Chemical modification of insulators by ion implantation, fundamental mechanisms and astrophysical implications

Abstract

Chemical processes upon ion implantation into insulators can be classified into two categories: high energy reactions of projectiles and secondary knock-on particles (so-called hot reactions), and the thermal reactions of radiation-induced radicals and defects. Hot reactions are characterized by the translational kinetic energy of at least some eV which the projectile carries into the reaction complex. This enables endothermic and high activation energy reactions, among them atom-molecule interactions. The often-used name Hot Atom Chemistry applies to the fact that due to neutralization processes most of the projectiles, at the end of their trajectories, reach the neutral charge state, irrespective of the charge of injection. These reactions can be studied (1) by ion implantation combined with optical spectroscopy, (2) via mass spectrometric analysis of molecules and molecular ions sputtered from surfaces, and (3) by nuclear recoil following nuclear reactions in conjunction with radiochromatographic techniques. Due to the fact that only the first method provides proper in-situ measurements, but operates in an extremely high dose regime, all three methods have to supplement each other. Furthermore, computer simulation of collision cascades may help to obtain information on the physical properties of the reaction medium.Superimposed on the primary hot reactions is the radiation chemistry of radicals and defects. This necessitates a detailed study of the dependence of chemical products on the total absorbed dose (fluence or integrated flux) and dose rate (flux) of the accompanying radiation. Doses below eV per target molecule in general do not result in measurable radiolytic changes. Between 0.1 and 10 eV per target molecule, radicals may survive recombination processes and may reduce or oxidize according to their reactivity, mobility and concentration. If the dose exceeds 10 eV per target molecule, decomposition may begin. The nuclear recoil method covers the dose regime between and lo2 eV per target molecule, whereas the ion implantation and chemical sputtering methods give access to the higher dose regime >I eV per target mokule.Systems treated in this review comprise simple inorganic salts, such as alkali and ammonium halides, LiH, LiNH2, MgO, A1203, etc., and ices such as frozen HzO, NH3 and CH4 at T<77 K.Special emphasis is given to the relatively new and exciting applications of hot atom chemistry to astrophysical problems : the specific formation of organic compounds and biomolecular precursors by impact of biogenic elements with cosmic dust grains, icy layers, comets and surfaces of outer planets and their satellites and the chemical reactions underlying the sputtering of ice or frozen volatiles. Hot atom chemistry, thus, may contribute to problems of chemical and prebiotic evolution by introducing the new type of energetic, endothermic and atom-molecule reactions into cosmic chemistry. 505Downloaded by [New York University] at 16:21 08 January 2015 22K. ROSSLER

A dose-survival model for low energy ion irradiation

“…This is the domain of hot atom chemistry (Matsuura 1984). Previous work has shown that the energetic 11 C implanted in CH4 formed 11 CH4, hot atom 14 C. At an energy of 5-10 eV the ion reacted with benzene or toluene to form 14 C-labelled molecules and, 11 C þ with an energy of 30 keV implanted into malonic acid will form 11 C-labelled malonic acid molecules (Rossler 1986). This could be regarded as restoration of damaged molecules at the molecular level, since the isotope-labelled molecule was the same as the original molecule in chemical properties.…”

“…Energetic ions or atoms penetrating into a solid dissipated their kinetic energy by inelastic interactions which led to electronic excitation and/or ionization of projectile and target atoms, and by elastic collisions producing displacements, replacements and other kinds of atomic defects (Rossler 1986). When the energy of the ions or atoms decreased to some tens or a few eV, chemical bonds could be formed.…”

“…Previously, the Energy transfer, Mass deposition and Charge exchange model (EMC model) based on the repair effects caused by mass deposition and charge exchange of low energy ions has been proposed and has given a good explanation of the experimental data (Shao & Yu 1997a). However, more recent studies on the physical and chemical processes of low energy ion irradiation of bioorganisms indicates that momentum transfer is the main repair physical process during low energy ion irradiation of biomolecules and bioorganisms (Huang et al 1998a, Rossler 1986, Huang et al 1998b, Huang et al 1998c, Huang et al 1998d, Huang 1995. Moreover, the EMC model does not consider the stopping range of keV ions in biomaterial.…”

Particle‐induced desorption in mass spectrometry. Part I. Mechanisms and processes

“…These excitations result in a wide variety of fast "hot atom" chemistry-bond breaking and formation, free radical and charged species creation, and a series of complex secondary reaction processes, as already stressed by Macfarlane more than a decade ago [2]. Fast nonequilibrium chemical processes induced by swift ion impacts have been studied extensively in connection with the radiation-induced modifications in polymers for technological purposes [173] as well as for modeling astrophysical processes-from prebiotic synthesis of biomolecules to the modification and erosion of ice covers of planets and their satellites [43,1741. Only the sputtered nascent (i.e., most energetic) ionic intermediates and reaction products are monitored by PDMS.…”

Saturn's inner satellites: Ice chemistry and magnetosphere effects

“…There have been few experiments on the chemistry resulting from the impact of nitrogen ions onto water ice, and more are needed. The only published results are those of Roessler [1985a, 1985b, 1986, 1988, 1992], Roessler and Nebeling [1987], Roessler and Eich [1988], and Strazzulla [1999]. Roessler used nitrogen ion beams or neutral nitrogen atoms to implant into the ice.…”