Greetings!
I have inherited a vanadium EXAFS data set (due to a group member
graduating - lucky dog!) that is giving me major fits. I am getting
consistent numbers with all my fits which visually look fantastic, that
is until I look at the numbers...one Debye-Waller factor is negative (a
V-O path) and the Eo's are large, the V-Cl path is on the order of -20
eV, the V=O path is on the order of 17 eV, and the V-O path falling at
-11 eV. ARe there any "tricks" outside of restraints or constraints
that I can attempt to get more realistic values? (I understand that a
large Eo can be realistic, but a negative Debye-Waller factor doesn't
make sense.) I can restrain the V-O Debye-Waller factor and get a
positive value, but then the V=O D-W factor is negative. If I restrain
both D-W factors, the fit falls apart, suggesting something else is
wrong that I just can't find.
Another issue I'm having involves something in Artemis that's confusing
me. Sometimes after a fit, the Artemis Pallet shows the V=O feature as
the one at 1.8 Angstroms and the V-O feature at 1.6 Angstroms,
completely reversed to the FEFF model. I have seen this when I
separated the chlorides as well (I attempted to model the Cl at
different distances as reported by Deguns, et. al. (attached) due to the
immediate environment around the V in our sample being similar to the
one they report in scheme 1-(I), p5008). Attached is a text file that
shows an Artemis Pallet output illustrating what I'm trying to describe
for the V=O and V-O paths. The Artemis project that generated the
output is also attached.
At this point, I'm wandering what I'm missing. Any help is greatly
appreciated!!
Many thanks in advance!
Rich Mayes
Barnes Group
450/452 Buehler Hall
Dept. of Chemistry
Univ. of Tennessee
Knoxville, TN 37996
Project title : Fitting SA_162deglitch-merge.chi
Comment : Fit #1
Prepared by :
Contact :
Started : 15:46:44 on 17 April, 2007
This fit at : 15:58:01 on 18 April, 2007
Environment : Artemis 0.8.007 using Windows XP, perl 5.006001, Tk 800.024, and Ifeffit 1.2.9
Figure of merit : 3
============================================================
Independent points = 14.027343750
Number of variables = 9.000000000
Chi-square = 45.640992098
Reduced Chi-square = 9.078550099
R-factor = 0.012557807
Measurement uncertainty (k) = 0.000687372
Measurement uncertainty (R) = 0.023895029
Number of data sets = 1.000000000
Guess parameters +/- uncertainties (initial guess):
enot = 17.1999060 +/- 3.4058600 (0.0000)
delr = 0.1588460 +/- 0.0081480 (0.0000)
ss = -0.0022720 +/- 0.0007180 (0.0030)
enot_O2 = -11.0384940 +/- 9.8688920 (guessed as enot)
delr_O2 = -0.2214820 +/- 0.0356500 (0.0000)
ss_O2 = 0.0027560 +/- 0.0021290 (0.0030)
enot_Cl = -16.0026700 +/- 3.5982940 (guessed as enot)
delr_Cl = -0.0118230 +/- 0.0104780 (0.0000)
ss_Cl = 0.0016470 +/- 0.0004220 (0.0030)
Set parameters:
amp = 0.93
CN_O1 = 1
CN_O2 = 1
CN_Cl = 2
Correlations between variables:
enot_Cl and delr_Cl --> 0.9479
enot_O2 and delr_O2 --> 0.9104
enot and enot_Cl --> 0.8766
enot and enot_O2 --> -0.8755
ss and delr_Cl --> 0.8737
delr_O2 and enot_Cl --> -0.8655
ss and enot_Cl --> 0.8469
enot_O2 and enot_Cl --> -0.8220
ss and delr_O2 --> -0.8025
enot and delr_Cl --> 0.7864
enot and delr_O2 --> -0.7825
delr_O2 and delr_Cl --> -0.7817
enot_O2 and ss_O2 --> 0.7535
delr_O2 and ss_O2 --> 0.7259
enot_O2 and delr_Cl --> -0.7080
ss and enot_O2 --> -0.6872
enot and ss --> 0.6795
delr_O2 and ss_Cl --> -0.6271
delr and ss_Cl --> -0.6217
ss_O2 and enot_Cl --> -0.6173
ss and ss_Cl --> 0.5740
ss_O2 and ss_Cl --> -0.5640
enot and ss_O2 --> -0.5314
enot_Cl and ss_Cl --> 0.5264
ss and ss_O2 --> -0.5084
enot_O2 and ss_Cl --> -0.4750
ss_O2 and delr_Cl --> -0.4747
delr_Cl and ss_Cl --> 0.4701
delr and ss_O2 --> 0.4134
enot and delr --> 0.3090
enot and ss_Cl --> 0.2693
All other correlations are below 0.25
===== Data set >>SA 162deglitch-merge<< ========================================
file: C:\Program Files\Ifeffit\horae\stash\artemis.project.2\chi_data\SA_162deglitch-merge.chi
title lines:
Self absorption correction of 162deglitch-merge using method of Troger et al.
+ V K edge
+ Formula=H24.75 C8.25 O21.48 Si12.125 Cl4.7 V1.48
Merge in e space of:
** D:/200600603/Barnes-lab/EXAFS/NSLS/EXAFS/MYL-JULY-2006/all-VOCl3/162-Me2SiCl2+V5/162.001
** D:/200600603/Barnes-lab/EXAFS/NSLS/EXAFS/MYL-JULY-2006/all-VOCl3/162-Me2SiCl2+V5/162.002
** D:/200600603/Barnes-lab/EXAFS/NSLS/EXAFS/MYL-JULY-2006/all-VOCl3/162-Me2SiCl2+V5/162.003
k-range = 2.388 - 15.000
dk = 2.000
k-window = hanning
k-weight = 2
R-range = 1.200 - 3.000
dR = 0.200
R-window = hanning
fitting space = R
background function = none
phase correction = none
R-factor for this data set = 0.01256
===== Paths used to fit SA 162deglitch-merge
FEFF1: Path 1: [O] ..
feff = C:\Program Files\Ifeffit\horae\stash\artemis.project.2\data0.feff1\feff0001.dat
id = reff= 1.6819, nlegs= 2, path= V<->O
label =
r = 1.840746
degen = 1.000000
s02 = 0.930000
e0 = 17.199906
dr = 0.158846
ss2 = -0.002272
3rd = 0.000000
4th = 0.000000
ei = 0.000000
FEFF1: Path 2: [O] ..
feff = C:\Program Files\Ifeffit\horae\stash\artemis.project.2\data0.feff1\feff0002.dat
id = reff= 1.8269, nlegs= 2, path= V<->O
label =
r = 1.605418
degen = 1.000000
s02 = 0.930000
e0 = -11.038494
dr = -0.221482
ss2 = 0.002756
3rd = 0.000000
4th = 0.000000
ei = 0.000000
FEFF1: Path 3: [Cl] ..
feff = C:\Program Files\Ifeffit\horae\stash\artemis.project.2\data0.feff1\feff0003.dat
id = reff= 2.1121, nlegs= 2, path= V<->Cl
label =
r = 2.100277
degen = 1.000000
s02 = 1.860000
e0 = -16.002670
dr = -0.011823
ss2 = 0.001647
3rd = 0.000000
4th = 0.000000
ei = 0.000000
