Spin–lattice relaxation time: T_1e

Spin–spin relaxation time: T_2e

If the spin–spin relaxation time is too fast then the

electrons will only remain in the upper state for a

very short period of time and give rise to a

broadening of the spectral line width as a consequence

of Heinsenberg’s uncertainty principle.

In general T_1e > T_2e and the line width depends

only on the spin–spin interactions. However, if,

in certain circumstances, both spin–spin and spin–

lattice relaxations contribute to the EPR line width

(ΔH), then the resonance line width can be simply

written as:

ΔΗ~ (1/T_1e) + (1/T_2e)

When T_1e becomes very short, below ~10^(–7) s, its effects

on the lifetime of the species in a given energy

level makes an important contribution to the linewidth.

From: Rowlands, C.C., and Murphy, D.M., 1999, EPR Spectroscopy, Theory, in Encyclopedia of Spectroscopy and Spectrometry (ed J. Lindon), 445–456, Elsevier, Oxford.

Thank you Dimitris, May I ask a little more? In a frustrated spin system, i.e. there are anti-ferromagnetic exchange interactions between Cu2+ ions( very far from each other), and also the Cu2+ experience Jahn-Teller distortion in a octahedral lattice, in this situation, the spin-spin interaction is very small, and the spin-lattice relaxation seems the major ways. In this situation, how to evaluate the effect of exchange interaction on the EPR linewidth?

For Cu (2) complexes

Delta H = 0.13129. 10^-10/ g. (1/2T1)

Where Delta H (the line width) and T1is the spin –lattice relaxation time( rate constant=1/T1) .Systems with long T1 are weakly coupled to the surroundings and vice versa. Further, lower the numerical value of SOC (lamda) of odd electron species, the longer is the value of T1 and vice versa.

Now Cu(+2) has quite high lamda= -830 cm^-1; meaning thereby Cu(+2) have quite spin-lattice relaxation time.This also lends credence to your view point that spin-spin interaction are weak as is the case of antiferromagnetic complexes of Cu(+2) like dimeric copper acetate monohydrate where there is present weak Cu---Cu delta bond.

Lastly, the very high value of the Jahn-Teller effect, causes distotion in its complexes and causes all the more broadening of the peaks.