Line shapes in inelastic electron tunneling spectroscopy of single-molecule junctions

摘要

Spectroscopic line-shape analyses for single-C6o vibrational modes are presented for two kinds of scanning tunneling microscope experiments. Inelastic electron tunneling spectroscopy is performed for C_(60) molecules with different adsorption geometries on Pb(111). Depending on the C_(60) adsorption site and rotational orientation, the lowest unoccupied molecular orbital exhibits varying degrees of overlap with vibrational energies. Concomitantly, the line shapes of vibrational modes are affected according to expectations for on-resonance inelastic electron tunneling. Inelastic electron transport is further studied for decreasing tip-C_(60) distances covering tunneling and contact ranges. Line-shape changes signaling a conductance increase rather than the expected decrease upon exciting vibrational quanta are observed. A phenomenological approach is suggested to understand the dissimilar behavior of the junction conductance in the different electron transport ranges.