Analysis of a frequency sweep S-Band spectrum of a triplet state at rt

[JacquiD]

Hi everybody =)

I simulated the frequency sweep spectrum of a triplet crystal system containing two molecule orientations at room temperature in the S-band.

1) 5 signals appear, which I cannot assign to certain transitions, does anyone know literature that can help?
2) Does the simulation look reasonable to you or do you spot a vulnerability?
3) Do I probably scratch the limits of EasySpin, so that artifacts appear?

All these questions arise because I don't know what signals to expect in the S-Band spectrum.

I am very grateful for all hints you have! Thanks!

Code: Select all

%Simulation of a frequency sweep spectrum of a triplet crystal at rt
%----------------------------------------------------------------------------------------

% Parameters 
SysFrqSwep = struct('S',1,'g',2.0023,'lwpp',[10 0.80],'D',[1395.57 , -53.35]);  



% Rotation of two molecules in the unit cell
Mol1_0 =[0 0 0];                                % starting postion   % Molz||B0    Molx||B1                         
Mol1_1 = rotatecrystal(Mol1_0, [0;1;0], pi/2); % Molx||B0    Molz||B1
Mol1_2 = rotatecrystal(Mol1_1, [0;0;1], pi/2);  % Molx||B0    Moly||B1

Mol2_1 = rotatecrystal(Mol1_2, [1;0;0], pi/3);  % Mol2 differs from Mol1 in a tilt angle of 60° in the Molx axes 


% Parameters simulation
ExpFrqSwep = struct('Field', 83.56, 'mwRange',[0 11.5],'Temperature', [0.08, 0.16, 0.76], 'Harmonic',0, 'CrystalOrientation', [Mol1_2  ;  Mol2_1]); 

% Simulation
[B0_FreqSwep,Simu_FreqSwep] = pepper(SysFrqSwep,ExpFrqSwep); 



plot(B0_FreqSwep,Simu_FreqSwep);
%-----------------------------------------------------------------------------------------------------------

% Export
%-------------------------
m(:,1)= B0_FreqSwep;
m(:,2)= Simu_FreqSwep;

save('Spectrum.txt','-ascii','m');

[thanasis]

Off the top of my head:
-Have you tried each orientation separately? Two resonances are from Mol1_2 and three from Mol2_1.
-The use of equilibrium populations gives rise to the negative peaks
-The line widths are very narrow for the default number of experimental points (1024).

Many more observations can be made by playing around with the parameters, but the main questions are:
-how you came up with that particular parameter set and whether it is physically meaningful
-precisely what is the experiment you are trying to simulate and whether the experimental parameters describe it correctly

[JacquiD]

Dear thanasis,

thank you soo much for your comments! =)
I already simulated both molecules independently, and can see which signals belong to which molecule!
I even understand the origin of most of the signals.
The used ZFS parameters and the ZF population was previously experimentally determined, as well as the two molecule orientations.

There is one thing which still bothers me.
The simulation of Mol1 alone shows me an emission and an absorption of different intensity, see the next Figure 1
https://imgur.com/a/yUjAmuP

The EPR intensity usually depends on the population of the states.
Taking a closer look at the energy diagram of this molecule and its specific populations under canonical orientation (see next Figure 2) I would expect an equally high intensity
https://imgur.com/E9QzbRA

Due to this simulated phenomenon I wonder how the populations look like in the S-Band. I am wondering if there are only 2 possible population states
a) in the ZFS with the population [0.08, 0.16, 0.76] (from the literature) and
b) under high field condition with a population of [0.12, 0.76, 0.12] at canonical orientation B||X (from the literature).

Or does an intermediate population exist for a triplet state in the S-band?
To explain in more detail what I mean take a look at the energy diagram in Figure 2.
Imagine that my B field in canonical orientation B||X is low enough to be located "before" the intersection points, how does the population look like?
Or does the origin of the different intensities lie somewhere else?

[thanasis]

Unfortunately, I still don't understand the details of the experiment (e.g. the geometry you use to scan the frequency domain, whether it's a real or hypothetical experiment) or the physics of the system (e.g. why you need non-Boltzmann populations, how those parameters were derived).

I don't think I can help just with this information. Maybe someone else may understand better.