ABSTRACT
The first discovered Overhauser effect relies on the relaxation of a saturated electron spin system that is driven by scalar and dipolar electron-nuclear interactions and results in the polarization of the nuclear spins. As shown in Fig. 3.2, this relaxation is a product of zero, single, and double quantum transitions (ZQ, SQ, and DQ) with corresponding probabilities W0, Wn,e, and W2. After saturation of the electron SQ transitions by microwave irradiation with e, the nuclear spin populations are affected by ZQ and DQ transitions, which leads to a non-equilibrium population of the nuclear spin states that itself results in NMR signals enhanced by a factor [17, 20, 21]. en= 1 – fs g g (3.2)The coupling parameter , the leakage factor f and the saturation factor s are defined as 0 z2 0 00 n 2 0n –– ,= = ; = ; =+ 2 + + S SW W f sW W W SW (3.3)where and are the cross-relaxation and auto-relaxation rates, and W n0 is the longitudinal nuclear relaxation rate in absence of a radical. Sz is the longitudinal nuclear magnetization and S0 is the corresponding value at thermal equilibrium [20]. The coupling factor can take values between −1 (pure scalar relaxation) and +0.5 (pure dipolar relaxation) [17]. The saturation factor s reflects the saturation of the electronic levels by irradiation.
