  Clarifying electronic and vibronic properties at individual molecule level
provides key insights to future chemistry, nanoelectronics, and quantum
information technologies. The single electron tunneling spectroscopy has been
used to study the charging/discharging process in single molecules. The
obtained information was, however, mainly on static electronic properties, and
access to their dynamical properties was very indirect. Here, we report on the
terahertz (THz) spectroscopy of single fullerene molecules by using a single
molecule transistor (SMT) geometry. From the time-domain THz autocorrelation
measurements, we have obtained THz spectra associated with the THz-induced
center-of-mass oscillation of the molecules. The observed peaks are finely
split into two, reflecting the difference in the van der Waals potential
profile experienced by the molecule on the metal surface when the number of
electrons on the molecule fluctuates by one during the single electron
tunneling process. Such an ultrahigh-sensitivity to the electronic/vibronic
structures of a single molecule upon adding/removing a single electron has been
achieved by using the THz spectroscopy in the SMT geometry. This novel scheme
provides a new opportunity for investigating ultrafast THz dynamics of sub-nm
scale systems.
