Adsorbed states and scanning tunneling microscopy

The authors have studied adsorption of acetylene on Cu͑110͒ by means of low-temperature scanning tunneling microscopy. Adsorbed molecules preferentially aggregate at 40 K to yield dimer, trimer, and larger islands on the surface. Isolated species ͑monomer͒ adsorbs on the fourfold hollow site with ϳsp 3 rehybridization as characterized by inelastic electron tunneling spectroscopy. Tunneling electron induces an acetylene molecule to migrate along the trough of Cu͑110͒. The migration proceeds in two steps: the molecule first hops to the adjacent long-bridge site and then to the next fourfold site. The voltage and current dependencies of the hopping probability show that the migration is induced by inelastic electron tunneling that causes vibrational excitation of mainly C–H stretch mode.