Heavy ion irradiation of condensed CO$\mathsf{_{2}}$: sputtering and molecule formation

Abstract

Context. Ices present in different astrophysical environments are exposed to ion irradiation from cosmic rays (H to heavier than Fe) in the keV to GeV energy range. Aims. The objective of this work is to study the effects produced in astrophysical ices by heavy ions at relatively high energies (MeV) in the electronic energy loss regime and compare them with those produced by protons. Methods. C 18 O 2 was condensed on a CsI substrate at 13 K and it was irradiated by 46 MeV 58 Ni 11+ up to a final fluence of 1.5 × 10 13 cm −2 at a flux of 2 × 10 9 cm −2 s −1 . The ice was analyzed in situ by infrared spectroscopy (FTIR) in the 5000−600 cm −1 range. Results. The CO 2 destruction was observed, as well as the formation of other species such as CO, CO 3 , O 3 , and C 3 . The destruction cross section of CO 2 is found to be 1.7 × 10 −13 cm 2 , while those for the formation of CO, CO 3 , and O 3 molecules are 1.6 × 10 −13 cm 2 , 4.5 × 10 −14 cm 2 , and 1.5 × 10 −14 cm 2 , respectively. The sputtering yield of the CO 2 ice is 4.0 × 10 4 molecules/impact, four orders of magnitude higher than for H projectiles at the same velocity. This allows us to estimate the contribution of the sputtering by heavy ions as compared to protons in the solar winds and in cosmic rays.Conclusions. The present results show that heavy ions play an important role in the sputtering of astrophysical ices. Furthermore, this work confirms the quadratic stopping power dependence of sputtering yields.

Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

“…At any rate, the effect of sputtering on N 1 (F) would be more evident at high fluences owing to thickness reduction, which is not seen in the current data (Seperuelo Duarte et al 2009. (iv) To obtain a solvable system of equations we have to make the assumption that the formation of a given daughter species (i = k) can be treated independently of the others; i.e., the projectile does not produce a daughter species from other pre formed daughter species: σ f,i j ≈ 0, where i and j hold for two daughters.…”

“…The experimental set-up has been described previously (Seperuelo Duarte et al 2009Pilling et al 2010). In the center of a high vacuum chamber, which is evacuated down to 10 −8 mbar, a closed cycle helium cryostat keeps the CsI substrate at 15 K. To analyze the ice sample, a Nicolet Magna 550 Fourier Transform Infrared Spectrometer running in transmission mode perpendicular to the sample surface was used.…”

“…Figure 6 shows the evolution of CH 4 column density as a function of 220 MeV oxygen beam fluence. The decreasing CH 4 column density is related to the formation of other species and to the sputtering induced by heavy ions (Seperuelo Duarte et al 2009Pilling et al 2010). The behavior of these data is assumed to be described by the system of differential equations:…”

“…Heavy galactic cosmic rays (HGCR) are considered as the main agent for keeping a significant fraction of gas phase material in molecular clouds (Leger et al 1985;Hasegawa & Herbst 1993;Willacy & Williams 1993;Bergin et al 1995;Shematovich et al 1997;Willacy & Millar 1998;Nguyen et al 2002). HGCRs are efficient for desorbing molecules from the surfaces (Seperuelo Duarte et al 2009. They also produce secondary UV photons inside the cloud, which may induce the photodesoption of molecules (Öberg et al 2007, 2009).…”

“…These desorption mechanisms compete with the gas condensation, thereby setting a steady-state chemistry condition within molecular clouds (Charnley et al 2001). Irradiation of astrophysical ices, such as H 2 O, CO 2 , CO, and NH 3 , by heavy ions have been recently investigated either by plasma desorption mass spectrometry (Collado et al 2004;Farenzena et al 2005;Ponciano et al 2006;Martinez et al 2006) or by infrared spectroscopy (FTIR) (Seperuelo Duarte et al 2009Pilling et al 2010). This kind of data may be useful for estimating desorption rates of the interstellar grains attributed to HGCR (Bringa & Johnson 2004).…”