TY - JOUR
AU - da Silveira, E, F
AB - ABSTRACT N-O-bearing molecules are considered to be important primary molecules of complex and prebiotic species in space, and therefore an understanding of the N-O chemistry is a fundamental step in linking the origin and evolution of prebiotic species to the evolution of interstellar and Solar System ices. With this in mind, we have started an extensive program of laboratory work to study and understand the effects of cosmic radiation on N-O-bearing molecules in space. Carbon dioxide (CO2) and nitrous oxide (N2O) are volatile molecules found in the Solar System and in the interstellar medium. Thus it is expected that both molecular species may freeze-out on dust grains and the surface of Solar System bodies and be exposed to cosmic radiation. The objective of the present work is to obtain an experimental analysis of the ion irradiation effects in an N2O:CO2 (1:2) ice mixture, at 11 K, when the mixture is irradiated by 90-MeV 136Xe23 +. Fourier-transformed infrared spectroscopy (FTIR) was the method of analysis used for this purpose. Eight product molecular species have been observed: CO, CO3, NO, NO3, N2O2, N2O3, N2O4 and O3; C−N compounds were not observed. The chemical evolution of the new molecules formed in the sample was followed by estimating the column densities of the primary molecule and products as a function of the beam fluence. This procedure allows the determination of their formation and dissociation cross-sections. The destruction cross-section of N2O is approximately twice that of CO2, which results in relatively large quantities of N, O, N2 and NO molecules in the ice. astrochemistry, molecular data, methods: laboratory: molecular, interplanetary medium, Cosmic rays, ISM: molecules 1 INTRODUCTION Ice and dust grains form the primary material out of which the Solar System (SS) was formed. During the past few years, great strides have been made in gaining an understanding of the composition of interstellar ice grains and of the role they play in interstellar chemistry. Such progress has been possible as a result of substantial advances in astronomical infrared spectroscopic techniques combined with the availability of spectra of realistic laboratory analogues. Astrophysical ices, such as those on interstellar grains and the surface of outer SS bodies, are continuously irradiated by cosmic ions and ultraviolet (UV) photons. Ions in the keV–GeV range impinging on these ices transfer energy to them, causing, in particular, electronic excitation and ionization. Molecular bonds are broken, and, in a very short time, the molecular fragments recombine, giving rise to chemical-physical changes (Palumbo, Baratta & Spinella 2006). In addition to the modifications of the chemical and lattice structure of the target material, new molecular species (i.e. species not present before irradiation) may be synthesized. The existence of the gas phase of N2O in the interstellar medium (ISM) was first confirmed by Ziurys et al. (1994) through the observation of molecular rotational lines at 75, 100, 125 and 150 GHz in the giant molecular cloud complex Sgr B2; gas-phase CO2 lines have been observed in numerous ISM environments (d’Hendecourt & de Muizon 1989; Gerakines et al. 1999; Bergin et al. 2005; Suhasaria et al. 2017). N2O has been proposed as the parent molecule of N2 and N|$_2^+$| species observed in comet 1P/Halley (Saxena 2004). N2 identification has recently been reported by Rubin et al. (2015) in the coma of comet 67P/Churyumov–Gerasimenko with the ROSINA mass spectrometer onboard the Rosetta spacecraft. Cochran & McKay (2018) recently reported strong emission from N|$_2^+$| in C/2016 R2 (PanSTARRS). Inside the SS, CO2 has been widely detected on the icy Galilean satellites Europa, Ganymede and Callisto and in the coma of comets (Carlson, Anderson & Johnson 1999; McCord et al. 1998; Mumma & Charnley 2011), on Triton (Quirico et al. 1999), and on the surface of Mars (Herr & Pimentel 1969; Larson & Fink 1972; Lewis et al. 2015). Given that the sublimation temperature of N2O, at 10−7 mbar, is about 75 K (fig. 1 of de Barros et al. 2016a) and that of CO2, at 10−8 mbar, is about 85 K (figs 4 and 7 of Ponciano et al. 2006), it is expected that both molecular species may freeze-out on ISM dust grains and the surface of SS bodies. In addition, solid CO2 has also been observed on interstellar grain mantles in interstellar clouds (Tielens & Hagen 1982; d’Hendecourt, Allamandola & Greenberg 1985; Öberg el a. 2011). From what has been said, then, CO2 is considered to play an important role in some of the physico-chemical processes occurring on ISM dust grains and on the surface of SS bodies. Because CO2 has been detected in several comets, such as in the coma of comet Halley (Combes et al. 1987, 1988; Krankowsky et al. 1986) and in 67P/Churyumov–Gerasimenko (Bockelée-Morvan et al. 2015), it is considered to play an important role in some of the physico-chemical properties of comets (e.g. Delsemme 1982). Furthermore, the study of N-O-containing molecules is particularly important because these molecules are essential for life on Earth and, supposedly, for extraterrestrial life. Besides NO, nitrogen dioxide (NO2) and nitrous oxide (N2O) are the smallest nitrogen oxides, they occur in the Earth’s stratosphere and troposphere (Ackerman et al. 1975; Noxon 1975; Shaw 1976; Gruzdev & Elokhov 2011). What has been stated so far is supported by laboratory studies of the irradiation of icy mixtures (H2O:N2, CO:N2, H2O:CO3 and N2:O2) and pure ices (CO2 and N2O) with various ions (13C2 +, Ar2 +, N+, 58Ni13 +, 136Xe23 + and H+) and various energies (from 200 to 15 × 104 keV), leading to the synthesis of new molecular species, such as NO, N2O, N2O2, N2O3, CO3, CO3, O3 and NO2 (Almeida et al. 2017; de Barros et al. 2016a; Boduch et al. 2012; Sicilia et al. 2012; Pilling et al. 2010; Seperuelo Duarte et al. 2009; Ponciano et al. 2005). With all this in mind, we have recently started an extensive laboratory program to study and understand the effects of cosmic irradiation on N-O-bearing molecules in space. In parallel to laboratory experiments focusing on pure N-O-bearing molecules (N2O, NO2, NO, etc.), we are also performing ion irradiation experiments on multi-component mixtures involving these N-O-bearing molecules mixed with some of the common ices found in the interstellar frozen grain mantles and/or on the frozen surfaces of SS bodies, such as CO2, H2O, CO and others. The current work is thus a contribution to the experimental analysis of the effects of heavy ion irradiation on multicomponent ices. The irradiation of the N2O:CO2 ice mixture, at typical ISM and outer SS temperatures (as low as 10 K), by energetic heavy ion projectiles was monitored by Fourier-transformed infrared (FTIR) spectroscopy. In Section 2, the experimental setup and the infrared analytical technique (FTIR) are briefly described; in Section 3 the spectroscopic modifications resulting from ion irradiation and the analysis of those results are presented; Section 4 is dedicated to the discussion of the results presented in the previous section, as the dissociation and formation cross-sections of the primaries and product species; final remarks, and the astrophysical implications and conclusions are given in Section 5. 