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...