Non-exponential tunneling in water near an electrode

We have measured the inverse decay length (κ) for tunneling in water between a Pt-Ir tip and an Au(111) surface. At 109 Ω κ = 0.92 ± 0.16 Å−1 (Au negative), almost independent of electrochemical potential. An unexpected drop is observed as the gap resistance is reduced below 108 Ω. The tunnel gap at 109 Ω is about 20 Å, 12 Å more than predicted by ab initio calculations. It is Ohmic for resistances between 109 and 106 Ω and voltages less than 0.1 V, implying that sharp resonances do not play a role.