2 EXPERIMENTAL SETUP The N2O:CO2 (1:2) mixture was condensed at 11 K on KBr substrate in a high-vacuum chamber, at 10−8 mbar, cooled by a closed-cycle helium cryostat. The ice mixture was irradiated by a beam of 90-MeV 136Xe23 + ions (0.66 MeV u−1). Infrared spectroscopy (FTIR) was used to analyse the chemical evolution as a function of fluence. IR absorbance was determined using a Nicolet FTIR spectrometer (Magna 550), covering the region 4000−600cm−1 with 1-cm−1 resolution. The experiments were carried out at the IRRSUD beamline of the heavy ion accelerator GANIL (Grand Accélérateur National d’Ions Lourds). The experimental setup has been described previously (de Barros et al. 2016a). Xenon ions are not a common constituent of cosmic rays but can be employed as an efficient beam to induce radiolysis (Seperuelo Duarte et al. 2010; Mejia et al. 2013). Destruction cross-sections produced by cosmic ray heavy ions can be deduced from the one determined with Xe ions. Fig. 1(a) represents the IR spectrum of the virgin sample in the range 4000 to 600 cm −1. This result is compared in Fig. 1(b) with the spectrum obtained after irradiating the ice mixture to 1 × 1012 ions cm−2. A decrease of the primary molecules is clearly observed in Fig.1(b), and the appearance of product species is better seen in the zoom of Fig. 1(c). Figure 1. Open in new tabDownload slide Infrared spectra showing radiolysis effects on N2O:CO2 ice: (a) spectrum of the non-irradiated ice mixture from 4000 to 600 cm−1; (b) spectrum after 1.0 × 1012 ions cm−2; and (c) zoom of the previous spectrum, in the region 2170 to 900 cm−1. The N2O3 band peaked at 1305 cm−1 appears as a shoulder band after de-convolution of the N2O fundamental band (1296 cm−1). Figure 1. Open in new tabDownload slide Infrared spectra showing radiolysis effects on N2O:CO2 ice: (a) spectrum of the non-irradiated ice mixture from 4000 to 600 cm−1; (b) spectrum after 1.0 × 1012 ions cm−2; and (c) zoom of the previous spectrum, in the region 2170 to 900 cm−1. The N2O3 band peaked at 1305 cm−1 appears as a shoulder band after de-convolution of the N2O fundamental band (1296 cm−1). In order to transform the integrated absorbance, S (the measured peak area, in molecules cm−1, of the i band observed in the IR spectrum), into column densities, N (in molecules cm−2), the Beer–Lambert law was used (e.g. d’Hendecourt et al. 1986): \begin{equation*} N = \ln10 \frac{S}{A_v}, \end{equation*} (1) where A|$v$| (in cm molecule−1, usually called the A-value) is the integrated absorption coefficient of the i band. In general, S and A|$v$| are quantities that vary with the beam fluence, F. Thus, equation (1) is better written as: \begin{equation*} N(F)= \ln10 \frac{S(F)}{A_v(F)}. \end{equation*} (2) The thickness (L) of the sample is determined by equation (3): \begin{equation*} L = \frac{N_0}{6.02 \times 10^{23}} \frac{M}{\rho } \times 10^4, \end{equation*} (3) where L is the primary molecule layer thickness (in |$\mu$|m),N0 is its initial column density (in molecules cm−2), M is its molar mass (44 g mol−1 for both N2O and CO2 molecules) andρ is its mass density in the solid phase as a pure substance. In this work, two reference bands have been used for the N0 determination of each primary molecule. For N2O, the 3516-cm−1 (ν1 + ν3 mode) and 2245-cm−1 (ν3 mode) bands yielded N0 = 1.4 and 1.6 × 1017 molecules cm−1, respectively. As a consequence, the mean value N0 = (1.5 ± 0.1) × 1017 molecules cm−1 was adopted. Similarly, for CO2, the 3708-cm−1 (ν1 + ν3 mode) and 2344-cm−1 (ν3 mode) bands were considered, and the value N0 = (3.4 ± 0.2) × 1017 molecules cm−1 was adopted. Based on these calculations, the measured CO2 / N2O molecular ratio in the mixture is 2.3. Taking into account that the densities of pure N2O and pure CO2 are 1.17 g cm−3 (Fulvio et al. 2009) and 1.82 g cm−3 (Ponciano et al. 2005), respectively, the estimated value for the total ice thickness is ∼ 0.23 |$\mu$|m. The predicted electronic and nuclear stopping powers for 90-MeV 136Xe23 + in the (1:2) N2O:CO2 ice mixture calculatedby the trim code (Ziegler et al. 2010) are: Se = 9.36 × 103 keV |$\mu$|m−1 and Sn = 6.70 × 101 keV |$\mu$|m−1, respectively. The penetration depth of the 136Xe23 + beam in N2O:CO2 ice is ∼ 17.8 |$\mu$|m. This length is approximately two orders of magnitude higher than the ice thickness calculated for the current experiment, indicating that the vast majority of incoming Xe ions traverse the ice target with velocities not much lower than the impinging one. The calculated electronic/nuclear stopping power ratio is 14.0, which reveals that projectile–ice interaction occurs basically through Xe ions interacting with target electrons; that is, most of the chemical modifications observed in the ice composition must be primarily a result of the energy delivered by projectiles to target electrons along the projectile ion track. A detailed description of the contaminants usually present in this particular analysing system is given in de Barros et al. (2016a). Inspection of the IR spectrum presented in Fig. 2(a) reveals no contamination by H2O, because its main band at 3300 cm−1 was not observed in the non-irradiated ice or during the ion irradiation. The absence of water also suggests no contamination by N2 and O2 on the sample surface, which in turn indicates the existence of sputtering during irradiation. Figure 2. Open in new tabDownload slide Experimental infrared spectra obtained for the N2O:CO2 ice at 11 K during irradiation (0–2.2 × 1012 ions cm−2). The ranges displayed are (a) 3750–3200 cm−1, (b) 2330–2200 cm−1, (c) 1310–1150 cm−1 and (d) 680–640 cm−1. The 3300-cm−1 water band is not observable in the experiment. Figure 2. Open in new tabDownload slide Experimental infrared spectra obtained for the N2O:CO2 ice at 11 K during irradiation (0–2.2 × 1012 ions cm−2). The ranges displayed are (a) 3750–3200 cm−1, (b) 2330–2200 cm−1, (c) 1310–1150 cm−1 and (d) 680–640 cm−1. The 3300-cm−1 water band is not observable in the experiment. 3 RESULTS 3.1 Primary molecule IR bands Table 2 presents the spectroscopic characteristics of the 11 bands observed in the virgin ice and attributed to the two primary molecules. Data include the vibrational assignments, the wavenumbers of the bands and their integrated absorption coefficient A|$v$|. The bands shown in bold are reference bands considered in the analysis developed below. Fig. 2 shows how the shapes of the main bands of the primary molecules CO2 and N2O evolve with irradiation; as can be seen, no peak shift is apparent. Fig. 3 displays the dependence of integrated absorbance (band area) on beam fluence for the primary molecule bands: they agree relatively well with each other when normalized at 1.0 × 1011 ions cm−2. Using equation (2) for both primary molecules and assuming a negligible variation of A|$v$|(F) with fluence, their column density dependence on fluence is obtained: \begin{equation*} N(F)= N_0\exp [-\sigma _{\rm d} F], \end{equation*} (4) where N0 is each initial column density for the non-irradiated ice(F = 0) and σd is the primary molecule's destruction cross-section. For this work, two bands of each primary molecule (shown in Table 2) were selected as reference, and the mean value of these bands was used for the determination of the N0 value. Figure 3. Open in new tabDownload slide Dependence of the primary molecule absorption bands on beam fluence. (a) N2O bands, and (b) CO2 bands. For all bands, data are normalized at 1.0 × 1011 ions cm−2. Figure 3. Open in new tabDownload slide Dependence of the primary molecule absorption bands on beam fluence. (a) N2O bands, and (b) CO2 bands. For all bands, data are normalized at 1.0 × 1011 ions cm−2. The N(F) evolutions for the two N2O and CO2 reference bands, fitted with equation (4), are presented in Fig. 4. The meanσd and N0 values for each primary molecular species are then determined and presented in Table 1. Figure 4. Open in new tabDownload slide Dependence of the N2O and CO2 band column densities on beam fluence. From the two fittings (solid lines) with equation (4), the average destruction cross-section (σd) for each primary molecule is determined. The inset shows the evolution of N at the beginning of irradiation. Figure 4. Open in new tabDownload slide Dependence of the N2O and CO2 band column densities on beam fluence. From the two fittings (solid lines) with equation (4), the average destruction cross-section (σd) for each primary molecule is determined. The inset shows the evolution of N at the beginning of irradiation. Table 