  The electronic properties of Fe-based superconductors are drastically
affected by deformations on their crystal structure introduced by doping and
pressure. Here we study single crystals of FeSe$_{1-x}$S$_{x}$ and reveal that
local crystal deformations such as atomic-scale defects impact the spectral
shape of the electronic core level states of the material. By means of scanning
tunnelling microscopy (STM) we image S-doping induced defects as well as
diluted dumbbell defects associated with Fe vacancies. We have access to the
electronic structure of the samples by means of X-ray photoemission
spectroscopy (XPS) and show that the spectral shape of the Se core levels can
only be adequately described by considering a principal plus a minor component
of the electronic states. We find this result for both pure and S-doped
samples, irrespective that in the latter case the material presents extra
crystal defects associated to doping with S atoms. We argue that the second
component in our XPS spectra is associated with the ubiquitous dumbbell defects
in FeSe that are known to entail a significant modification of the electronic
clouds of surrounding atoms.
