  The formation of extended electron states in one-dimensional nanostructures
is of key importance for the function of molecular electronics devices. Here we
study the effects of strong electron-phonon interaction on the formation of
extended electronic states in intentionally created Cl vacancy pairs and chains
in a NaCl bilayer on Cu(111). The interaction between the vacancies was
tailored by fabricating vacancy pairs and chains of different orientation and
separation with atomic precision using vertical manipulation. Small separation
of divacancies led to the formation of symmetric and antisymmetric vacancy
states and localized interface-states. By scanning tunneling spectroscopy (STS)
we measured their energy splitting and broadening as a function of the
inter-vacancy separation. Unexpectedly, the energy splitting between the
vacancy states is enlarged by level repulsion resulting from phonon dressing of
the electronic states, as evidenced by theory. Already for a few coupled
vacancies we observe an emerging band structure of the defect band.
