Breathing: A simple and intuitive approach relates changes in bond lengths of core-hole ionized states to the extent of vibrational fine structure, or Franck–Condon broadening (ΔE), in X-ray photoelectron spectra of ionic compounds. For the extreme case of a monolayer of MgO on Ag(100), the difference in Franck–Condon broadening compared to bulk MgO reflects the different binding situation in the supported oxide (see picture, r=MgO distance). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.