1. Destruction cross-section (σd), initial column density (N0) and theA-value of the primary N2O and CO2 molecules. Two reference bands per primary species are employed in the determination of average σd and N0. Primary molecule Reference band (cm−1) σd (10−13 cm2) N0 (1017 molecules cm−2) A|$v$| (10−18 cm molecule1) N2O 3516 12.4 1.4 1.9a N2O 2245 12.6 1.6 56.9b N2O mean – 12.5* 1.5 – CO2 3708 7.8 3.7 1.4c CO2 2344 6.7 3.1 76.0d CO2 mean – 7.3* 3.4 – Primary molecule Reference band (cm−1) σd (10−13 cm2) N0 (1017 molecules cm−2) A|$v$| (10−18 cm molecule1) N2O 3516 12.4 1.4 1.9a N2O 2245 12.6 1.6 56.9b N2O mean – 12.5* 1.5 – CO2 3708 7.8 3.7 1.4c CO2 2344 6.7 3.1 76.0d CO2 mean – 7.3* 3.4 – a de Barros et al. (2016a, 2011), bFulvio et al. (2009), cGerakines et al. (1995), cYamada & Person (1964). *σd adopted values for product species column density determination (see next section). Open in new tab Table 1. Destruction cross-section (σd), initial column density (N0) and theA-value of the primary N2O and CO2 molecules. Two reference bands per primary species are employed in the determination of average σd and N0. Primary molecule Reference band (cm−1) σd (10−13 cm2) N0 (1017 molecules cm−2) A|$v$| (10−18 cm molecule1) N2O 3516 12.4 1.4 1.9a N2O 2245 12.6 1.6 56.9b N2O mean – 12.5* 1.5 – CO2 3708 7.8 3.7 1.4c CO2 2344 6.7 3.1 76.0d CO2 mean – 7.3* 3.4 – Primary molecule Reference band (cm−1) σd (10−13 cm2) N0 (1017 molecules cm−2) A|$v$| (10−18 cm molecule1) N2O 3516 12.4 1.4 1.9a N2O 2245 12.6 1.6 56.9b N2O mean – 12.5* 1.5 – CO2 3708 7.8 3.7 1.4c CO2 2344 6.7 3.1 76.0d CO2 mean – 7.3* 3.4 – a de Barros et al. (2016a, 2011), bFulvio et al. (2009), cGerakines et al. (1995), cYamada & Person (1964). *σd adopted values for product species column density determination (see next section). Open in new tab Fig. 5presents the evolution of the column density ratio between N2O and CO2, determined by equation (1). The column densities used in the ratio N(N2O)/N(CO2) were determined by the mean values between the two reference bands for each primary molecule (defined in Tables 1 and 2). Data displayed in Fig. 3 show that N2O is destroyed faster than CO2 throughout the experiment ; in particular, the N2O:CO2 concentration ratio at the end of the irradiation is about 1:8, namely about 4 times less then the initial concentration (1:2). Astrophysical ices are usually formed by the condensation of gas-phase molecules onto dust grains or the surface of minor bodies in the outer SS, owing to the low temperatures found in these regions. Thus, they are expected to have a porous structure at the beginning of their formation. In the laboratory, the porosity is generally recognized in IR spectra as an abrupt variation of the vibration mode absorbance at the beginning of the irradiation (Almeida et al. 2017; Leto & Baratta 2003; Mejia et al. 2015). The column density evolution with irradiation shown in Fig. 4 does not exhibit such behaviour, and, therefore, it is concluded that the sample probably has no porosity. Figure 5. Open in new tabDownload slide Relative concentrations of the ice mixture in relation to the fluence: the N2O molecule is destroyed to a much greater extent throughout the experiment than CO2, for which the final concentration is 4 times lower than at the beginning of the experiment. Figure 5. Open in new tabDownload slide Relative concentrations of the ice mixture in relation to the fluence: the N2O molecule is destroyed to a much greater extent throughout the experiment than CO2, for which the final concentration is 4 times lower than at the beginning of the experiment. Table 2. IR band characteristics for an N2O:CO2 ice mixture at 11 K: band assignment, wavenumber, and A-value for the non-irradiated ice (A|$v$|(0)). Data in bold correspond to the two reference bands for each primary molecule. Molecule Assignment Wavenumber (this work) (cm−1) Literature wavenumber (cm−1) Absorption coefficient (× 10−18 cm molecule−1) CO2 υ1 + υ3 3708 3708 1.4a CO2 2υ2 + υ3 3600 3600 0.45a N2O υ1 + υ3 3516 3509 1.9b N2O 2υ1 2585 2581 1.5c N2O υ1 + 2υ2 2471 2468 2.7b CO2 υ3 2344 2343 76.0a 13CO2 υ3 2282 2283 78.0a N2O υ3 2245 2240 56.9c N2O υ1 1296 1295 10.7c N2O 2υ2 1165 1165 0.32b CO2 υ2 655 660 11.0a Molecule Assignment Wavenumber (this work) (cm−1) Literature wavenumber (cm−1) Absorption coefficient (× 10−18 cm molecule−1) CO2 υ1 + υ3 3708 3708 1.4a CO2 2υ2 + υ3 3600 3600 0.45a N2O υ1 + υ3 3516 3509 1.9b N2O 2υ1 2585 2581 1.5c N2O υ1 + 2υ2 2471 2468 2.7b CO2 υ3 2344 2343 76.0a 13CO2 υ3 2282 2283 78.0a N2O υ3 2245 2240 56.9c N2O υ1 1296 1295 10.7c N2O 2υ2 1165 1165 0.32b CO2 υ2 655 660 11.0a a Gerakines et al. (1995), bde Barros et al. (2016a, 2011), cFulvio et al. (2009). Open in new tab Table 2. IR band characteristics for an N2O:CO2 ice mixture at 11 K: band assignment, wavenumber, and A-value for the non-irradiated ice (A|$v$|(0)). Data in bold correspond to the two reference bands for each primary molecule. Molecule Assignment Wavenumber (this work) (cm−1) Literature wavenumber (cm−1) Absorption coefficient (× 10−18 cm molecule−1) CO2 υ1 + υ3 3708 3708 1.4a CO2 2υ2 + υ3 3600 3600 0.45a N2O υ1 + υ3 3516 3509 1.9b N2O 2υ1 2585 2581 1.5c N2O υ1 + 2υ2 2471 2468 2.7b CO2 υ3 2344 2343 76.0a 13CO2 υ3 2282 2283 78.0a N2O υ3 2245 2240 56.9c N2O υ1 1296 1295 10.7c N2O 2υ2 1165 1165 0.32b CO2 υ2 655 660 11.0a Molecule Assignment Wavenumber (this work) (cm−1) Literature wavenumber (cm−1) Absorption coefficient (× 10−18 cm molecule−1) CO2 υ1 + υ3 3708 3708 1.4a CO2 2υ2 + υ3 3600 3600 0.45a N2O υ1 + υ3 3516 3509 1.9b N2O 2υ1 2585 2581 1.5c N2O υ1 + 2υ2 2471 2468 2.7b CO2 υ3 2344 2343 76.0a 13CO2 υ3 2282 2283 78.0a N2O υ3 2245 2240 56.9c N2O υ1 1296 1295 10.7c N2O 2υ2 1165 1165 0.32b CO2 υ2 655 660 11.0a a Gerakines et al. (1995), bde Barros et al. (2016a, 2011), cFulvio et al. (2009). Open in new tab 3.2 Product molecule IR bands No new features have been seen in the region 4500−2200cm−1; indeed, product molecule bands are observed in the region 2200−1000cm−1. As an example, Fig. 1(c) highlights the new bands appearing after 1 × 1012 ion cm−2 fluence. All IR bands attributed to product species are listed in Table 3. Table 3. IR-band characteristics for the observed product species: assignment, wavenumber position, wavelengths, and absorption coefficient (A-value). Molecule Assignment Wavenumber (this work) (cm−1) Wavelengths (⁠|$\mu$|m) Wavenumber literature value (cm−1) Absorption coefficient (10−18 cm molecule−1) CO υ1 2138 4.7 2139a 11.0a CO3 υ3 2045 4.9 2040b 89.0b NO υ1 1848→ 1869* 5.3 1869d 4.5c N2O4 υ9 1741 5.7 1741e 5.0e N2O2/N2O4 υ5 1765 5.7 1764f − N2O4 υ2 1621 6.1 1628f 59.5f NO3 υ1 1460 6,8 1471g 7.6g N2O3 υ1/υ3 1305 7.6 1303g 46.3g N2O4 υ11 1260 7.9 1262h 85.0h O3 υ1 1037 9.6 1040g 14.0g Molecule Assignment Wavenumber (this work) (cm−1) Wavelengths (⁠|$\mu$|m) Wavenumber literature value (cm−1) Absorption coefficient (10−18 cm molecule−1) CO υ1 2138 4.7 2139a 11.0a CO3 υ3 2045 4.9 2040b 89.0b NO υ1 1848→ 1869* 5.3 1869d 4.5c N2O4 υ9 1741 5.7 1741e 5.0e N2O2/N2O4 υ5 1765 5.7 1764f − N2O4 υ2 1621 6.1 1628f 59.5f NO3 υ1 1460 6,8 1471g 7.6g N2O3 υ1/υ3 1305 7.6 1303g 46.3g N2O4 υ11 1260 7.9 1262h 85.0h O3 υ1 1037 9.6 1040g 14.0g a Palumbo & Strazulla (1993), bBennet et al. (2004), cSicilia et al. (2012), dFateley, Bent & Crawford (1959), eFulvio et al. (2009), fJamieson, Mebel & Kaiser (2006), gJamieson et al. (2005), hGerakines et al. (2001). *The arrow means blue shift of the NO band along the irradiation. Open in new tab Table 3. IR-band characteristics for the observed product species: assignment, wavenumber position, wavelengths, and absorption coefficient (A-value). Molecule Assignment Wavenumber (this work) (cm−1) Wavelengths (⁠|$\mu$|m) Wavenumber literature value (cm−1) Absorption coefficient (10−18 cm molecule−1) CO υ1 2138 4.7 2139a 11.0a CO3 υ3 2045 4.9 2040b 89.0b NO υ1 1848→ 1869* 5.3 1869d 4.5c N2O4 υ9 1741 5.7 1741e 