Abstract
Forbidden transitions are not observed in the continuous-wave electron paramagnetic resonance (EPR) spectrum nor in the free induction decay because, unlike allowed transitions, their coherences have no observable magnetic moment and are spectroscopically silent. Yet, the paramagnetic relaxation described by Redfield theory can cause coherence transfer between any types of transitions. Coherence transfer between allowed transitions is now known to cause noticeable changes in EPR spectra, but coherence transfer involving forbidden transitions has long been considered to be negligible because those coherences are silent and unseen. However, our simulations of a simple model system indicate that coherence transfer with silent transitions can introduce new features into EPR spectra. The EPR-silent coherence of a forbidden transition can be transferred to an allowed transition by paramagnetic relaxation. A silent coherence can have consequences felt in the EPR spectrum.
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Acknowledgments
We thank Prof. K. M. Salikhov for reigniting our interest in the coherence transfer caused by paramagnetic relaxation. This work was supported by the RFBR N0. 14-03-93180 (AGM) and the NSF 1416238 (MKB).
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Dedicated to Professor James Hyde on the occasion of his 85th birthday. He has demonstrated many times that it is possible to measure very subtle effects provided they can be encoded or modulated to distinguish them from other responses and noise.
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Maryasov, A.G., Bowman, M.K. Unseen Coherences Can Be Felt. Appl Magn Reson 48, 1447–1459 (2017). https://doi.org/10.1007/s00723-017-0949-y
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DOI: https://doi.org/10.1007/s00723-017-0949-y