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