Atomic resolution images of Cu(111) and Cu(100) surfaces obtained in ultrahigh vacuum with a combined scanning tunneling (STM)/atomic force microscope (AFM) are presented. Scan lines recorded in the noncontact AFM mode and the constant current mode show enhanced corrugation similar to STM experiments. Frequency shift versus distance curves are analyzed to determine the forces between probing tip and sample using the force separation method. The short-range chemical forces are in good agreement with an exponential law. The decay lengths of these chemical forces are found to be twice as large as the decay length of the tunneling current, as predicted by a simple theoretical model. However, both lengths are significantly larger than expected for metallic adhesive interactions.

• Received 15 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.16944