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QENS Fitting of Ionic Liquid H2NC(dma)2

Graphs of HWHM for various temperatures in the four models considered.
Fitting results for room temperature ionic liquid H2NC(dma)2. The X axis represents Q values and the Y axis represents Half-Width Half-Max(HWHM) values. Open symbols are for the sequential fitting the narrow Lorentzian component (left panels), broad Lorentzian component (right panels), and the difference between the two HWHMs (middle panels). Dashed lines in the left panels (indistinguishable from the solid lines) are unconstrained translational jump diffusion model. Dashed lines in the middle panels are fits with the constrained diffusion model. Red dotted lines in the middle panels are fits with the unconstrained translational jump diffusion model. Solid dots/lines are similar, but for a global fitting across all Q values simultaneously.

Achievement

This work has demonstrated the use of the QClimax machine learning package for fitting of QENS experimental data.

Significance and Impact

The improvement of QClimax's machine learning global fits over classical models has been demonstrated via application to experimental data. 

Research Details

  • Fit ionic fluid experimental data with physical constraints on the model.
  • Compared the result to naive, unconstrained, local fittings to demonstrate the increased accuracy.

Overview

Modern Quasi-Elastic Neutron Scattering(QENS) spectrometers produce experimental data at high energy ranges that cannot be accurately described by a single component. Rather, the data is binned into discrete Q values, which are then fit by multiple components defined by formulas with a small number of variables. Classical models fit these bins on their own, which loses the dependence of the component parameters on Q. The QClimax software package, part of the Integrated Computational Environment for Modeling and Analysis of Neutrons (ICEMAN) workbench, avoids this by fitting all bins at once and allowing a functional constraint to be defined on parameters.

QClimax has been used in the creation of a model for the room temperature ionic liquid H2NC(dma)2 for comparison to the classical method of sequential fitting. In both cases, two Lorentzian components were used alongside a linear background component. A jump diffusion model is then imposed onto the Lorentzians' parameters. As demonstrated in the plots to the right, the imposition of the jump diffusion constraint within the global fitting produced a model that, while slightly less good at some individual Q values, was ultimately much better fit to the experimental data than the original jump diffusion, while avoiding the over-fitting caused by ignoring the parameter Q dependence, which cause physically incorrect results for the model in the sequential, unconstrained case.