Theory of electron spin-echo envelope modulation in isotropic tunneling methyl rotor systems

The first-order A- and E-state mixing in tunneling methyl rotors as a mechanism for observable electron spin-echo envelope modulation (ESEEM) spectra is demonstrated. Explicit formulas for the ESEEM frequencies and intensities are obtained using a secular model. At low temperatures in systems where the hyperfine coupling a is exceeded by the tunneling frequency 3Δ, the results simplify to the two positive weight components at 3Δ±a/4 and the two negative weight components at 6Δ and a/2 of half intensity. Numerical ESEEM simulations based on the diagonalization of the full spin-rotation Hamiltonian support these findings. The generalization to higher spin rotors is pointed out.