Dear Claudia,
We think that restraining the DWF's might be a good approach. Unfortunately, the crystalline and amorphous systems do not have similar DWF's due to the large amount of disorder in the amorphous phase. From previous studies it is known, that the DWF of an amorphous semiconductor is roughly (!) twice the DWF of the crystalline phase. Hence, fixing the In-P DWF to twice the crystalline value (from a standard we also measured) would be a a first approach. My question is whether there maybe is a better way of relating/restraining the DWFs, as for example suggested by Crozier, Rehr and Ingalls (X-ray Absorption, Koningsberger and Prins, Wiley & Sons, 1988). They derive a formula for the DWF that contains the reduced mass and an integral over the projected density of states. If one could make a reasonable assumption about the integral (which is the problem) it would be possible to correlate the two DWF's. Or maybe there is another way to make an educated guess (by computing with FEFF or so).
If I might jump here, I think this question is closely related to the one from Ricardo Faccio earlier today, and other questions about standards and reference compounds we've seen recently. It is generally difficult to get accurate and meaningful amplitude (values for N and sigma2) from an a priori modeling of an EXAFS spectrum. There are many reasons for this, including experimental problems (that is, measuring the amplitude of chi(k) with high accuracy) and theoretical problems (that is, understanding with high accuracy all the physical terms other than N and sigma2 that influence the amplitude of chi(k)). It seems natural to want to improve the accuracy of N and sigma2 through "better modeling" including trying different k-weights for the fit. This can even work to a small extent, and it is impressive that it does work and that Artemis helps you do this easily. But it won't improve the accuracy of N and sigma2 by a lot. But since you bring up some older work on Debye-Waller Factors and are working on a classic EXAFS problem like site disorder in an amorphous semiconductor, I'd like to bring up a point that at one time was taught earlier on in EXAFS analysis: Absolute measures of N and sigma2 have much larger uncertainties than relative measures of N and sigma2. In the pre-FEFF days, very few people would measure a single spectra and expect (dare) to publish N and sigma2 for it. Instead, two spectra would be measured with similar instrumentation, analyzed together, and N and sigma2 would be compared. One common approach was to vary the temperature, and if there was no phase change, N could be assumed to be invariant, and you would extract the temperature dependence of sigma2 (within a constant offset). Another was to measure a well-characterized crystal (say, crystalline InP) and compare N and sigma2 between the two phases. Even with FEFF, these are good approaches. The drawback to these approach are that they're limited to single scattering and need a good reference sample and spectra with which to compare your unknown. But it would certainly be possible to analyze spectra of amorphous semiconductor in this way. The key is to look for relative differences. For sure, start by modeling data for crystalline InP to get some idea of the thermal component of sigma2 for the In-P bond. Then, if at all possible, measure InP at a few different low temperatures: All the variation between those spectra will be due to the thermal part of sigma2, not the static disorder, and not N. Once you get that far, you'll probably want to do a multi-data-set fit constraining N for In-In and In-P to be the same for all temperatures. Hope that helps, --Matt