Role of rotational thermal bath excitation on the EPR transient regime: A theoretical discussion

The excitation of rotational thermal baths can result in long‐lived effects on spin relaxation in the slow motional region. An attempt at obtaining their quantitative evaluation is successfully made by enriching the Mori–Kivelson–Ogan (MKO) theory through comparison with the Kubo–Freed (KF) one and vice versa. This comparison, in turn, is allowed by suitably extending the MKO theory so as to deal with spin excitation by external radiation fields. The MKO, furthermore, is limited to the second‐order Mori truncation while the KF one is related to the two‐jump case. It is shown, then, that in the usual kind of steady‐state experiments, the MKO theory provides the same result as the KF one. In contrast, when spin relaxation is monitored in the presence of a fluctuating force whose time average intensity is not vanishing, the two theories can result in quite different physical effects.