Electric Conductance of Single Ethylene and Acetylene Molecules Bridging between Pt Electrodes

We have investigated the conductance and atomic structure of single ethylene and acetylene molecule junctions on the basis of the conductance measurement and vibration spectroscopy of the single molecule junction. Single :molecule junctions have a conductance comparable to that of metal atomic functions (around 0.9G(0): G(0) = 2e(2)/h) due to effective hybridization between metal and the pi molecular orbital. The ethylene molecules are bound to Pt electrodes via a di-sigma bond, while the acetylene molecules are bound to Pt electrodes via di-sigma and pi bonds. By using the highly conductive single molecule junctions, we investigated the characteristics of vibration spectroscopy of the single molecule junction in an intermediate regime between tunneling and contact. The vibration modes that could modify the conduction orbital were excited for the ethylene and acetylene molecule junctions. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, whereas clear Crossover was not observed for the acetylene molecule junction, reflecting the number of conduction orbitals in the single molecule junction.