  Upon the discovery of the superatom states in endohedral metallofullerenes
(EMFs), the superatom properties have become attractive, because ultrafast
motion of the trapped atom modifies the charge distribution in the fullerene
cage. However, the observation of ultrafast atom motion in the fullerene cage
is very challenging, since dynamical processes take place in the terahertz
(THz) frequency range in a picometer region. Here, we report on the THz
spectroscopy of single Ce@C82 EMF molecules by using the single molecule
transistor geometry. Due to the vibron-assisted tunneling process, two broad
photocurrent peaks are observed in the THz spectra and are ascribed to the
bending and stretching motions of the encapsulated single Ce atom. This work
demonstrates that THz spectroscopy using nanogap electrodes can detect a motion
of a single atom, opening a door to ultrafast THz nanoscience on the
picometer-scale.