5.0e N2O2/N2O4 υ5 1765 5.7 1764f − N2O4 υ2 1621 6.1 1628f 59.5f NO3 υ1 1460 6,8 1471g 7.6g N2O3 υ1/υ3 1305 7.6 1303g 46.3g N2O4 υ11 1260 7.9 1262h 85.0h O3 υ1 1037 9.6 1040g 14.0g Molecule Assignment Wavenumber (this work) (cm−1) Wavelengths (⁠|$\mu$|m) Wavenumber literature value (cm−1) Absorption coefficient (10−18 cm molecule−1) CO υ1 2138 4.7 2139a 11.0a CO3 υ3 2045 4.9 2040b 89.0b NO υ1 1848→ 1869* 5.3 1869d 4.5c N2O4 υ9 1741 5.7 1741e 5.0e N2O2/N2O4 υ5 1765 5.7 1764f − N2O4 υ2 1621 6.1 1628f 59.5f NO3 υ1 1460 6,8 1471g 7.6g N2O3 υ1/υ3 1305 7.6 1303g 46.3g N2O4 υ11 1260 7.9 1262h 85.0h O3 υ1 1037 9.6 1040g 14.0g a Palumbo & Strazulla (1993), bBennet et al. (2004), cSicilia et al. (2012), dFateley, Bent & Crawford (1959), eFulvio et al. (2009), fJamieson, Mebel & Kaiser (2006), gJamieson et al. (2005), hGerakines et al. (2001). *The arrow means blue shift of the NO band along the irradiation. Open in new tab In order to analyse the evolution along the irradiation of the observed product features, some selected ranges of the FTIR spectra are presented in Fig. 6. Notice that only the NO product band at 1869 cm−1 presented a shift in wavenumber along the irradiation. This characteristic will be discussed in the next section. Figure 6. Open in new tabDownload slide IR spectra obtained for the N2O:CO2 ice at 11 K during irradiation (0–2.2 × 1012 ions cm−2). The evolution product species is presented in the ranges (a) 2300–1700cm−1, (b) 1700–1315 cm−1 and (c) 1315–900 cm−1. Figure 6. Open in new tabDownload slide IR spectra obtained for the N2O:CO2 ice at 11 K during irradiation (0–2.2 × 1012 ions cm−2). The evolution product species is presented in the ranges (a) 2300–1700cm−1, (b) 1700–1315 cm−1 and (c) 1315–900 cm−1. The column densities of product species are also determined by using equation (2) with the A-values listed in Table 3. Fig. 7 compares the decrease in the column densities of both primary molecules (N2O in Fig. 7a and CO2 in Fig. 7b) with the evolution of their product species, along the beam fluence. Figure 7. Open in new tabDownload slide Column-density evolutions of primary and product molecules. (a) for N2O and products and (b) for CO2 and products. Solid lines are aids to visulaization. Figure 7. Open in new tabDownload slide Column-density evolutions of primary and product molecules. (a) for N2O and products and (b) for CO2 and products. Solid lines are aids to visulaization. The column-density evolution of each product species behaves according to the expression used previously in a similar analysis (de Barros et al. 2011; Mejia et al. 2015): \begin{equation*} N_i (F) \approx N_0 \frac{\sigma _{{\rm f},i}}{\sigma _{\rm d} - \sigma _{{\rm d},i}} [\exp (-\sigma _{{\rm d},i} F) -\exp (-\sigma _{\rm d} F)], \end{equation*} (5) where Ni(F) is the column density of the product species i, and N0 is the initial column density of the primary molecule from which the product originated. The quantities σf, i and σd, i are its formation and destruction cross-sections, respectively, and are extracted by fitting the evolution of the column density Ni(F) by equation (5). The quantities σd and N0 were obtained previously by fitting their primary molecule evolutions (see Table 1). Table 4 presents the results obtained for all product species fitted by equation (5), and Fig. 8 shows the ratio evolution (Ni(F)/N0) as the beam fluence increases; the solid lines correspond to fittings with equation (5). Table 4 presents two distinct values found for the O3 molecule: because the primary molecule that formed it is not known, the O3 cross-section values were calculated for the two primary molecules integrally and independently of each other using equation (5). The σd, i value for O3, when formed 100 per cent by N2O, equals 10.5 × 10−13 cm2, and, when formed 100 per cent by CO2 is equal to 16.9 × 10−13 cm2. Regardless of the correct proportion of primary molecules in the formation of O3, itsσd, i must be between these two values. Figure 8. Open in new tabDownload slide Evolution of the column densities of product species produced by the N2O:CO2 radiolysis. Solid curves are fittings performed with equation (5). Data are normalized to N0 and exhibited in log-log scale to stress the linear dependence on F at low fluences. (a) For N2O product species and (b) for CO2 product species. Figure 8. Open in new tabDownload slide Evolution of the column densities of product species produced by the N2O:CO2 radiolysis. Solid curves are fittings performed with equation (5). Data are normalized to N0 and exhibited in log-log scale to stress the linear dependence on F at low fluences. (a) For N2O product species and (b) for CO2 product species. Table 4. Characteristics of N2O product species formed in the N2O:CO2 radiolysis. Species Position (cm−1) A|$v$| (10−18 cm molec−1) σf, i (10−13 cm2) σd, i (10−13 cm2) NO 1869 4.5 9.0 3.9 N2O4 1741 5.0 2.9 8.4 NO3 1460 7.6 2.1 13.1 N2O3 1305 46.3 2.2 3.1 O3* 1037 14.0 0.5 10.5 Species Position A|$v$| σf, i σd, i (cm−1) (10−18 cm molec−1) (10−13 cm2) (10−13 cm2) CO 2138 11.0 2.5 35.6 CO3 2045 10.0 0.05 64.7 O3** 1037 14.0 0.2 16.9 Species Position (cm−1) A|$v$| (10−18 cm molec−1) σf, i (10−13 cm2) σd, i (10−13 cm2) NO 1869 4.5 9.0 3.9 N2O4 1741 5.0 2.9 8.4 NO3 1460 7.6 2.1 13.1 N2O3 1305 46.3 2.2 3.1 O3* 1037 14.0 0.5 10.5 Species Position A|$v$| σf, i σd, i (cm−1) (10−18 cm molec−1) (10−13 cm2) (10−13 cm2) CO 2138 11.0 2.5 35.6 CO3 2045 10.0 0.05 64.7 O3** 1037 14.0 0.2 16.9 *Values obtained under the hypothesis that O3 was formed 100% by N2O, and **values obtained under the hypothesis that O3 was formed 100% by CO2. Open in new tab Table 4. Characteristics of N2O product species formed in the N2O:CO2 radiolysis. Species Position (cm−1) A|$v$| (10−18 cm molec−1) σf, i (10−13 cm2) σd, i (10−13 cm2) NO 1869 4.5 9.0 3.9 N2O4 1741 5.0 2.9 8.4 NO3 1460 7.6 2.1 13.1 N2O3 1305 46.3 2.2 3.1 O3* 1037 14.0 0.5 10.5 Species Position A|$v$| σf, i σd, i (cm−1) (10−18 cm molec−1) (10−13 cm2) (10−13 cm2) CO 2138 11.0 2.5 35.6 CO3 2045 10.0 0.05 64.7 O3** 1037 14.0 0.2 16.9 Species Position (cm−1) A|$v$| (10−18 cm molec−1) σf, i (10−13 cm2) σd, i (10−13 cm2) NO 1869 4.5 9.0 3.9 N2O4 1741 5.0 2.9 8.4 NO3 1460 7.6 2.1 13.1 N2O3 1305 46.3 2.2 3.1 O3* 1037 14.0 0.5 10.5 Species Position A|$v$| σf, i σd, i (cm−1) (10−18 cm molec−1) (10−13 cm2) (10−13 cm2) CO 2138 11.0 2.5 35.6 CO3 2045 10.0 0.05 64.7 O3** 1037 14.0 0.2 16.9 *Values obtained under the hypothesis that O3 was formed 100% by N2O, and **values obtained under the hypothesis that O3 was formed 100% by CO2. Open in new tab 4 DISCUSSION 4.1 Primary species radiolysis The dominant effect produced by a MeV-projectile traversing a solid target formed by condensed gases is the electronic excitation of molecules (including ionization). Following impact by multi-ionized projectiles, many bonds of the same molecule may be broken, causing strong repulsion among the fragments (particularly of the anions) in a phenomenon called Coulomb explosion. High-temperature plasma (103−104 K) is then formed along the projectile trajectory (infratrack) on a picosecond time-scale (Fleischer, Price & Walker 1965; Vineyard 1976; Sundqvist et al. 2012). The secondary electrons emitted by the infratrack cross a much larger peripheral region (ultratrack), in which they produce other excitations and/or are captured (Mejia et al. 2013, section 6.1). During the relaxation (102−103 ps), the molecular synthesis occurring in the infra- and ultratrack regions may be distinct owing to different chemical conditions. In the former, the region is positively charged, and the ionization rate, temperature rise and atomization level are very high. In the latter, most of the region is negatively charged; because the temperature rise is not as high as in other case, the kinetic energies of molecular fragments are comparable to the activation energies of the dissociation potential barriers. Moreover, the