  We measure electron tunneling in single-molecule transistors made from
C_{140}, a molecule with a mass-spring-mass geometry chosen as a model system
to study electron-vibration coupling. We observe vibration-assisted tunneling
at an energy corresponding to the stretching mode of C_{140}. Molecular
modeling provides explanations for why this mode couples more strongly to
electron tunneling than the other internal modes of the molecule. We make
comparisons between the observed tunneling rates and those expected from the
Franck-Condon model.
