Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices

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 astrophys-ical 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 ∼ 4 − 10 × 10 −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.
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Géraldine Féraud, Mathieu Bertin, Claire Romanzin, Rémi Dupuy, Franck Le Petit, et al.. Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices. ACS Earth and Space Chemistry, ACS, In press, ⟨10.1021/acsearthspacechem.9b00057⟩. ⟨hal-02136048⟩

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