Browsing by Author "Frota Da silveira, Enio"

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A simple model for Ice compaction data induced by low energy Ion irradiation
(2015) Ferreira de Barros, Ana Lucía; Martínez Morgado, Welles Antonio; Frota Da silveira, Enio; Mejía Guamán, Christian Fernando; Almeida, L. F.
A connection between the compaction of amorphous solid water (ASW) during energetic ion irradiation and the disappearing of water dangling bonds (OH-db) has been analyzed particularly by the Palumbo et al. and by Baragiola et al. In this work, a further discussion of the process for inducing the compaction is presented. Simple models for OH-db evolution for irradiated water ice are discussed. Literature results on the OH-db disappearance in ices bombarded by 100–200 keV H+ ions and on the comparison of porosity and OH-db results for 200 keV Ar+ ions are revisited. It is observed that for both, porosity decrease (compaction) and OH-db absorption signal decrease, experimental data can be well fitted by the sum of two decreasing exponentials with similar sets of parameters. Although a clear explanation for this correlation cannot be extracted, it suggests strongly that compaction and OH-db destruction are both triggered by two different processes.
Swift heavy ion irradiation of water ice from MeV to GeV energies
(2013) Mejía Guamán, Christian Fernando; Frota Da silveira, Enio; Godard, Marie; Domaracka, Alicja; Chabot, Marin; Brunetto, Rosario; Boduch, Philippe; Ferreira De barros, Ana Lucía; Ding, Jing Jie; Dartois, Enmanuel; Pino, Thomas; Thomas, Jean Charles; Rothard, Hermann
Context. Cosmic ray ion irradiation affects the chemical composition of and triggers physical changes in interstellar ice mantles in space. One of the primary structural changes induced is the loss of porosity, and the mantles evolve toward a more compact amorphous state. Previously, ice compaction was monitored at low to moderate ion energies. The existence of a compaction threshold in stopping power has been suggested. Aims. In this article we experimentally study the effect of heavy ion irradiation at energies closer to true cosmic rays. This minimises extrapolation and allows a regime where electronic interaction always dominates to be explored, providing the ice compaction cross section over a wide range of electronic stopping power. Methods. High-energy ion irradiations provided by the GANIL accelerator, from the MeV up to the GeV range, are combined with in-situ infrared spectroscopy monitoring of ice mantles. We follow the IR spectral evolution of the ice as a function of increasing fluence (induced compaction of the initial microporous amorphous ice into a more compact amorphous phase). We use the number of OH dangling bonds of the water molecule, i.e. pending OH bonds not engaged in a hydrogen bond in the initially porous ice structure as a probe of the phase transition. These high-energy experiments are combined with lower energy experiments using light ions (H, He) from other facilities in Catania, Italy, and Washington, USA. Results. We evaluated the cross section for the disappearance of OH dangling bonds as a function of electronic stopping power. A cross-section law in a large energy range that includes data from different ice deposition setups is established. The relevant phase structuring time scale for the ice network is compared to interstellar chemical time scales using an astrophysical model. Conclusions. The presence of a threshold in compaction at low stopping power suggested in some previous works seems not to be confirmed for the high-energy cosmic rays encountered in interstellar space. Ice mantle porosity or pending bonds monitored by the OH dangling bonds is removed efficiently by cosmic rays. As a consequence, this considerably reduces the specific surface area available for surface chemical reactions.