Swift heavy ion irradiation of water ice from MeV to GeV energies

Abstract

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.

Comparison of UV and high-energy ion irradiation of methanol:ammonia ice

“…Nevertheless, as mentioned in Sect. 1, these ions are expected to be an important component of cosmic rays in space; in the past five years several experiments simulated their effects in various molecular ice components (Seperuelo Duarte et al 2009Pilling et al 2010a,b;Dartois et al 2013). We conclude that in our selected ice mixture where both components, methanol and ammonia, are dissociated easily by vacuum-UV photons, similar photon and ion energy doses led to similar chemical reactions, based on the similarity of the IR spectra of the processed ices.…”

“…The results obtained in these experiments were often extrapolated to higher energies (on the order of 1 GeV) for astrophysical ices. In recent years, some experiments have reached energies near 1 GeV (Seperuelo Duarte et al 2009Pilling et al 2010a,b;Dartois et al 2013).…”

Pore evolution in interstellar ice analogues

“…The binding energy of a water molecule increases with its number of neighbours and is estimated at 0.22 eV per hydrogen bond (Brill & Tippe 1967;Isaacs et al 1999;Dartois et al 2013). In fact, the hydrogen bond strength depends on the number of water molecules involved as donors and acceptors in the neighbourhood (Huš & Urbic 2012).…”

Cosmic-ray slowing down in molecular clouds: Effects of heavy nuclei

“…We report on calculations performed with dE/dx powers α = 1 and α = 2. For bulk modifications, such as dehydrogenation of carbonaceous material (Godard et al 2011), or ice compaction (Dartois et al 2013), the slope α has been found close to unity in experiments. For desorption processes (i.e.…”