Re: [Ifeffit] XANES - relative peak energies

Thanks Josh and everyone else for the useful information! Here's another question: How precise should the relative peak energies be compared to experiment? If they don't match up, are there any parameters I can change that might stretch the spectrum or shift peaks relative to one another? Thanks Ross On 09/17/13, Joshua Kas wrote:

Hi Ross, FEFF calculates the Edge energy, as well as the Fermi level. The edge energy is accurate to about 1%, which means 1 to 10 eV for most edges and systems. Feff also calculates the Fermi level, but does not give a good estimate of the ionization potential.

Also, in reference to your first question, as Bruce suggested, the LDOS is a good quantity to look at. If a peak shows up strongly in the Ca and the O LDOS, but only weakly in the C LDOS, that is an indication that the peak is due to hybridization (bonding) between the Ca and O states.

Cheers, Josh

...

What about ionization potential? Does FEFF calculate it, or is there a way to extract it from the spectra?

Thanks Ross

On 09/13/13, Bruce Ravel wrote:

...

On 09/13/2013 03:25 PM, Ross Devol wrote:

...

Does anyone know whether FEFF can be used to assign peaks? In other words, is there a way to tell which peaks in a calcite O K-edge spectrum are due to CO bonds and which to CaO bonds? Also, does FEFF calculate the ionization potential and if so, where does it output this?

Feff is a real-space multiple-scattering code. From the RSMS, your question may not make a lot of sense. The XAS spectrum is a measure of how the photoelectron interacts with its surroundings. In general, peaks in the spectrum have something to do with all of the surroundings.

One way to get a handle, in some sense, on your question is to run Feff with successively larger values of the FMS parameter. That is, see what features appear or change in the calculation as you increase the radius of the cluster considered in the FMS calculation.

Also of interest will be the partial densities of state as a function of cluster size. That gives some hints of how Feff sees bonding in your cluster.

HTH, B

-- Bruce Ravel ------------------------------------ bravel@bnl.gov

National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973

Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

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Message: 2 Date: Tue, 17 Sep 2013 11:50:55 -0400 From: Bruce Ravel To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] XANES - Peak assignment and ionization potential with FEFF Message-ID: <52387A5F.8@bnl.gov> Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 09/17/2013 01:42 AM, Ross Devol wrote:

...

What about ionization potential? Does FEFF calculate it, or is there a way to extract it from the spectra?

Are you asking about edge energy on an absolute energy scale? Some measure of that can be found in xmu.dat, but I am not sure what the current state of Feff's ability to calculate that reliably is. Perhaps John, Kevin, or Josh might comment on that...?

B

-- Bruce Ravel ------------------------------------ bravel@bnl.gov

National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973

Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel

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