Simultaneous UV- and ion processing of astrophysically relevant ices

Abstract

Context. Interstellar ices are known to be simultaneously processed by both cosmic-ray bombardment and UV photolysis. Our knowledge of the effects of energetic processing on relevant icy samples is mainly based on laboratory investigations. In the past 35 years many experiments have been performed to study these effects separately but, to the best of our knowledge, never simultaneously. Aims. The aim of this work is to study the effects of simultaneous processing of ices by both cosmic rays and UV photons to investigate to what extent the combined effect of ion bombardment and UV photolysis influences the chemical pathways. Methods. We carried out the simultaneous processing of CH3OH:N2 ice held at 16 K by 200 keV H + ions and Lyman-alpha 10.2 eV UV photons. The samples were analyzed by in situ transmission infrared spectroscopy. The un-combined processes of UV irradiation and bombardment by H + ions of CH3OH:N2 ice were also studied. This mixture was chosen because the effects of ion bombardment and UV photolysis on methanol and nitrogen have been extensively studied in previous investigations. This mixture enables one to investigate whether simultaneous processing (a) influences the destruction of original species; (b) influences the formation of new species; or (c) causes synergistic effects since Lyman-alpha photons have a very low efficiency in breaking the dinitrogen bond because N2 is almost transparent at Lyman-alpha wavelengths. Results. After processing a CH3OH:N2 sample, the intensity of the methanol bands was observed to decrease at the same rate in all cases. After ion bombardment, species such as CO2 ,C O, H2CO, CH4 ,N 2O, HNCO, and OCN − are formed in the ice mixture. After UV photolysis, species such as CO2 ,C O, H2CO, and CH4 are formed, but no N-bearing species are detected. Spectra of ices processed by both UV photons and ions were compared with spectra of ices bombarded only by ions. We find that there are no differences in the band area and profile of N-bearing species for the two types of experiment at the same ion fluence; therefore, the addition of UV irradiation to ion bombardment does not affect the abundance of N-bearing species. The initial formation rate of CH4, within the experimental uncertainties, is the same in all cases studied, while the saturation value of CH4 is higher for UV photolysis than for ion bombardment when they act separately. In the case of simultaneous processing, when the dose (eV/16u) given by UV photons is similar to the dose given during ion bombardment, the saturation value of CH4 reaches a value intermediate between the value obtained after UV photolysis and ion bombardment separately. Conclusions. Our results confirm that when UV photolysis and ion bombardment act separately, their effects are very similar from a qualitative point of view, while significant quantitative difference may exist. In the case of simultaneous processing we did not detect any synergistic effect, but in some instances the behavior of newly formed species (such as CH4) can significantly depend on the UV/ions dose ratio.