low atomization level and the relatively long time for interactions in the ultratrack allow quasi-equilibrium processes and a treatment similar to a 'thermodynamics-based' may be attempted. their Recalling that electron–electron processes are very fast (fs-ns), collective interaction between molecules (characteristic of solid-state properties) may be neglected on this time-scale; therefore, molecular dissociations in the ultratrack should resemble those of the gas phase, in which molecules are fairly independent. Assuming that this process efectively occurs and restricting the current discussion to excitation energies, the most likely dissociation channels are as follows: (a) the neutral excited primary molecule dissociates into two neutral fragments; (b) the primary molecule is a singly ionized unstable cation that dissociates into a cation and a neutral fragment; and (c) a secondary electron is captured by a molecule that dissociates into an anion and a neutral. Dissociation energies may be used as a procedure to sort the preferential dissociation channels. The dissociation energies relevant for the N2O + CO2 system are summarized in Table 5. Table 5. Dissociation and ionization energies of the CO2 and NO2 primary molecules and products. Process Energy (eV) Neutral N2O* → N2 + O 1.67a primary molecule N2O* → NO + N 4.90b dissociation CO2* → CO + O 7.59c CO2* → C + O2 11.44c Primary molecule N2O → N2O+ 12.89c ionization CO2 → CO|$_2^+$| 13.78c NO* → N + O 2.07c Product O2* → O + O 5.07c dissociation N2* → N + N 10.36c CO* → C + O 11.10c NO → NO+ 9.26c C → C+ 11.25c Product O2 → O|$_2^+$| 12.07c ionization O → O+ 13.62c CO → CO+ 14.01c N → N+ 14.53c N2 → N|$_2^+$| 15.58c Ionized CO|$_2^+$|* → CO + O+ 7.79c primary molecule CO|$_2^+$|* → CO+ + O 8.18 dissociation N2O+* → NO+ + N 16.50c N2O+* → N|$_2^+$| + O 18.11c Ionized CO+* → C+ + O 7.80c product O|$_2^+$|* → O+ + O 18.73c dissociation NO+* → N + O+ 20.00c N|$_2^+$|* → N+ + N 20.00c Process Energy (eV) Neutral N2O* → N2 + O 1.67a primary molecule N2O* → NO + N 4.90b dissociation CO2* → CO + O 7.59c CO2* → C + O2 11.44c Primary molecule N2O → N2O+ 12.89c ionization CO2 → CO|$_2^+$| 13.78c NO* → N + O 2.07c Product O2* → O + O 5.07c dissociation N2* → N + N 10.36c CO* → C + O 11.10c NO → NO+ 9.26c C → C+ 11.25c Product O2 → O|$_2^+$| 12.07c ionization O → O+ 13.62c CO → CO+ 14.01c N → N+ 14.53c N2 → N|$_2^+$| 15.58c Ionized CO|$_2^+$|* → CO + O+ 7.79c primary molecule CO|$_2^+$|* → CO+ + O 8.18 dissociation N2O+* → NO+ + N 16.50c N2O+* → N|$_2^+$| + O 18.11c Ionized CO+* → C+ + O 7.80c product O|$_2^+$|* → O+ + O 18.73c dissociation NO+* → N + O+ 20.00c N|$_2^+$|* → N+ + N 20.00c aParkes et al. (1967), bReuben & Linnett (1959), cLinstrom & Mallard (2001). Open in new tab Table 5. Dissociation and ionization energies of the CO2 and NO2 primary molecules and products. Process Energy (eV) Neutral N2O* → N2 + O 1.67a primary molecule N2O* → NO + N 4.90b dissociation CO2* → CO + O 7.59c CO2* → C + O2 11.44c Primary molecule N2O → N2O+ 12.89c ionization CO2 → CO|$_2^+$| 13.78c NO* → N + O 2.07c Product O2* → O + O 5.07c dissociation N2* → N + N 10.36c CO* → C + O 11.10c NO → NO+ 9.26c C → C+ 11.25c Product O2 → O|$_2^+$| 12.07c ionization O → O+ 13.62c CO → CO+ 14.01c N → N+ 14.53c N2 → N|$_2^+$| 15.58c Ionized CO|$_2^+$|* → CO + O+ 7.79c primary molecule CO|$_2^+$|* → CO+ + O 8.18 dissociation N2O+* → NO+ + N 16.50c N2O+* → N|$_2^+$| + O 18.11c Ionized CO+* → C+ + O 7.80c product O|$_2^+$|* → O+ + O 18.73c dissociation NO+* → N + O+ 20.00c N|$_2^+$|* → N+ + N 20.00c Process Energy (eV) Neutral N2O* → N2 + O 1.67a primary molecule N2O* → NO + N 4.90b dissociation CO2* → CO + O 7.59c CO2* → C + O2 11.44c Primary molecule N2O → N2O+ 12.89c ionization CO2 → CO|$_2^+$| 13.78c NO* → N + O 2.07c Product O2* → O + O 5.07c dissociation N2* → N + N 10.36c CO* → C + O 11.10c NO → NO+ 9.26c C → C+ 11.25c Product O2 → O|$_2^+$| 12.07c ionization O → O+ 13.62c CO → CO+ 14.01c N → N+ 14.53c N2 → N|$_2^+$| 15.58c Ionized CO|$_2^+$|* → CO + O+ 7.79c primary molecule CO|$_2^+$|* → CO+ + O 8.18 dissociation N2O+* → NO+ + N 16.50c N2O+* → N|$_2^+$| + O 18.11c Ionized CO+* → C+ + O 7.80c product O|$_2^+$|* → O+ + O 18.73c dissociation NO+* → N + O+ 20.00c N|$_2^+$|* → N+ + N 20.00c aParkes et al. (1967), bReuben & Linnett (1959), cLinstrom & Mallard (2001). Open in new tab Considering the neutral excited primary molecules, the following conclusions can be deduced from this analysis: N2O dissociation is much more likely than CO2 dissociation; N2O dissociation into N2 + O is preferred to that into NO + N, nevertheless the two processes are energetically competitivee; if N2 is formed, this molecule is very stable (triple bond); CO2 dissociation into CO + O is more likely than that into C + O2; if CO is formed, this molecule is very stable (triple bond); both CO2 and N2O dissociations deliver (active) atomic oxygen in the ice; the production of free carbon atoms requires more energy than that of free nitrogen atoms; NO2 ionization is more likely than CO2 ionization. From the ionized primary molecules and fragments, on deduce that: CO|$_2^+$| dissociation is much more likely than N2O+ dissociation; CO|$_2^+$| dissociation into CO + O+ and that into CO+ + O are competitive; CO+ dissociation is much more likely than CO dissociation; N2O+ dissociation into NO+ + N is more likely than that into N|$_2^+$| + O; N|$_2^+$| and NO+ dissociations are unlikely since both species are very stable. From the experimental results presented in Table 1, it follows that the N2O destruction cross-section is more than 3 times higher than that of CO2. Based on the Table 5 data, the faster destruction rate of N2O molecules may be interpreted from the fact that the N2O molecule has a single N-O bond, whereas CO2 has two double bonds (O=C=O). In other words, the dissociation CO2|$\longrightarrow$| CO + O needs 7.59 eV to occur, while N2O |$\longrightarrow$| N2 + O requires only 1.67 eV (Parkes et al. 1967). Furthermore, the projectile generates secondary electrons inside the target; if they are captured by neighbouring N2O molecules, the reaction N2O−|$\longrightarrow$| N2 + O− is triggered, enhancing the N2O concentration decrease and delivering more N2 molecules around the projectile track (Xia et al. 2012). The distinct destruction rates can be confirmed by analysing the ice-mixture relative concentration presented in Fig. 5, where the ice-mixture concentration at the end of the irradiation is 4 times lower than the initial concentration. 4.2 Product species 4.2.1 Product species formation and radiolysis From Fig. 8, it can be seen that the column densities of all product species increase linearly in the low-fluence regime (up to F ∼ 5.0 × 1011 ions cm−1). Indeed, at the beginning of irradiation, because the projectile tracks do not overlap each other, the observed compounds are direct products from the primary molecule dissociation and from chemical reactions between the product species, which occur only inside the projectile track. The product column densities reach a maximum for two reasons: the decrease of the primary molecules' column densities with fluence and the increase of the destruction rate of product species (e.g. see equation 5 in Almeida et al. 2017). Equation (5) predicts that the maximum of Ni(F) should occur for all product species at Fmax ∼ 1/σd. Using the cross-sections presented in Tables 1 and 4, Fmax is approximately 1 × 1012 ions cm−2. This is, in fact, the value of fluence observed for the data displayed in Figs 8(a) and (b). Table 4 shows that σd,i values of CO2 product species are almost one order of magnitude higher than those of the N2O species. This characteristic is verified in Fig. 7(b), which shows the column densities of the CO2 product regime. Two possible explanations for the faster destruction of the carbon-content molecules are: (a) the relative low disponibility of free carbon