Radiolysis of NH3:CO ice mixtures – implications for Solar system and interstellar ices

Abstract

Experimental results on the processing of NH3:CO ice mixtures of astrophysical relevance by energetic (538 MeV 64Ni24 +) projectiles are presented. NH3 and CO are two molecules relatively common in interstellar medium and solar system; they may be precursors of amino acids. 64Ni ions may be considered as representative of heavy cosmic ray analogues. Laboratory data were collected using mid-infrared Fourier transform (FTIR) spectroscopy and revealed the formation of ammonium cation (NH$_4^+$), cyanate (OCN−), molecular nitrogen (N2), and CO2. Tentative assignments of carbamic acid (NH2COOH), formate ion (HCOO−), zwitterionic glycine (NH$_3^+$CH2COO−), and ammonium carbamate (NH$_4^+$NH2COO−) are proposed. Despite the confirmation on the synthesis of several complex species bearing C, H, O, and N atoms, no N-O bearing species was detected. Moreover, parameters relevant for computational astrophysics, such as destruction and formation cross sections, are determined for the precursor and the main detected species. Those values scale with the electronic stopping power (Se) roughly as σ ∼ a S$_e^n$, where n ∼ 3/2. The power law is helpful for predicting the CO and NH3 dissociation and CO2 formation cross-sections for other ions and energies; these predictions allow estimating the effects of the entire Cosmic ray radiation field.

On the synthesis of N–O bearing species in astrophysical ices – an infrared spectroscopic study using heavy-ion irradiation of solid N2:CO samples

“…species containing at least one N-O bond, one hydrogen, and one carbon), which were not detected in our previous investigation on the processing of CO:NH3 ices (de Barros et al 2020). In fact, de Barros et al (2020) have not detected any molecule containing both nitrogen and oxygen, bonded, or otherwise.…”

“…Among the 15 charge-neutral species found, 7 are nitrogen oxides, 5 of which were detected in our experiment according to reference infrared bands (see Tables 1 and 2). Thus, there are potentially eight hydrogen-free, charge-neutral isomeric species containing both at least one carbon atom and one N-O bond, none of which was detected either in this study, in the study of de Barros et al (2020), or in the interstellar medium so far. Clearly, without proper reference of their infrared bands, the possibility that any given isomer had been synthesized in laboratory cannot be completely excluded, but one may at least say that there are no obvious signs of their presence as well.…”

“…A previous investigation by de Barros et al (2020) on the processing of an CO:NH3 ice mixture of astrophysical interest showed no evidence of formation of molecules containing both N and O. Despite the potential large availability of oxygen and nitrogen atoms and radicals within the sample CO:NH3, no species with general formulas HxCyNzOk, CyNzOk, or NzOk (with x, y, z, k 1) were observed.…”

“…It is worth to mention that, as discussed by e.g. de Barros et al (2020), it is usually easier to fit any given region by adding several peaks to the fit until the sum of peaks matches the actual spectrum; however, the experience shows that this approach is, in most cases, just a mathematical fluke. Therefore, the fit presented here was done by adding the fewest possible number of peaks which would still result in a good fit, which was evaluated based on the residual value of the sum of peaks.…”

“…Some physiochemical parameters, relevant to astrochemical models, have been calculated using the methodology described by de Barros et al (2011Barros et al ( , 2020. Fig.…”