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

A determination of the ruby high-pressure scale is presented using all available appropriate measurements including our own. Calibration data extend to 150 GPa. A careful consideration of shock-wave-reduced isotherms is given, including corrections for material strength. The data are fitted to the calibration equation P = ͑A / B͓͒͑ / 0 ͒ B −1͔ ͑GPa͒, with A = 1876± 6.7, B = 10.71± 0.14, and is the peak wavelength of the ruby R1 line.

A [2+2] cycloaddition cap-to-cap C70 dimer with C 2 h molecular symmetry was synthesized in high yield by pressure treatment of polycrystalline C70 at 1 GPa and 200 °C. It was separated from unreacted monomers by chromatography and characterized by 13C NMR, Raman, and infrared spectroscopy, and other methods. Remarkably, only one isomer was produced out of the five possible [2+2] cycloaddition products which have equally low formation energies according to semiempirical modeling calculations. The dimer obtained is the one favored when C70 molecules adopt an ordered packing with parallel D 5 axes. The intercage bonding in C140, its thermal stability, and intercage vibrational modes are similar to those found for the C60 dimer, C120. Both dimers photodissociate to the monomers in solution, probably via excited triplet states. The UV absorption and fluorescence properties of C140 are not very different from those of C70, suggesting only weak electronic interactions between the two cages of C140. In comparison, the pressure-induced dimerization of C60, under the conditions used for C70, results mainly in C60 oligomers and polymeric chains, but the dimer C120 could be isolated at low yield when short reaction times (≤5 min) were used.