atoms in the ice mixture and (b) that, after ionization, NO+ ions have a much higher dissociation barrier (20 eV) than CO+ ions (7.8 eV), as displayed in Table 5. Taking into account that the total number of each atomic species must be preserved in radiolysis, the sum of all related product molecule formation cross-sections σfi should not exceed the primary molecule destruction cross-section (de Barros et al. 2016a, b). In other words, during the whole irradiation, the number of N−, C− and O− atoms existent in all product molecules cannot be higher than those of the dissociated primary molecules. This fact can be checked by atomic budget analysis (Bennet et al. 2006; de Barros et al. 2011), in which the total number of atoms rearranged during the irradiation should be conserved. Table 6 presents the atomic budget for the current measurement, where it can be observed that the column-density percentage of the product species does not exceed that of the primary molecules carbon and oxygen. Alternatively, the same conclusion is obtained by inspecting data in Figs 9(a), (b) and (c), which show the dependence on fluence of the nitrogen, carbon and oxygen column density, respectively. Comparing, for these three atomic species, the behaviour of the atomic column density of the primary molecules (N0 - N) and of the product species (Ni) throughout the irradiation, it is also verified that the atomic column densities of the product species do not exceed those of the primary molecules. Table 6. Summary of nitrogen, carbon and oxygen atom budgets: primary and product molecule species, atomic column-density variation (ΔN = Nf - N0, in 1016 molec cm−2) and atomic yield (number of atoms destroyed or produced per projectile) in 104. The fluence at the end of irradiation was Ff = 2.2 × 1012 ions cm−2. A-values are given in Tables 2 and 3. Species ΔN nitrogen ΔN carbon ΔN oxygen Yield nitrogen Yield carbon Yield oxygen primary molecules N2O −30.42 - −15.21 −13.83 - −6.91 CO2 - −23.91 −47.81 − −10.87 −21.73 Formed Species* NO 5.76 – 5.76 2.62 – 2.62 N2O4 2.12 – 4.24 0.96 – 1.93 N2O2 0.05 – 0.05 0.02 – 0.02 N2O4 0.06 – 0.12 0.03 – 0.05 NO3 0.18 – 0.55 0.08 – 0.25 N2O5 3.02 – 7.55 1.37 – 3.43 N2O4 0.13 – 0.27 0.06 – 0.12 CO – 0.48 0.48 – 0.22 0.22 CO3 – 0.005 0.015 – 0.002 0.007 O3 – – 0.89 – – 0.40 Total 9.6 7.9 16.1 4.4 0.2 7.3 32% 2% 26% Species ΔN nitrogen ΔN carbon ΔN oxygen Yield nitrogen Yield carbon Yield oxygen primary molecules N2O −30.42 - −15.21 −13.83 - −6.91 CO2 - −23.91 −47.81 − −10.87 −21.73 Formed Species* NO 5.76 – 5.76 2.62 – 2.62 N2O4 2.12 – 4.24 0.96 – 1.93 N2O2 0.05 – 0.05 0.02 – 0.02 N2O4 0.06 – 0.12 0.03 – 0.05 NO3 0.18 – 0.55 0.08 – 0.25 N2O5 3.02 – 7.55 1.37 – 3.43 N2O4 0.13 – 0.27 0.06 – 0.12 CO – 0.48 0.48 – 0.22 0.22 CO3 – 0.005 0.015 – 0.002 0.007 O3 – – 0.89 – – 0.40 Total 9.6 7.9 16.1 4.4 0.2 7.3 32% 2% 26% *For molecular species with more than one nitrogen or oxygen atom, the atomic column density for each species is obtained by multiplying the molecular column density by the number of atoms of that species in the molecule. Open in new tab Table 6. Summary of nitrogen, carbon and oxygen atom budgets: primary and product molecule species, atomic column-density variation (ΔN = Nf - N0, in 1016 molec cm−2) and atomic yield (number of atoms destroyed or produced per projectile) in 104. The fluence at the end of irradiation was Ff = 2.2 × 1012 ions cm−2. A-values are given in Tables 2 and 3. Species ΔN nitrogen ΔN carbon ΔN oxygen Yield nitrogen Yield carbon Yield oxygen primary molecules N2O −30.42 - −15.21 −13.83 - −6.91 CO2 - −23.91 −47.81 − −10.87 −21.73 Formed Species* NO 5.76 – 5.76 2.62 – 2.62 N2O4 2.12 – 4.24 0.96 – 1.93 N2O2 0.05 – 0.05 0.02 – 0.02 N2O4 0.06 – 0.12 0.03 – 0.05 NO3 0.18 – 0.55 0.08 – 0.25 N2O5 3.02 – 7.55 1.37 – 3.43 N2O4 0.13 – 0.27 0.06 – 0.12 CO – 0.48 0.48 – 0.22 0.22 CO3 – 0.005 0.015 – 0.002 0.007 O3 – – 0.89 – – 0.40 Total 9.6 7.9 16.1 4.4 0.2 7.3 32% 2% 26% Species ΔN nitrogen ΔN carbon ΔN oxygen Yield nitrogen Yield carbon Yield oxygen primary molecules N2O −30.42 - −15.21 −13.83 - −6.91 CO2 - −23.91 −47.81 − −10.87 −21.73 Formed Species* NO 5.76 – 5.76 2.62 – 2.62 N2O4 2.12 – 4.24 0.96 – 1.93 N2O2 0.05 – 0.05 0.02 – 0.02 N2O4 0.06 – 0.12 0.03 – 0.05 NO3 0.18 – 0.55 0.08 – 0.25 N2O5 3.02 – 7.55 1.37 – 3.43 N2O4 0.13 – 0.27 0.06 – 0.12 CO – 0.48 0.48 – 0.22 0.22 CO3 – 0.005 0.015 – 0.002 0.007 O3 – – 0.89 – – 0.40 Total 9.6 7.9 16.1 4.4 0.2 7.3 32% 2% 26% *For molecular species with more than one nitrogen or oxygen atom, the atomic column density for each species is obtained by multiplying the molecular column density by the number of atoms of that species in the molecule. Open in new tab From the carbon data behaviour presented in in Fig. 9(b), it can be seen that, at the end of irradiation, the carbon column density of all product species presents only 2 per cent of the destroyed CO2 column density. The reason for such a high discrepancy is not clear but must be related to the fact that the column densities of the observed C-bearing species achieve a maximum and then decrease with fluence (Fig. 8b). Possible explanations are: C and C2 production – these species cannot be observed since they are not IR-active; C3 production – this species is characterized by a small band at 2045 cm−1, which unfortunately coincides with that of CO3, at 2044 cm−1; re-formation of CO2 from CO + CO or from C + CO; sublimation of CO near the projectile impact site (CO ice sublimates at 27 K for high vacuum pressures; Ponciano et al. 2008); the A-value considered here for the CO band is not adequate, because it has been determined for pure CO or CO + H2O ices (and not for CO immersed in a N2O + CO2 matrix); this point is also pertinent for the calculation of the CO formation cross-section, which is A-value-dependent. Figure 9. Open in new tabDownload slide Atomic budget for the rearrangement of: (a) nitrogen atoms, (b) carbon atoms and (c) oxygen atoms. The solid curves correspond to the column-density variation (N0 - N(F)) for the primary molecules, and the dashed ones to the column density summed over all productspecies (∑Ni). Figure 9. Open in new tabDownload slide Atomic budget for the rearrangement of: (a) nitrogen atoms, (b) carbon atoms and (c) oxygen atoms. The solid curves correspond to the column-density variation (N0 - N(F)) for the primary molecules, and the dashed ones to the column density summed over all productspecies (∑Ni). There is a small discrepancy relative to nitrogen data: at the end of irradiation, the sum of the nitrogen column densities owing to product species represents 32 per cent of the nitrogen column density arising from the N2O destruction. Again, a possible explanation is N and N2 production – the latter species having very low IR activity in ice. Another explanation is the N diffusion near the track towards the ice surface and N2 desorption around the impact site (N2 ice sublimates at 37 K) (Ponciano et al. 2006). 