Electron spin echo envelope modulation (ESEEM) has been observed for the first time from a coupled heterospin pair of electron and nucleus in liquid solution. Previously, modulation effects in spin-echo experiments have only been described in liquid solutions for a coupled pair of homonuclear spins in nuclear magnetic resonance or a pair of resonant electron spins in electron paramagnetic resonance. We observe low-frequency ESEEM (26 and ) due to a new mechanism present for any electron spin with that is hyperfine coupled to a nuclear spin. In our case these are electron spin and nuclear spin in the endohedral fullerene . The modulation is shown to arise from second-order effects in the isotropic hyperfine coupling of an electron and nucleus.
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These flip-flop transitions are allowed only to third order—the transition probability is proportional to , and thus is negligibly small at for .
The molecule has been proposed as an electron spin-based qubit in several quantum information processing schemes.17,18 At the very least, the slow evolution within the sublevels of the system, which is responsible for the observed ESEEM must be taken into account when designing pulse sequences to perform a quantum algorithm. However, it could also be exploited to provide a separate family of gates for performing operations between sublevels, increasing the potential of as a single quantum bit.
