portion of the p band. That portion of the density of states is not flat. It has structure -- peaks and troughs. A peak is place where there is higher state density and vice versa for a trough. Consequently, if the incident photon has the amount of energy needed to raise the photoelectron to an energy at which there is a peak in the density of states, then it is relatively more likely to be absorbed than for a photon with an energy that takes the photoelectron to a trough. The wiggles in the XAS follow the ups and downs of the density of states. Now suppose that one were interested in calculating an XAS spectrum. Well, there are many theoretical frameworks for making such a calculation. Feff (and, as a consequence, Ifeffit and Artemis) use an approach called "real space multiple scattering". In this approach we need to know two things -- the function that describes how an electron travels between points in space and the function that describes how an electron scatters off of an atom. Putting these two functions together, we can now describe how an electon leaves the point in space occupied by the absorbing atom, travels to a neighbor, scatters off that neighbor, and continues traveling. We are interested in computing the absorption at a particular atom. The photoelecton starts in the deep core of the absorber and is promoted to a higher lying state OF THE ABSORBER. Thus, the thing that is relevant is to compute the density of states OF THE ABSORBER. In the RSMS approach, the density of states is computed from the overlap of the wavefunctions of the outwardly propagating photoelectron with the functions of the various scattered waves. The part of the overlap that is relevant to computing the density AT THE ABSORBER is the bit that happens at the position OF THE ABSORBER. So, the interaction in question is "a deep core electron is promoted to an occupied state of the absorber". The computational tool used to compute that interaction is RSMS. Happily, the parameters of the RSMS approach (R, sigma^2, N) map readily onto the things that we want to know when we do an XAS experiment. Here are all the details, as implemented in Feff: http://rmp.aps.org/abstract/RMP/v72/i3/p621_1 It's a dense read but it's also an excellent and very rewarding paper. B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

Dear Jatin, I tried to answer both of these question in chapter 3 of http://xafs.org/Tutorials?action=AttachFile&do=view&target=Newville_xas_fundamentals.pdf Does the explanation there help? If not, do you have any suggestions for making it clearer? The basic physics is that an absorbed x-ray creates a photo-electron which will scatter from its neighbors. A portion of the scattered portion (so, a small amount) of that photo-electron will return to the absorbing atom. But Pauli's exclusion principle tells us that if an electron is at the absorbing atom, and has the same energy (and other quantum numbers) as the outgoing photo-electron, the outgoing photo-electron cannot be created in the first place. Thus, the scattering of the photo-electron from the neighbors affects whether the photo-electron can be created, and so whether the x-ray will be absorbed. The effect can be either destructive or constructive, as with nearly any wave-like phenomenon. It does seem a bit circular that the photo-electron scattering affects the existence of the photo-electron, but quantum mechanics leads to many much stranger behaviors. --Matt