4.2.2 Relevance of the CN and OCN species The CN- and OCN-bearing species (e.g. CN, XCN, R−OCN, HNC, O=C=N, O−C≡N, C≡N−O ions or radicals) acquired astrophysical interest when the 216-cm−1 (4.62-|$\mu$|m) IR band, discovered in the ISM by Soifer et al. (1979), was attributed to the XCN functional group (Lacy & Popper 1984). Because these compounds are primary molecules for amino acids and other bio-molecules, their astrobiological significance was also recognized (e.g. Palumbo et al. 2000). The synthesis of the CN and OCN species by irradiating ice mixtures with different ionizing beams has been performed in several laboratories. For instance, Palumbo et al. (2000) used a keV heavy ion beam, Moore & Hudson (2003) and Gerakines, Moore & Hudson (2004) used UV and MeV proton beams, while de Barros et al. (2016a) employed MeV heavy ions; all of the researchers followed the induced chemical reactions with FTIR spectroscopy. Martinez et al. (2014) bombarded CO + NH3 ice at 25 K with 252Cf fragments (MeV heavy ions) but analysed the desorbed ions with time-of-flight mass spectrometry; they found that CN- and OCN- ions are abundantly produced and emitted just after (ps) the projectile impact; in fact, the two most intense peaks in the mass spectrum correspond to these species, demonstrating their very fast synthesis. In a different approach, Raunier et al. (2003) produced OCN-bearing ions by the co-deposition of HCNO and NH3 ice at 10 K, without irradiation. Depending on the chemical environment (ice mixture), IR bands of the CN and OCN species are shifted; their wavenumbers reported in these works are listed in Table 7. Table 7. Infrared bands of CN- and OCN-bearing species reported in literature. Wavenumber (cm−1) Species Mode Ice mixture Ref. 3565 HNC NH stretch HNCO a, b 3365 - 3245 HNCO νas NCO stretch NH stretch HNCO b 3286 - 3245 HCN ν3 CH stretch CH4 + N2 (1:100) a N2 + CO + CH4 (100:1:1) 2266 - 2252 HNCO ν2 NCO N2 + CO + CH4 (100:1:1) a, b 2168 - 2151 OCN νas N2 + CO + CH4 (100:1:1) a, c, d NH3 + HCN (5:1) 2100 - 2096 HCN ν2 CN stretch NH3 + HCN (5:1) a, d N2 + CO + CH4 (100:1:1) 1942 - 1934 OCN ∑+3 asym. stretch N2 + CO (100:1) a 1798 HCN2 rad − N2 + CO + CH4 a 1335 - 1300 OCN− 2ν2 OCN bend NCO stretch NH|$_4^+$|OCN− in solid NH3 b 1212 OCN− ν1 OCN symmetric stretch NH|$_4^+$|OCN− in solid NH3 b 630 OCN− ν2 OCN bend ν6 NCO NH|$_4^+$|OCN− in solid NH3 b Wavenumber (cm−1) Species Mode Ice mixture Ref. 3565 HNC NH stretch HNCO a, b 3365 - 3245 HNCO νas NCO stretch NH stretch HNCO b 3286 - 3245 HCN ν3 CH stretch CH4 + N2 (1:100) a N2 + CO + CH4 (100:1:1) 2266 - 2252 HNCO ν2 NCO N2 + CO + CH4 (100:1:1) a, b 2168 - 2151 OCN νas N2 + CO + CH4 (100:1:1) a, c, d NH3 + HCN (5:1) 2100 - 2096 HCN ν2 CN stretch NH3 + HCN (5:1) a, d N2 + CO + CH4 (100:1:1) 1942 - 1934 OCN ∑+3 asym. stretch N2 + CO (100:1) a 1798 HCN2 rad − N2 + CO + CH4 a 1335 - 1300 OCN− 2ν2 OCN bend NCO stretch NH|$_4^+$|OCN− in solid NH3 b 1212 OCN− ν1 OCN symmetric stretch NH|$_4^+$|OCN− in solid NH3 b 630 OCN− ν2 OCN bend ν6 NCO NH|$_4^+$|OCN− in solid NH3 b (a) Moore & Hudson (2003); (b) Raunier et al. (2003); (c) Palumbo et al. (2000); (d) Gerakines et al. (2004). Open in new tab Table 7. Infrared bands of CN- and OCN-bearing species reported in literature. Wavenumber (cm−1) Species Mode Ice mixture Ref. 3565 HNC NH stretch HNCO a, b 3365 - 3245 HNCO νas NCO stretch NH stretch HNCO b 3286 - 3245 HCN ν3 CH stretch CH4 + N2 (1:100) a N2 + CO + CH4 (100:1:1) 2266 - 2252 HNCO ν2 NCO N2 + CO + CH4 (100:1:1) a, b 2168 - 2151 OCN νas N2 + CO + CH4 (100:1:1) a, c, d NH3 + HCN (5:1) 2100 - 2096 HCN ν2 CN stretch NH3 + HCN (5:1) a, d N2 + CO + CH4 (100:1:1) 1942 - 1934 OCN ∑+3 asym. stretch N2 + CO (100:1) a 1798 HCN2 rad − N2 + CO + CH4 a 1335 - 1300 OCN− 2ν2 OCN bend NCO stretch NH|$_4^+$|OCN− in solid NH3 b 1212 OCN− ν1 OCN symmetric stretch NH|$_4^+$|OCN− in solid NH3 b 630 OCN− ν2 OCN bend ν6 NCO NH|$_4^+$|OCN− in solid NH3 b Wavenumber (cm−1) Species Mode Ice mixture Ref. 3565 HNC NH stretch HNCO a, b 3365 - 3245 HNCO νas NCO stretch NH stretch HNCO b 3286 - 3245 HCN ν3 CH stretch CH4 + N2 (1:100) a N2 + CO + CH4 (100:1:1) 2266 - 2252 HNCO ν2 NCO N2 + CO + CH4 (100:1:1) a, b 2168 - 2151 OCN νas N2 + CO + CH4 (100:1:1) a, c, d NH3 + HCN (5:1) 2100 - 2096 HCN ν2 CN stretch NH3 + HCN (5:1) a, d N2 + CO + CH4 (100:1:1) 1942 - 1934 OCN ∑+3 asym. stretch N2 + CO (100:1) a 1798 HCN2 rad − N2 + CO + CH4 a 1335 - 1300 OCN− 2ν2 OCN bend NCO stretch NH|$_4^+$|OCN− in solid NH3 b 1212 OCN− ν1 OCN symmetric stretch NH|$_4^+$|OCN− in solid NH3 b 630 OCN− ν2 OCN bend ν6 NCO NH|$_4^+$|OCN− in solid NH3 b (a) Moore & Hudson (2003); (b) Raunier et al. (2003); (c) Palumbo et al. (2000); (d) Gerakines et al. (2004). Open in new tab It turns out from this short review that, depending on the ice mixture and temperature, CN- and OCN-bearing species may be produced spontaneously (from HNCO + NH3), by irradiation (N2 + CH4 or CO + NH3 targets) or by irradiation and warming up afterwards (the 2168-cm−1 OCN peak appears from irradiated N2 + CH4 + CO ice after being warmed up to 35 K). For the mixture studied in the current work, N2O:CO2 (1:2) at 11 K, the formation of CN or OCN species was not observed. Fig.10 shows IR spectra for relevant ranges and, indeed, the 2165-, 2083-, 1937- and 1212-cm−1 bands are definitely not present. No peaks in the range 645−630cm−1 are seen. Unfortunately, no IR spectrum was acquired during the warming of the irradiated sample, so we do not know if one of these bands would appear. Figure 10. Open in new tabDownload slide Representation of the absence of CN- and OCN-bearing species in the ice IR spectrum: (a) range 2280–2140cm−1, (b) range 2110–1100cm−1. Figure 10. Open in new tabDownload slide Representation of the absence of CN- and OCN-bearing species in the ice IR spectrum: (a) range 2280–2140cm−1, (b) range 2110–1100cm−1. The following observations can be made from the above results. Intense atomization occurs in the core of the projectile track when MeV heavy ions traverse solids constituted by molecules. Once the local plasma cools down, the ions and free radicals should form chemical species according to the thermodynamical conditions. In particular, triple-bond species such as C≡N and O−C≡N are expected to be formed from C- and N-bearing molecules present in the irradiated ice. Indeed, mass spectrometry confirms that CN and OCN anions are the most abundant anions ejected after MeV impact Martinez et al. (2014). From (i), we would expect to observe CN and the CNO formation, but we did not. An issue for the contradiction with the previous discussion is that the vast majority of the synthesized molecules are not produced in the core track but around it. In this peripheral region, the local temperature is not so high just after the projectile passage and thermal variations are not so fast, so that quasi-equilibrium processes support thermodynamical analysis. Assuming that hot chemistry is valid around the track, it is expected that N2O and CO2 dissociate preferentially into N2 + O and CO + O, respectively. Because N2 and CO are triple-bond molecules, the chemical reaction between both is not likely and may be neglected with respect to other possible reactions. When irradiating N2 + CH4 + CO at 13 K with 0.8-MeV protons, Moore & Hudson (2003) observed HCN and HNCO formation, but not OCN. The OCN IR peak only appears after warming up the ice to 35 K, when the HNCO decomposes. A possible explation for this finding is that the OCN gathering is triggered by the presence of hydrogen for the formation of the stable isocyanic acid, HNCO; its decomposition into H+ + NCO− is easily performed by heating. In the current experiment, hydrogen is not available and, therefore, the above pathway for the OCN− formation does not exist. 4.3 The shift of the NO molecule peak Pure N2O ice was irradiated under the same conditions as the N2O:CO2 ice analyzed in current work (de Barros et al. (2016a); a shift was found for the 1869-cm−1 NO band. For the ice mixture, among all the analysed bands for both primaries and product species, the only feature that presents a shift along the irradiation is the same NO band. Comparing the two results, a similar blueshift behaviour (shift to a higher wavenumber) is found, with the maximum the wavenumber variation equal to 29 cm−1. Fig. 11 shows the profile of the NO band shift along the irradiation for both cases. Note that in Fig. 11 (b) the band shift reaches its maximum value and remains constant at the fluence 1 × 1012 ions cm−2. As shown in Fig. 11 (a), the shift occurs gradually as fluence increases and only reaches its maximum value at the fluence 2 × 1012 ions cm−2. Figure 11. Open in new tabDownload slide Evolution of the NO 1869-cm−1 band shift during irradiation: (a) for the N2O:CO2 ice at 11 K (this work), and (b) for pure N2O ice at 11 K (de Barros et al. 2016a). Figure 11. Open in new tabDownload slide Evolution of the NO 1869-cm−1 band shift during irradiation: (a) for the N2O:CO2 ice at 11 K (this work), and (b) for pure N2O ice at 11 K (de Barros et al. 2016a). In order to investigate the origins of the shift phenomenon, a relationship between the shift and the relative concentration of NO in both cases (N2O:CO2 and pure N2O) was sought. For pure N2O ice, the relative concentration (RC(F)) of the NO molecule was determined as the ratio of its column density (NNO(F)) and the sum of the column densities of the N2O fundamental band (N|$_{\rm N_2O}$|(F)) and of all the other product species generated by N2O : \begin{equation*} RC(F) = \frac{N_{\rm NO}(F)}{N_{\rm N_2O}(F)+\sum _i N_i(F)}.