Effect of Hstrain in coupled system

[AlbCi]

Dear all,
I have a question regarding the effect of Hstrain when considering a coupled system of three 1/2 spins.
Simulating the powder spectrum, all the three Hstrain components have an effect.
Simulating the epr spectrum of a crystal, along the axis perpendicular to the plane of the triangle (Exp.SampleFrame = [0 0 0]), only Hstrain_z counts, which I consider right.
Trying to simulate the epr spectrum along the a axis of the crystal (i.e. in the plane of the triangle, Exp.SampleFrame = [0 pi/2 0]), I find that only Hstrain_x counts, while Hstrain_y has no effect.
This is quite unexpected to me: can you help me to understand the reason?
Thanks in advance.

Here the code I use

Code: Select all

clear all 
close all

Exp.mwFreq = 9.4;
Exp.Temperature = 30;
Exp.ModAmp = 0.5;
Exp.Range = [231.545 398.555];
Exp.nPoints = 2048;

gxA = 2.0266;
gyA = 2.0577;
gzA = 2.189;
gA = [gxA gyA gzA];

JA = 281*29.98e3;

Hstrainx = 600;
Hstrainy = 0;
Hstrainz = 1200;

Exp.SampleFrame = [0 pi/2 0]; 
Exp.MolFrame = [0 0 0];

g3A = [5 0 0];

% Triangle A 
Sys1.S = [1/2 1/2 1/2];
Sys1.g = [gA; gA; gA];
Sys1.HStrain = [Hstrainx Hstrainy Hstrainz];
Sys1.gFrame = [g3A; g3A+[120 0 0]; g3A+[240 0 0]]*pi/180;
Sys1.J = [JA JA JA];

[x,y1]=pepper(Sys1,Exp);


plot(x*10,y1,'k','LineWidth',3)

xlabel('Magnetic field (G)','FontSize',16,'FontWeight','bold')
ylabel('EPR signal (arb. units)','FontSize',16,'FontWeight','bold')

[thanasis]

That's a nice observation.

If you turn the triangle to the third direction (i.e. Exp.SampleFrame = [0 pi/2 pi/2]), you will see that the effect is reversed: Hstrain_x has no effect and only Hstrain_y counts.

I don't have a complete explanation in mind, but it might help to note that:

• In Exp.SampleFrame = [0 pi/2 0] the magnetic field is parallel to one triangle's side S2-S3, and also to the HStrain_x component of S1

• In Exp.SampleFrame = [0 pi/2 pi/2] it is normal to side S2-S3 and parallel to the HStrain_y component of S1

(You may also see that orientations [4*pi/3 pi/2 0] and [8*pi/3 pi/2 0] give the same results as [0 pi/2 0]. You may also compare [0 pi/2 pi/2+4*pi/3] and [0 pi/2 pi/2+8*pi/3] to [0 pi/2 pi/2])

It might be that the projections of the x/y components of S2 ans S3 cancel out at the orientation you selected and only the corresponding projection of S1 is important.