  Electron spin echo envelope modulation (ESEEM) has been observed for the
first time from a coupled hetero-spin 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
NMR or a pair of resonant electron spins in EPR. We observe low-frequency ESEEM
(26 and 52 kHz) due to a new mechanism present for any electron spin with S>1/2
that is hyperfine coupled to a nuclear spin. In our case these are electron
spin (S=3/2) and nuclear spin (I=1) in the endohedral fullerene N@C60. The
modulation is shown to arise from second order effects in the isotropic
hyperfine coupling of an electron and 14N nucleus.
