The formation of a Pd atomic chain in a hydrogen atmosphere was investigated by measurements of conductance and vibrational spectroscopy of a single molecular junction and the theoretical calculation. While atomic chains were not formed for clean 3d and 4d metals, in the case of Pd ͑a 4d metal͒ atomic chains could be formed in the presence of hydrogen. Stable atomic chains with two different atomic configurations were formed when the Pd atomic contact was stretched in a H 2 atmosphere: highly conductive short hydrogenadsorbed atomic chain and low conductive long hydrogen incorporated atomic chain.

We have investigated Au atomic contacts in H 2 and D 2 environment by conductance measurement and dI / dV spectroscopy. A single H 2 or D 2 molecule was found to bridge Au electrodes. In the case of the Au/ H 2 / Au junction, symmetric peaks were observed in dI / dV spectra while they were not observed for the Au/ D 2 / Au junction. The shape of the peaks in dI / dV spectra originated from the structural change in the single-molecule junction induced by the phonon excitation. The structural change could occur only for the Au/ H 2 / Au junction. The difference in the two single-molecule junctions could be explained by larger zeropoint energy of Au-H 2 vibration mode than that in the Au/ H 2 / Au junction.

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We have investigated the conductance of a single 4,4'-bipyridine molecule anchored on Au electrodes under electrochemical potential control. Appearance of the peak in the conductance histogram suggests the formation of a single molecular junction. The conductance of the single 4,4'-bipyridine molecular junction was estimated from the peak value of 0.01 G 0 (G 0 =2e 2 /h) in the histogram. The conductance of the molecular junction did not change with the electrochemical potential. On the other hand, the intensity of the 0.01 G 0 peak in the conductance histogram decreased as the potential of the Au electrode scanned from 0.2 V to -0.4 V vs. Ag/AgCl. The electrochemical potential dependence of the intensity of the peak in the conductance histogram could be explained by the change in the adsorption properties of 4,4'-bipyridine on the Au electrode.

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