We measure electron tunneling in 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 to the other internal modes of the molecule. We make comparisons between the observed tunneling rates and those expected from the Franck−Condon model. When electrons travel through molecules, vibrational modes of the molecules can affect current flow. Molecular-vibrationassisted tunneling was first measured in the 1960s using devices whose tunnel barriers contained many molecules. 1 Recently, effects of vibrations in single molecules have been measured using scanning tunneling microscopes, 2 singlemolecule transistors, 3,4 and mechanical break junctions. 5 Theoretical considerations suggest that different regimes may exist depending on whether tunneling electrons occupy resonant energy levels on the molecule, and also on the relative magnitudes of the rate of electron flow, the vibrational frequency, and the damping rate of vibrational energy. [6][7][8][9][10][11][12][13][14] A quantitative analysis of electron-vibration interactions has been difficult to achieve in previous molecular-transistor experiments. In transistors made from cobalt coordination complexes, 4 neither the precise nature of the vibrational modes nor their energies was determined independently of transport measurements. In transistors made from C 60 , 3 the "bouncing-ball" mode of a single C 60 molecule against a gold surface was observed, a mode not intrinsic to the molecule itself. In this letter we study single-molecule transistors made using a molecule, C 140 , with low-energy internal vibrational modes that are well understood. We observe clear signatures

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