Radiolysis of ammonia-containing ices by energetic, heavy, and highly charged ions inside dense astrophysical environments

¹ Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rua Marquês de São Vicente 225, CEP 22453-900, Rio de Janeiro, Brazil e-mail: sergiopilling@yahoo.com.br
² Instituto de Pesquisa & Desenvolvimento (IP&D), Universidade do Vale do Paraíba (UNIVAP), Av. Shishima Hifumi 2911, CEP 12244-000, São José dos Campos, SP, Brazil
³ 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
⁴ Grupo de Física e Astronomia, CEFET/Química de Nilópolis, Rua Lúcio Tavares 1052, CEP 2653-060, Nilópolis, Brazil

Received: 4 April 2009
Accepted: 20 October 2009

Abstract

Deep inside dense molecular clouds and protostellar disks, interstellar ices are protected from stellar energetic UV photons. However, X-rays and energetic cosmic rays can penetrate inside these regions triggering chemical reactions, molecular dissociation, and evaporation processes. We present experimental studies of the interaction of heavy, highly charged, and energetic ions (46 MeV ⁵⁸Ni¹³⁺) with ammonia-containing ices H₂O:NH₃ (1:0.5) and H₂O:NH₃:CO (1:0.6:0.4) in an attempt to simulate the physical chemistry induced by heavy-ion cosmic rays inside dense astrophysical environments. The measurements were performed inside a high vacuum chamber coupled to the IRRSUD (IR radiation SUD) beamline at the heavy-ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared spectrometer (FTIR) at different fluences. The average values of the dissociation cross-section of water, ammonia, and carbon monoxide due to heavy-ion cosmic ray analogs are ~, , and  cm², respectively. In the presence of a typical heavy cosmic ray field, the estimated half life of the studied species is 2– years. The ice compaction (micropore collapse) produced by heavy cosmic rays seems to be at least 3 orders of magnitude higher than that produced by (0.8 MeV) protons. The infrared spectra of the irradiated ice samples exhibit lines of several new species including HNCO, N₂O, OCN^-, and NH. In the case of the irradiated H₂O:NH₃:CO ice, the infrared spectrum at room temperature contains five bands that are tentatively assigned to vibration modes of the zwitterionic glycine (NHCH₂COO^-).