orisel

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

Syntax

orisel(Sys,Par)
orisel(Sys,Par,Opt)
Weights = ...
[Weights,Trans] = ...

Description

This function calculates weights for orientation selectivity in powders and frozen solutions (disordered systems). At a given magnetic field not all, but only centres with certain orientations relative to the external magnetic field have transitions that are on resonance with the spectrometer frequency and can be 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.

Trans is an array listing the transitions included in the computation, with one transition per row. A transition is specified by the numbers of the two levels participating, with the lowest-energy level having label 1, etc.

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, 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.

Field

Magnetic field in mT.

mwFreq

Spectrometer frequency in GHz.

In addition, there are a few optional parameters.

ExciteWidth

FWHM bandwidth of the Gaussian excitation profile, in MHz. If not specified, it is assumed to be zero.

Orientations

2xn 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 Symmetry and GridSize given in the Options structure.

Opt is a structure containing optional settings

GridSize

Number of orientations, as in pepper (optional, default 46)

GridSymmetry

Point-group symmetry of the orientational grid, as in pepper. If not given, the grid symmetry is determined automatically from the spin system (see hamsymm).

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 grid symmetry manually to C1

Opt = struct('GridSymmetry','C1');
orisel(Sys,Exp,Opt);