DiffSphere¶

Description¶

Summary¶

This fitting function models the dynamics structure factor of a particle undergoing continuous diffusion but confined to a spherical volume. According to Volino and Dianoux 1,

$S(Q,E\equiv \hbar \omega) = A_{0,0}(Q\cdot R) \delta (\omega) + \frac{1}{\pi} \sum_{l=1}^{N-1} (2l+1) A_{n,l} (Q\cdot R) \frac{x_{n,l}^2 D/R^2}{[x_{n,l}^2 D/R^2]^21+\omega^2}$

$A_{n,l} = \frac{6x_{n,l}^2}{x_{n,l}^2-l(l+1)} [\frac{QRj_{l+1}(QR) - lj_l(QR)}{(QR)^2 - x_{n,l}^2}]^2$

Because of the spherical symmetry of the problem, the structure factor is expressed in terms of the $j_l(z)$ spherical Bessel functions. Furthermore, the requirement that no particle flux can escape the sphere leads to the following boundary condition2:

$\frac{d}{dr}j_l(rx_{n,l}/R)|_{r=R}=0 \,\,\,\, \forall l$

The roots of this set of equations are the numerical coefficients $x_{n,l}$ .

The fit function DiffSphere has an elastic part, modelled by fitting function ElasticDiffSphere and an inelastic part, modelled by InelasticDiffSphere.

When using InelasticDiffSphere, he value of Q can be obained either though the Q attribute or can be calucated from the input workspace using the WorkspaceIndex property. The value calculated using the workspace is used whenever the Q attibute is empty.

Attributes (non-fitting parameters)¶

Name	Type	Default	Description
NumDeriv
Q

Properties (fitting parameters)¶

Name	Default	Description
f0.Height	1.0	Scaling factor to be applied to the resolution.
f0.Centre	0.0	Shift along the x-axis to be applied to the resolution.
f0.Radius	2.0	Sphere radius
Intensity	1.0	scaling factor
Radius	2.0	Sphere radius, in Angstroms
Diffusion	0.05	Diffusion coefficient, in units of A^2THz, if energy in meV, or A^2PHz if energy in ueV
Shift	0.0	Shift in domain

Categories: FitFunctions | QuasiElastic

Source¶

C++ source: DiffSphere.cpp

C++ header: DiffSphere.h