title: Trans-cismolecular photoswitching in interstellar Space.
abstract: As many organic molecules, formic acid (HCOOH) has two conformers (transandcis). The energy barrier to internal conversion fromtranstocisis 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 ofcis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a lowtrans-to-cisabundance 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 withab initioquantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation.
authors.affiliation: Grupo de Astrofísica Molecular. Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors.forename: S
authors.initials: S
authors.lastname: Cuadrado
authors.affiliation: Grupo de Astrofísica Molecular. Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors.forename: J R
authors.initials: JR
authors.lastname: Goicoechea
authors.affiliation: Instituto de Física Fundamental (IFF-CSIC). Calle Serrano 123, E-28006 Madrid, Spain.
authors.forename: O
authors.initials: O
authors.lastname: Roncero
authors.affiliation: Facultad de Ciencias, Unidad Asociada de Química-Física Aplicada CSIC-UAM, Universidad Autónoma de Madrid, E-28049, Madrid, Spain.
authors.forename: A
authors.initials: A
authors.lastname: Aguado
authors.affiliation: Grupo de Astrofísica Molecular. Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors.forename: B
authors.initials: B
authors.lastname: Tercero
authors.affiliation: Grupo de Astrofísica Molecular. Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Cernicharo
keywords: Astrochemistry; ISM: clouds; ISM: molecules; Line: identification; Photon-dominated region (PDR)