Our quasielastic fitting utility, TANQENS, was written by Taner Yildirim. I shall give most of the details to use TANQENS here, but there may be something I have overlooked and you need to know what is going on, if so mail Taner.

The Initial Window

You can launch TANQENS in two ways, either type 'tanqens' if you have the alias defined or press the 'TANQENS' button after running TREAT.

Setting up

Looking at the file initially will help to choose sensible options for the fitting ranges and which detectors to include. A file is also dumped, 'sqw.xy', that contains the raw scattering functions for the treated detector. If you plan on using your own fitting/plotting software this is a useful file to have.

More Parameters can be used to force elastic peak position and background values to be within a specified range.

S(Q,w) can be defined at the detector or at given Q-values. If you choose 'detectors' the elastic peak Q values is used for further analysis.

Fitting Functions are currently limited to :

• A Gaussian and a Lorentzian.
• A Gaussian and two Lorentzians.
• A Gaussian and a stretched exponential.
• A Gaussian and two stretched exponentials.
• Isotropic rotational diffusion.
• You own model (still in progress..).

Start Fitting

If there are problems while fitting check youir numbers. It is easy to accidentally exclude detectors if the intensity window for fitting is not correctly set!

S(Q,w) The final fits (including contributions and difference curve) can be viewed using 'tanplot' (a tcl plotter that scrolls through the spectra) or XMGR (all the spectra are on one graph and while it looks strange you can see the data by zooming to the region of interest). Note that each S(Q,w) is scaled here and arbitrarily placed on an intensity scale.

EISF The EISF is defined as the fraction of the total quasielastic intensity contained in the elastic peak. The Q dependence of the EISF holds information on the characteristic model of dynamics in the system, such as jump distances and time scales.

FWHM The full-width at half-maximum for all peaks is plotted against Q using XMGR.

TAU The decay constant for any stretched exponential is plotted against (XMGR).

BETA The exponent for any stretched exponential is plotted against (XMGR).

<TAU> The integral of the stretched exponential function.

PLOT You can choose any of the output parameters to plot against each other by selecting their 'column numbers' that are listed at the start of each fitting cycle:
1= Q, 2= Peak Position (meV), 3= EISF, etc.