Isotopic and structural signature of experimentally irradiated organic matter
Résumé
The effects of electron irradiation on the structure and the D/H signature of a synthetic analogue of extraterrestrialinsoluble organic matter (IOM) were studied. Polyethylene terephthalate (PET) was chosen because it contains both aliphaticand aromatic functional groups. A 900 nm-thick film was irradiated with electrons within the energy range 4–300 keV, fordifferent run durations. Temperature influence was also tested. Irradiated residues were structurally and isotopically characterizedby infrared spectroscopy (IR), electronic paramagnetic resonance (EPR), and Secondary Ion Mass Spectrometry(SIMS). With increasing energy deposition, spectroscopic results indicate (i) a gradual amorphization with chain scissions,(ii) an increase of CH2/CH3 and (iii) the formation of quinones. The EPR study shows that mono- and biradicals (organicspecies with one or several unpaired valence electrons) are also formed during irradiation. As these structural modificationsoccur, the dD (initially at 33& relative to SMOW) decreases first during a transient step and then stabilizes at +300&.There is a strong correlation between the changes recorded by the different methods and the electron dose. Deposited energyappears to be the key parameter to induce these modifications. In this respect a low-energy electron irradiation causes moredamages than high energy ones. Based on our data and considering the current solar electron flux, the irradiation at moderateenergy (1–10 keV) can produce significant D-enrichments of the IOM in a timescale compatible with the evolution of a typicalprotoplanetary disk