Triplet ESR in aromatic molecules

[woodywood]

Hi there,

I am a little confused with triplets in magnetic filed and ESR measurements on them.

Triplet levels in aromatic molecules are indexed Tx, Ty and Tz and if magnetic field is applied they start to mix. If magnetic filed is along the x direction of the crystal, then Tx level remains the same while Ty and Tz are being mixed and we obtain T+ To and T-, where To=Tx (see attached file). Tx state has 91% of electron population.

Now, each of triplet states have total spin of 1, and its projection to the magnetic field can be -1, 0, +1. I assume that these newly created states T+ To and T- correspond to +1, 0, -1 projection of the spin to the magnetic field.

After this one starts to do ESR on a high field transition, so one induces transitions To -> T+. This is the point I do not understand very well, if electron had 0 spin projection to the magnetic field and then after the Pi/2 pulse it goes to the T+ state and then it has +1 projection to the external magnetic field. After this Pi/2 pulse electrons start to precess and do free induction decay.

Basically I do not understand how can electron that is parallel to the magnetic field induce FID and therefore observe the signal of it?

I understand how ESR works in terms of magnetization, but for some reason I cannot figure out how this would work :(

 

[eljose79]

I can understand your confusion with triplets in magnetic field and ESR measurements. Let me try to explain it in a simpler way.

In aromatic molecules, the triplet levels (Tx, Ty, and Tz) have a total spin of 1. This means that the electrons in these levels have three possible spin projections to the magnetic field: -1, 0, and +1. When a magnetic field is applied, these levels start to mix, resulting in new states T+ and T-, with spin projections of +1 and -1 respectively.

Now, during ESR measurements, a high field transition is induced from the To state to the T+ state. This means that the electron, which originally had a spin projection of 0 to the magnetic field, now has a spin projection of +1. This transition is induced by a Pi/2 pulse, which essentially flips the electron's spin.

After this Pi/2 pulse, the electron starts to precess and undergoes free induction decay (FID). This is because the electron now has a non-zero spin projection to the magnetic field (+1), and it is this precession that produces the ESR signal.

To summarize, the electron's spin projection to the magnetic field changes from 0 to +1 due to the Pi/2 pulse, and it is this change that induces the FID and allows us to observe the ESR signal.

I hope this helps to clarify your understanding. If you have any further questions, please feel free to ask.