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 10⁻² 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 10⁻⁴ and 10² eV per target molecule, whereas the ion implantation and chemical sputtering methods give access to the higher dose regime ≫ eV per target mokule.

Systems treated in this review comprise simple inorganic salts, such as alkali and ammonium halides, LiH, LiNH₂, MgO, Al₂O₃, etc., and ices such as frozen H₂O, 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.