Field-induced transfer of an atom between two closely spaced electrodes

The transfer of an adsorbed atom from one electrode to another in close proximity, with a potential difference between the electrodes, is analyzed theoretically. Calculations for the case of a Si atom are in accord with results on the transfer of Si atoms in the scanning tunneling microscope under ultrahigh-vacuum conditions at room temperature. Most of the activation-barrier lowering that permits a measurable transfer rate at this temperature is a chemical effect due to the proximity of the electrodes, but the bias provides an additional barrier lowering and a directional driving force. For conditions relevant to experiment, the atom acquires a distance-dependent charge no larger than several tenths of a unit charge (e). © 1992 The American Physical Society.