Colloquia

H₂N has one unpaired electron and three nuclei of non-zero spin. The four H₂N isotopes from ¹H, ²H, ¹⁴N and ¹⁵N have corresponding sets of hyperfine interactions. Measurements of these constrain calculations of electronic wavefunctions and energies, and provide basic knowledge for application to more complex systems. Nuclear spin-state mixing arises from the off-diagonal elements of the nuclear energy matrix, e.g. M_xx ≡ σκ 〈ψ|Σ (S_kzx²_kn/r⁵_kn + S_k'zx²_k'n/r⁵_k'n|Ψ〉 (Airne and Brill, Phys. Rev.A 63 052511). The principle hyperfine A-values can be expressed in terms of the M’s, e.g. A_zz = A_Fermi - (4/3σ)( M_xx + Myy - 2 M_zz), thereby simplifying the energy matrices. In the absence of nuclear spin-state mixing (i.e. each state pure m_I) there are, e.g. 10 epr transitions in D₂¹⁵N and 15 in D₂¹⁴N, all Δm_I = 0 fully allowed. In the presence of mixing there are 243 in D₂¹⁵N and 729 in D₂¹⁴N, with large differences in probability among transitions. Because of numerous, at least partially allowed, overlapping transitions, useful information can be obscured in H₂N magnetic resonance spectra. Research is required to arrive at effective experimental conditions. The wide range of transition probabilities will cause H₂N resonances to exhibit a corresponding range of microwave power saturation behavior. Simulations display remarkable effects which call for experimental verification by employing a wide range of powers. The nuclear Zeeman interaction (proportional to B) perturbs both the energy and state mixing of nuclear levels, thereby affecting the separation and probability of resonances. Of special interest are the fields B_cross at which pairs of hyperfine levels draw closest. A spectrometer with microwave frequency scanning at fixed B would be useful for centers like H₂N in which on-diagonal hyperfine energy matrix elements depend significantly upon B.