\end{equation*} (6) For the case of N2O:CO2, the sum of the fundamental bands of N2O and CO2 with all the product species generated was performed: \begin{equation*} RC(F) = \frac{N_{\rm NO}(F)}{N_{\rm N_2O}(F)+N_{\rm CO_2}(F)+\sum _i N_i(F)}. \end{equation*} (7) Fig. 12 shows the behaviour of the NO shift as a function of its relative concentration for both the pure N2O ice matrix and the N2O:CO2 mixture. Figure 12. Open in new tabDownload slide Behaviour of the NO shift relative concentration for N2O:CO2 (this work) and pure N2O (de Barros et al. 2016a). Figure 12. Open in new tabDownload slide Behaviour of the NO shift relative concentration for N2O:CO2 (this work) and pure N2O (de Barros et al. 2016a). The two curves presented in Fig. 12 show a decrease of the NO peak shift as the NO relative concentration increases; however, the decrease in pure N2O seems to be roughly exponential, whereas the dependence in N2O:CO2 is linear. Thus, the initial hypothesis that the shift of this molecule was directly influenced by the change in its relative concentration was discarded, creating space for future discussions about this phenomenon. 5 CONCLUSIONS AND ASTROPHYSICAL IMPLICATIONS From the results presented in Section 3 and discussed in Section 4, we can draw the following conclusions. None of the N-C compounds were found in the N2O:CO2 radiolysis. Even when undergoing radiolysis in a quite homogenous mixture, N and C atoms do not react with each other, at least significantly. In particular, the bands HNCO at 2266 cm−1 (Moore & Hudson 2003), R-OCN at 2165 cm−1 (Palumbo et al. 2000), CN− at 2083 cm−1 (Moore & Hudson 2003), OCN at 1934 cm−1 (Strazzulla & Palumbo 2001) and 1212 cm−1 (Raunier et al. 2003) are not seen in the spectra presented in Fig. 10. The N2O product species formation cross-sections are 4 times larger than those found by de Barros et al. (2016a) when irradiating pure N2O. This fact can be explained by the extra supply of active oxygen from CO2 dissociation existing in our ice mixture. Among the produced N-O-containing molecules, NO and N2O4 are the main species produced by ion irradiation of the N2O:CO2 mixture. The production of NO is almost one order of magnitude higher than that of N2O4. Other minor N-O-bearing species produced by ion irradiation are NO3 and N2O3. Among all the produced N-O-containing species, only NO presented a peak shift as a function of ion irradiation. The shift was at most equal to 29 cm−1 and it is comparable in magnitude of the shift to the one observed for the NO produced when irradiating pure N2O by 90-MeV 136Xe23 + (studied by de Barros et al. 2016a). The N2O destruction cross-section is approximately 2 times that of CO2. This fact could be caused by the decrease of the initial mixture concentration, namely 4 times less than the initial concentration. Among the C-O product species, CO is the main species produced by ion irradiation of the N2O:CO2 mixture considered in this study. Besides this, the O molecule made available by the ion irradiation goes in particular to O3 formation. The atomic budget analysis shows a relatively good agreement between the primary and product atoms. However, a systematic discrepancy (factor of 2 or 3) is observed for the nitrogen atoms. This is attributed to the production of N2, which is hardly seen in IR spectroscopy. N-O molecules are considered to be important astrochemical primary molecules of complex and prebiotic species, and therefore the results presented in this study will help to unveil the chemistry of N-O in space. In fact, N-O chemistry in space is at present poorly understood. For instance, there are a number of open questions regarding the lack of detection of N-O-bearing molecules in the solid phase in space, despite the fact that in the gas phase NO, N2O and HNO have already been detected (so far in low- and high-mass star-forming regions). Indeed, it is expected that in the ISM some of these N-O-bearing species could freeze-out on to dust grains. It is also reasonable to expect that some N-O-bearing species could be formed in interstellar frozen grain mantles and in the frozen surface of outer SS bodies as a consequence of the radiation processing of commonly identified ices such as CO, N2, CH4 and H2O (e.g. Boduch et al. 2012; Sicilia et al. 2012). Such ices on interstellar grain mantles and on the surface of outer SS bodies are continually exposed to galactic cosmic rays, solar wind ions and UV photons. These reactions may be simulated in the laboratory by bombarding the selected ice or icy mixtures with keV/MeV ion beams (see, for instance, Kaiser & Roessler 1998; Hudson, Moore & Raines 2009; Fulvio et al. 2010; Lv et al. 2012; Raut & Baragiola 2013; de Barros et al. 2016b). With all this in mind, we have recently started an extensive laboratory investigation aimed at understanding the effects of cosmic irradiation on N-O-bearing molecules in space. In parallel to laboratory experiments focusing on ices of pure N-O-bearing molecules (N2O, NO2, NO, etc.) we started, with the current work, to perform ion irradiation experiments on multi-component icy mixtures of N-O-bearing molecules mixed with some of the common ices found in interstellar frozen grain mantles and/or on the frozen surfaces of SS bodies, such as CO2, H2O, CO, among others. We have thus presented here a detailed study of the effects induced by 90-MeV 136Xe23 + ion irradiation of N2O:CO2 (1:2) ice mixture, at 11 K, and the consequent formation of product containing N-O- and C-O-bearing molecules. Taking into account the results of this study and also other ion irradiation experiments performed on pure N-O-bearing ices or icy mixtures such as H2O:N2, H2O:N2:O2, N2:O2 and CO:N2, it is seen that NO always represents one of the main irradiation products (Boduch et al. 2012; Sicilia et al. 2012; Fulvio et al. 2018). In particular, we point out that in a parallel laboratory study we have recently estimated that, in the outer SS and ISM, ion irradiation of N2O ice and NO2:N2O4 ice mixtures should produce relatively large amounts of NO ice within time-scales of about 105–107 yr. The detection, in space, of gas-phase NO and the simultaneous lack of any detection of solid NO therefore represents an intriguing observation that only dedicated irradiation experiments performed on NO ice can help us to finally unveil and understand (Fulvio et al. 2018). ACKNOWLEDGEMENTS This work was supported by the French–Brazilian exchange program CAPES-COFECUB. We are grateful to T. Been, C. Grygiel, A. Domaracka and J. M. Ramillon for their invaluable support. The Brazilian agencies CNPq (INEspaço), CNPq (301868/2017-4), CAPES (BEX 5383/15-3) and FAPERJ (E-26/213.577/2015, E-26/216.730/2015, E-05/2015-214814 and E-03/2017-203.204) are also acknowledged. REFERENCES Ackerman M. , Fontanella J. 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TI - Radiolysis of N2O:CO2 ice by heavy ions: simulation of cosmic ray effects
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/sty1519
DA - 2018-08-21
UR - https://www.deepdyve.com/lp/oxford-university-press/radiolysis-of-n2o-co2-ice-by-heavy-ions-simulation-of-cosmic-ray-bk5o7cX8a1
SP - 4939
VL - 478
IS - 4
DP - DeepDyve
ER -