orisel
Introduced in 0.99
Synopsis

Compute orientation selectivity weights for ENDOR and ESEEM simulations of disordered systems

Weights = orisel(Sys,Par)
Weights = orisel(Sys,Par,Opt)
orisel(...)
Description

This function calculates weights for orientation selectivity in powders and frozen solutions. At a given magnetic field and spectrometer frequency not all, but only centres with certain orientations relative to the external magnetic field are excited. orisel determines which orientations are excited and how strongly.

Weights is a vector with the selectivity weights for a set of orientations. The larger the weight, the stronger the selectivity. The weights are not normalized. The maximum values for resonant excitation of a transition depend on the number of spins and the g tensor(s). E.g., for an S=1/2 spin system with an isotropic g factor, a value of 1231 results if the magnetic field is centered on the EPR line.

The orientations are computed internally, but are not returned by orisel. To obtain the orientations associated with the weights, call sphgrid with the symmetry and number of knots given in the command window output of orisel.

These weights can directly be used in the options structure in salt.

If the function is called with no output argument (last syntax above), the weights are plotted as a function of orientation.

Sys is a spin system structure. Be sure to include the EPR line width in the field HStrain, since it is used in the selectivity computation.

Par is the parameter structure containing the following mandatory fields.

FieldMagnetic field in mT.
mwFreqSpectrometer frequency in GHz.

In addition, there are a few optional parameters.

ExciteWidthFWHM bandwidth of the Gaussian excitation profile, in MHz. If not specified, it is assumed to be zero.
Orientations2xn vector [phi;theta] giving the polar angles that define the orientations. This is an optional field. If not given, orientations are computed internally using sphgrid with the values of Symemtry and nKnots given in the Options structure.

Opt is a structure containing optional settings

nKnotsNumber of orientations, as in pepper (optional, default 46).
SymmetryPoint-group symmetry of the spin Hamiltonian, as in pepper. If not given, the symmetry is determined automatically (see symm).

The total effective excitation bandwidth is the combination of Par.ExciteWidth and the anisotropic EPR line width Sys.HStrain. Strains (gStrain, AStrain and DStrain) are not included in the computation.

Examples

Orientation selection in the spin system

Sys = struct('S',1/2,'g',[2.1 2.2 2],'HStrain',[10 10 10]);
Sys = nucspinadd(Sys,'1H',[5 4 6]*1e2);

can be computed using

Exp = struct('mwFreq',9.5,'Field',325,'ExciteWidth',100);
Weights = orisel(Sys,Exp);

The orientation selection can easily be visualised by omitting the output argument

orisel(Sys,Exp);

If the full orientational sphere should be plotted, set the symmetry manually to C1

Opt = struct('Symmetry','C1');
orisel(Sys,Exp,Opt);
See also

salt, sham