Vacuum ultraviolet photodesorption and photofragmentation of formaldehyde-containing ices

Géraldine Féraud; Mathieu Bertin; Claire Romanzin; Rémi Dupuy; Franck Le Petit; Evelyne Roueff; Laurent Philippe; Xavier Michaut; Pascal Jeseck; Jean-Hugues Fillion

doi:10.1021/acsearthspacechem.9b00057

paper

Vacuum ultraviolet photodesorption and photofragmentation of formaldehyde-containing ices

arXiv:1905.08223 · doi:10.1021/acsearthspacechem.9b00057

Abstract

Non-thermal desorption from icy grains containing H $_{2}$ CO has been invoked to explain the observed H $_{2}$ CO gas phase abundances in ProtoPlanetary Disks (PPDs) and Photon Dominated Regions (PDRs). Photodesorption is thought to play a key role, however no absolute measurement of the photodesorption from H $_{2}$ CO ices were performed up to now, so that a default value is used in the current astrophysical models. As photodesorption yields differ from one molecule to the other, it is crucial to experimentally investigate photodesorption from H $_{2}$ CO ices. We measured absolute wavelength-resolved photodesorption yields from pure H $_{2}$ CO ices, H $_{2}$ CO on top of a CO ice (H $_{2}$ CO/CO), and H $_{2}$ CO mixed with CO ice (H $_{2}$ CO:CO) irradiated in the Vacuum UltraViolet (VUV) range (7-13.6~eV). Photodesorption from a pure H $_{2}$ CO ice releases H $_{2}$ CO in the gas phase, but also fragments, such as CO and H $_{2}$ . Energy-resolved photodesorption spectra, coupled with InfraRed (IR) and Temperature Programmed Desorption (TPD) diagnostics, showed the important role played by photodissociation and allowed to discuss photodesorption mechanisms. For the release of H $_{2}$ CO in the gas phase, they include Desorption Induced by Electronic Transitions (DIET), indirect DIET through CO-induced desorption of H $_{2}$ CO and photochemical desorption. We found that H $_{2}$ CO photodesorbs with an average efficiency of $\sim 4 - 10 \times 1 0^{- 4}$ molecule/photon, in various astrophysical environments. H $_{2}$ CO and CO photodesorption yields and photodesorption mechanisms, involving photofragmentation of H $_{2}$ CO, can be implemented in astrochemical codes. The effects of photodesorption on gas/solid abundances of H $_{2}$ CO and all linked species from CO to Complex Organic Molecules (COMs), and on the H $_{2}$ CO snowline location, are now on the verge of being unravelled.

ACS Earth and Space Chemistry; Complex Organic Molecules (COMs) in Star-Forming Regions special issue

Vacuum ultraviolet photodesorption and photofragmentation of formaldehyde-containing ices

Abstract

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