TUNNELING SPECTROSCOPY OF HALOGEN ADLAYERS ON AG(111) SURFACES

Tunneling spectroscopy measurements performed with a scanning tunnelin g microscope (STM) of Ag(111) surfaces covered with a monolayer of hal ogen atoms (F, Cl, Br, and 1) are reported. At positive sample biases, the tunneling current rises sharply from the nanoampere range well in to the microampere range for all Ag(111)-halogen systems. The threshol d voltage of this diodelike response is a function of the halogen spec ies and correlates with vibrational energies of the respective Ag-halo gen system determined from resonant Raman scattering. The minimum tunn el junction resistance is approximately piHBAR/e2, in agreement with t heoretical predictions based on models of a point-contact single-atom junction or a double-barrier tunnel junction. The results are discusse d in terms of a point contact formed between the tip and surface halog en atom and resonant tunneling coupled to the vibrational modes of the Ag-halogen bond.