  As many organic molecules, formic acid (HCOOH) has two conformers (trans and
cis). The energy barrier to internal conversion from trans to cis is much
higher than the thermal energy available in molecular clouds. Thus, only the
most stable conformer (trans) is expected to exist in detectable amounts. We
report the first interstellar detection of cis-HCOOH. Its presence in
ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation
region), with a low trans-to-cis abundance ratio of 2.8+-1.0, supports a
photoswitching mechanism: a given conformer absorbs a stellar photon that
radiatively excites the molecule to electronic states above the interconversion
barrier. Subsequent fluorescent decay leaves the molecule in a different
conformer form. This mechanism, which we specifically study with ab initio
quantum calculations, was not considered in Space before but likely induces
structural changes of a variety of interstellar molecules submitted to UV
radiation.
