[Ifeffit] Fitting Commercial Molybdenum Disulfide
Michael A Groves
dingoboy at u.washington.edu
Thu Nov 4 14:29:09 CST 2004
Dan,
I too have had problems with positive sigma-squared values. I think it may
indicate a deeper problem with your fit. Two things I can see that might be
causing it are:
1. How did you come up with the value for amp (0.7) If your amp is too small,
your debye-waller factors get smaller, and sometimes even become negative to
compensate. One way to check this is fitting in different k-weights. If your
ss variables stay relatively constant at different k-weights, your amp value is
probably not the problem. If they change much, you might look at
http://leonardo.phys.washington.edu/~ravel/course/notes.pdf
page 37.
2. How about background subtraction? I don't know if Mo has a large white
line, but if it does, k=2 is probably too close to the edge for the background
subtraction to be trusted. Trying a larger k will get you away from the range
where the background subtraction is questionable.
Good luck,
Mike
On Thu, 4 Nov 2004, Kelly, Shelly D. wrote:
> Hi Dan,
>
> Sigma2 can not be negative. It is the mean square displacement of the
> half path length. You can force it to be positive by using abs(ss) on
> the definition page for each path in the fit.
>
>
> Shelly
>
>> -----Original Message-----
>> From: dmc at pdx.edu [mailto:dmc at pdx.edu]
>> Sent: Thursday, November 04, 2004 10:19 AM
>> To: XAFS Analysis using Ifeffit
>> Subject: Re: [Ifeffit] Fitting Commercial Molybdenum Disulfide
>>
>> Hi List,
>>
>> Here's my most recent data. I was able to look at the individual
>> paths
>> and changed my R-range to 2-4.5 angstroms on the first set of data, I
> set
>> the
>> second set of data to the same range, although 4 works better as a
> maximum
>> for
>> that set. The k-range on the second range had to be truncated though.
>>
>> Six paths are used for each fit, same parameters, except for amp (0.7
> for
>> one
>> ~0.8 for the other). The seperate data sets still have a few
> parameters
>> that
>> are outside of the error bars of the other (ss2,ss3,ss4,alpha,
>> alpha2,alpha5).
>> What can I do to get these beamlines to agree?
>>
>> delr_# define alpha# * reff
>> all other parameters guessed.
>> ss for each path, only one e0 for all
>>
>> Thanks for the help, Dan
>>
>>
>> First Data set -- Full k-range works best, Full range used for fit.
>> Project title : Fitting merge 10_15_04.chi
>>
>> Guess parameters +/- uncertainties (initial guess):
>> e0 = 0.8686930 +/- 1.1038290 (guessed as 0.868694
>> (1.103841))
>> ss = 0.0041150 +/- 0.0004990 (guessed as 0.004115
>> (0.000499))
>> ss2 = 0.0044360 +/- 0.0004450 (guessed as 0.004436
>> (0.000445))
>> ss3 = 0.0243560 +/- 0.0201400 (guessed as 0.024368
>> (0.020159))
>> ss4 = -0.0065470 +/- 0.0047820 (guessed as -0.006545
>> (0.004786))
>> ss5 = -0.0016100 +/- 0.0050490 (guessed as -0.001611
>> (0.005047))
>> ss6 = 0.0059980 +/- 0.0139390 (guessed as 0.005996
>> (0.013935))
>> alpha = -0.0011360 +/- 0.0029380 (guessed as -0.001136
>> (0.002938))
>> alpha2 = 0.0071480 +/- 0.0019780 (guessed as 0.007148
>> (0.001978))
>> alpha3 = -0.0060900 +/- 0.0273580 (guessed as -0.006078
>> (0.027378))
>> alpha4 = -0.0198570 +/- 0.0154200 (guessed as -0.019851
>> (0.015430))
>> alpha5 = 0.1399050 +/- 0.0148530 (guessed as 0.139906
>> (0.014848))
>> alpha6 = 0.0199630 +/- 0.0268690 (guessed as 0.019961
