  We investigate atomistic details of a single atom extraction process realized
by using scanning tunneling microscope (STM) tip-cluster interaction on a
Ag(111) surface at 6 K. Single atoms are extracted from a silver cluster one
atom at a time using small tunneling biases less than 35 mV and a threshold
tunneling resistance of 47 kOhm. A combination of total energy calculations and
molecular dynamics simulations shows a lowering of the atom extraction barrier
upon approaching the tip to the cluster. Thus, a mere tuning of the proximity
between the tip and the cluster governs the extraction process and is
sufficient to extract an atom. The atomically precise control and
reproducibility of the process are demonstrated by repeatedly extracting single
atoms from a silver cluster on an atom-by-atom basis.
