Radiolysis of astrophysical ices by heavy ion irradiation: Destruction cross section measurement

Abstract

Many solar system objects, such as planets and their satellites, dust grains in rings, and comets, are known to either be made of ices or to have icy surfaces. These ices are exposed to ionizing radiation including keV, MeV and GeV ions from solar wind or cosmic rays. Moreover, icy dust grains are present in interstellar space and in particular in dense molecular clouds. Radiation effects include radiolysis (the destruction of molecules leading to formation of radicals), the formation of new molecules following radiolysis, the desorption or sputtering of atoms or molecules from the surface, compaction of porous ices, and phase changes. This review discusses the application of infrared spectroscopy FTIR to study the evolution of the chemical composition of ices containing the most abundant molecular species found in the solar system and interstellar medium, such as H 2 O, CO, CO 2 and hydrocarbons. We focus on the evolution of chemical composition with ion fluence in order to deduce the corresponding destruction and formation cross sections. Although initial approach focused on product identification, it became increasingly necessary to work toward a comprehensive understanding of ice chemistry. The abundances of these molecules in different phases of ice mantles provide important clues to the chemical processes in dense interstellar clouds, and therefore it is of importance to accurately measure the quantities such as dissociation and formation cross sections of the infrared features of these molecules. We also are able to obtain the scaling of these cross sections with deposited energy.

Long-Range Structures of Amorphous Solid Water

“… 15 , 21 − 23 The effect of irradiation on the structure of amorphous ice, and in particular on the dangling OH groups, has been studied using UV, 24 IR, 17 , 25 and heavy ion bombardment. 26 − 28 Formation of high-density ASW has been reported to occur at very low deposition temperatures (<30 K). 29 − 31 When the deposition temperature is increased from 77 to 200 K, the deposited ice phase changes from porous ASW (p-ASW) 1 to collapsed ASW (c-ASW) and eventually to crystalline ice.…”

Radiolysis of pyridine in solid water

“…The destruction cross sections scale with a power n of Se, d ~ Se n , with a stronger than linear dependence. This behavior is universal and has been observed for many simple and more complex molecules [8,16,17,27].…”

“…For this experiment at GANIL (Grand Accélérateur National d'Ions Lourds; Caen, France), we have used two different ion beam lines: the low energy line ARIBE delivered an 16 O 6+ beam at 90 keV, and the medium energy SME beam line 70 Zn 26+ at 650 MeV. The CASIMIR set-up (Chambre d'Analyse par Spectroscopy Infrarouge des Molécules IRradiées), dedicated to FTIR spectroscopy with low temperature samples can be mounted on both of these beam lines.…”

“…The disappearance of COMs was quantified by the "destruction cross section" , which was obtained from the exponential decrease of the peak area (A) as a function of the fluence F (i.e. the number of projectile ions per unit of area) [8,13,[15][16][17][18][19]:…”

Reactions of Photoionization-Induced CO–H2O Cluster: Direct Ab Initio Molecular Dynamics Study

“…A number of experiments have been carried out in mixtures of CO–H 2 O systems. 22 − 25 The HOCO radical was detected as one of the products in these experiments. Therefore, the study of the formation mechanism of HOCO is strongly correlated to the elucidation of the chemical evolution in the universe.…”