Role of electron capture in ion-induced electronic sputtering of insulators

Abstract

Measurements of the sputtering yield of solid O 2 by 25-240-keV H + show that it is double valued in its dependence on electronic stopping power. We propose that this is because the electronic sputtering yield is dominated by repulsion of ions in the ionization track of the projectile which, at low velocities, is augmented near the surface due to the additional ionization resulting from electron captures. This process may also be responsible for enhanced radiation damage in insulators, in particular in the production of fission tracks. Ionization effects in insulators by fast ions are evident in a wide range of areas, including sputtering, dating by nuclear fission tracks, destruction of interstellar matter by cosmic rays, and medical therapy using ion beams. Some of the most basic information on these effects has come from the study of the sputtering of condensed gases 1 and fission tracks, 2 both occurring through the conversion of the electronic energy stored in electronic excitations and electron-hole pairs into repulsion between lattice atoms or molecules. Atoms set in motion when repulsive states evolve can be ejected directly or initiate a collision cascade of recoil atoms in the solid leading to sputtering or the amorphization of the ion track. The transfer of electronic to recoils depends strongly on specific details of the material, and its description can only be done in very few cases, due to the lack of knowledge of the excited electronic states in the solid state. For this reason, the understanding of ionization effects is usually limited to their correlation to the linear energy loss or electronic stopping cross section S e = dE / Ndx as the ion enters the solid. Here, dE / dx is the energy loss per unit path length and N is the atomic density. In the case of sputtering, it is often assumed that the yield Y ͑molecules ejected per incident ion͒ is proportional to the energy deposited near the surface ͑several monolayers, depending on the material͒, which can be considered to be proportional to the energy lost by the ion in that region. However, there is evidence that parameters other than the stopping power affect the sputtering yields for a given material. Since S e increases at low E, passes through a maximum, and then falls at high E, there are two ion energies for a given value of S e and one would expect the same sputtering yield in both cases if the deposited energy was the only ion property of importance. Instead, what is found is that there is a "loop" in the Y͑S e ͒ dependence where, for the same S e , Y is double valued, being larger at low than at high ion energies, as well documented for Ar ͑Ref. 1͒ and H 2 O. 3 In addition, Y is often not proportional to S e but depends on a higher power n ͑Y ϰ S e n ͒, with n up to 4, depending on the type of condensed gas and the type and energy of the projectile. 1 For instance, n = 1 for Ar, and n = 2 for water ice, a case of great astrophysical importance because of the role of sputtering in the erosion of surfaces and formation of atmospheres arou...

Laboratory Studies on the Formation of Ozone (O3) on Icy Satellites and on Interstellar and Cometary Ices

“…4); the oxygen matrix can be seen coming off, where presumably the signal at $40 K ($170 minutes) represents the bulk amorphous oxygen ice, whereas the crystalline phase should be more strongly bound. Finally, the ozone starts subliming at temperatures between 58 and 73 K. The ion current peaks of the subliming oxygen at 38 K and of ozone at 67 K are consistent with those of a previous study, where the sublimation temperatures were determined to be 35 and 63 K for oxygen and ozone, respectively (Fama et al 2002). Since molecular oxygen and ozone can both result in signal at m/e ¼ 32 and 48, it is imperative to actually calculate the true partial pressure of ozone via matrix interval algebra (Kaiser et al 1995) using electron impact ionization cross sections from Cosby (1993; O 2 ) and Newson et al (1995;O 3 ).…”

“…This paper is the first in a series of a systematic research program that aims to investigate the formation of ozone using varied energetic particles (electrons, protons, and photons), target compositions (neat oxygen and binary mixtures with water), and temperatures (10 K, relevant to molecular clouds and comets in the Oort cloud, to 150 K, relevant to the Jovian satellites). Previous studies on condensed oxygen include energetic processing by UV photons (Crowley & Sodeau 1989;Schriver-Mazzuoli et al 1995;Gerakines et al 1996;Dyer et al 1997), keV protons (Baragiola et al 1999;Fama et al 2002), and electrons (Lacombe et al 1997 and references therein) and will be discussed in this context.…”

Physical and chemical effects on crystalline H2O2 induced by 20 keV protons

“…The peak sputtering yield observed in these experiments is much higher than that measured in previous keV proton sputtering experiments on solid H 2 O at 80 K ͑Ref. 36͒ and on solid O 2 at 5 K. 37 While H 2 O is compositionally similar to H 2 O 2 , the amount of O 2 produced in irradiation of water ice is at least an order of magnitude lower for proton irradiation of water ice than in our experiments, which translates into a much lower sputtering yield. The peak sputtering yield in our experiments is approximately three times higher than measured on solid O 2 .…”

Ion-induced electrostatic charging of ice at 15–160 K

“…The charged icy surfaces might be capable of deflecting or even reflecting low energy magnetospheric projectiles, so that charging might affect the current flow into the icy surfaces. The return of secondary electrons to a positively charged surface can have significant effects in the sputtering of insulators [2]. Limited laboratory studies have focused on the charging of ice films due to charge imbalance upon ion [3] or electron [4,5] injection.…”