Trans-cis molecular photoswitching in interstellar Space

Abstract

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.

Manuscript accepted for publication in A&A

References in corpus (5)

• Complex Chemistry in Star-Forming Regions: An Expanded Gas-Grain Warm-up Chemical Model
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• The penetration of FUV radiation into molecular clouds
• Nascent bipolar outflows associated with the first hydrostatic core candidates Barnard 1b-N and 1b-S

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• A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry
• New molecular species at redshift z=0.89
• Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
• Anharmonic Vibrational Spectrum and Experimental Matrix Isolation Study of Thioformic Acid Conformers -- Potential Candidates for Molecular Cloud and Solar System Observations?
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• Multi-line Observations, Models, and Data Needed to Understand the Nature of UV-irradiated Interstellar Matter
• A meta-analysis of molecular spectroscopy databases, and prospects of molecule detection with some future facilities