  The effects of a voltage pulse on the localization probability for a Xe atom
prepared in a pure state localized on the STM surface at 0 temperature is
investigated by numerically integrating the time-dependent Schroedinger
equation. In these calculations the environmental interactions are neglected,
and voltage pulses of 20 and 7 ns with symmetric triangular and trapezoidal
shapes are considered. The atom dynamics at an environmental temperature of 4 K
is studied in the frame of a stochastic, non-linear Liouville equation for the
density operator. It is shown that the irreversible transfer from surface to
tip may be explained by thermal decoherence rather than by the driving force
acting during the application of the voltage pulse.
