Heavy ion irradiation of condensed CO: sputtering and molecule formation

¹ Centre de Recherche sur les Ions, les Matériaux et la Photonique (CEA /CNRS /ENSICAEN /Université de Caen–Basse Normandie), CIMAP – CIRIL – Ganil, Boulevard Henri Becquerel, BP 5133, 14070 Caen Cedex 05, France e-mail: esduarte@ganil.fr, esduarte@gmail.com
² Physics Department, Pontifícia Universidade Católica, Rua Marquês de S. Vicente 225, 22453-900 Rio de Janeiro, Brazil
³ Grupo de Física e Astronomia – CEFET/Química de Nilópolis, Rua Lúcio Tavares 1045, Centro, 26530-060 Nilópolis, Brazil
⁴ Institut d'Astrophysique Spatiale, Astrochimie Expérimentale, UMR-8617 Université Paris-Sud, bâtiment 121, 91405 Orsay, France
⁵ Physics Department, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil

Received: 16 November 2008
Accepted: 28 April 2009

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. C¹⁸O₂ was condensed on a CsI substrate at 13 K and it was irradiated by 46 MeV ⁵⁸Ni¹¹⁺ up to a final fluence of  cm^-2 at a flux of  cm^-2 s^-1. The ice was analyzed in situ by infrared spectroscopy (FTIR) in the  cm^-1 range.

Results. The CO₂ destruction was observed, as well as the formation of other species such as CO, CO₃, O₃, and C₃. The destruction cross section of CO₂ is found to be  cm², while those for the formation of CO, CO₃, and O₃ molecules are  cm²,  cm², and  cm², respectively. The sputtering yield of the CO₂ ice is  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.