Compute orientation selectivity weights for ENDOR and ESEEM simulations of disordered systems
Weights = orisel(Sys,Par) Weights = orisel(Sys,Par,Opt) orisel(...)
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.
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
Symemtry and nKnots given in the Options structure. |
Opt
is a structure containing optional settings
nKnots | Number of orientations, as in pepper (optional, default 46). |
Symmetry | Point-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.
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);