Origin of the Nuclear Modulation Effect

The nuclear modulation effect was first observed by Rowan, Hahn, and Mims [16], and the theory was later developed by Mims in 1972 [17]. The origin of the nuclear modulation effect can be understood with a semi-quantitative discussion using a two-spin model system consisting of one electron spin (S = Vz) and one nuclear spin (/ = Vi). Assuming an isotropic g-matrix and an anisotropic hyperfme interaction, the spin Hamiltonian in the rotating frame can be written as 

T and iso are the dipolar and the isotropic hyperfine coupling and 9 is the angle between the electron-nuclear vector and the external static magnetic field Bq. 

JEFFREY HARMER, GEORGE MITRIKAS, and ARTHUR SCHWEIGER 

The transition probabilities of the allowed (4) and forbidden (4) EPR transitions are given by Eq. (13), where 2rj is the angle between the nuclear quantization axes in the two manifolds with respect to Bq [7]  

For the isotropic coupling case T= 0) the B term vanishes and 4 = 1,4 = 0. Then the EPR stick spectrum (Fig. 4, right) consists of only two lines corresponding to the allowed transitions split by co =. The addition of an anisotropic part to the 

---