  Backscattering of delocalized electrons has been recently established [Phys.
Rev. A 105, L041101 (2022)] as a mechanism to enhance high-order harmonic
generation (HHG) in periodic systems with broken translational symmetry. Here
we study this effect for a variable spatial gap in an atomic chain. Propagating
the many-electron dynamics numerically, we find enhanced HHG and identify its
origin in two mechanisms, depending on the gap size, either backscattering or
enhanced tunneling from an impurity state. Since the gapped atomic chain
exhibits both impurities and vacancies in a unified setting, it provides
insight how periodicity breaks influence HHG in different scenarios.
