Ab initio absorption cross-sections and photoisomerization probabilities computed in this work. Top panel: trans- and cis-HCOOH absorption cross-sections for photons with E< 5 eV (those leading to fluorescence). Middle panels: normalized probabilities of bound-bound decays producing isomerization (trans → cis and cis → trans). Bottom panel: standard interstellar dust extinction curve (blue). Black and red curves show the effect of an increased PAH abundance.