>> (0.026862))
>>
>> Def parameters:
>> delr_1 = -0.0051820
>> delr_2 = 0.0326180
>> delr_3 = -0.0277930
>> delr_4 = -0.0906110
>> delr_5 = 0.6384270
>> delr_6 = 0.0910950
>>
>> Set parameters:
>> amp1 = .7
>>
>>
>> Correlations between variables:
>> e0 and alpha --> 0.8170
>> e0 and alpha2 --> 0.6726
>> ss and ss2 --> 0.6591
>> alpha and alpha2 --> 0.5516
>> alpha5 and alpha6 --> -0.4856
>> e0 and alpha3 --> 0.4083
>> alpha3 and alpha4 --> 0.3588
>> ss5 and ss6 --> -0.3581
>> alpha and alpha3 --> 0.3100
>> alpha2 and alpha3 --> 0.3045
>> ss2 and alpha6 --> 0.2745
>> e0 and ss3 --> 0.2649
>> ss and ss3 --> 0.2627
>> ss2 and ss3 --> 0.2626
>> ss6 and alpha5 --> -0.2510
>> All other correlations are below 0.25
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Second Data Set -- Trucated k-range (2-13.5)
>> Project title : Fitting merge 10_15_04.chi
>>
>> Independent points = 25.270507813
>> Number of variables = 14.000000000
>> Chi-square = 1202.598221935
>> Reduced Chi-square = 106.703108852
>> R-factor = 0.057229121
>> Measurement uncertainty (k) = 0.000827712
>> Measurement uncertainty (R) = 0.022111003
>> Number of data sets = 1.000000000
>>
>>
>> Guess parameters +/- uncertainties (initial guess):
>> amp = 0.7976070 +/- 0.1361170 (guessed as 0.776348
> (0.139898))
>> ss = 0.0037770 +/- 0.0015530 (guessed as 0.003589
>> (0.001617))
>> e0 = 0.5608430 +/- 1.8018380 (guessed as 0.642687
>> (1.861307))
>> alpha = -0.0061290 +/- 0.0049080 (guessed as -0.005966
>> (0.005078))
>> ss2 = 0.0029050 +/- 0.0009970 (guessed as 0.002686
>> (0.001078))
>> ss3 = 0.0075410 +/- 0.0094220 (guessed as 0.005992
>> (0.008858))
>> ss4 = -0.0110250 +/- 0.0035850 (guessed as -0.011776
>> (0.004653))
>> ss5 = -0.0042000 +/- 0.0078070 (guessed as -0.007610
>> (0.005250))
>> alpha2 = 0.0038110 +/- 0.0030450 (guessed as 0.003762
>> (0.003187))
>> alpha3 = -0.0037820 +/- 0.0145580 (guessed as 0.001200
>> (0.015976))
>> alpha4 = -0.0212550 +/- 0.0107080 (guessed as -0.011852
>> (0.014417))
>> alpha5 = 0.1151980 +/- 0.0236640 (guessed as 0.113430
>> (0.014973))
>> ss6 = 0.0028560 +/- 0.0166050 (guessed as -0.007250
>> (0.003624))
>> alpha6 = 0.0193530 +/- 0.0299640 (guessed as 0.001791
>> (0.008195))
>>
>> Def parameters:
>> delr_1 = -0.0279670
>> delr_2 = 0.0173920
>> delr_3 = -0.0172570
>> delr_4 = -0.0969910
>> delr_5 = 0.5256830
>> delr_6 = 0.0883130
>>
>>
>> Correlations between variables:
>> amp and ss --> 0.8382
>> e0 and alpha --> 0.8048
>> amp and ss2 --> 0.7964
>> e0 and alpha2 --> 0.7533
>> ss3 and alpha5 --> 0.7397
>> ss5 and alpha4 --> -0.6999
>> ss and ss2 --> 0.6591
>> alpha and alpha2 --> 0.6159
>> alpha5 and ss6 --> 0.6116
>> ss5 and alpha3 --> -0.5790
>> ss5 and alpha6 --> -0.5023
>> ss3 and ss6 --> 0.4804
>> alpha4 and alpha5 --> -0.4375
>> e0 and alpha3 --> 0.4083
>> ss4 and ss5 --> -0.4047
>> ss3 and alpha4 --> -0.3972
>> alpha5 and alpha6 --> 0.3827
>> alpha3 and alpha6 --> 0.3498
>> ss3 and ss5 --> -0.3278
>> amp and ss3 --> 0.3209
>> ss4 and ss6 --> 0.3181
>> alpha and alpha3 --> 0.3100
>> alpha2 and alpha3 --> 0.3045
>> ss4 and alpha3 --> 0.2948
>> amp and ss5 --> 0.2820
>> ss2 and alpha6 --> 0.2745
>> ss and ss3 --> 0.2627
>> ss2 and ss3 --> 0.2626
>> amp and alpha6 --> 0.2589
>> alpha2 and alpha5 --> 0.2581
>> All other correlations are below 0.25
>>
>>
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