J.W. Boring

Erosion and molecule formation in condensed gas films by electronic energy loss of fast ions

Abstract Fast ions erode films of molecular and rare gas solids as a result of electronic energy loss of the incident particles. The dependence of the low temperature erosion yield on electronic stopping power is nonlinear and approximately quadratic for light ions. Thermal spike and Coulomb repulsion models for erosion in this temperature regime are discussed. Measurements of ejected species from D 2 O ice eroded by MeV He + ions show a constant and dominant yield of D 2 O molecules below 135 K and a monotonically increasing yield of D 2 and O 2 molecules from 55 to 150 K. The new molecules must result from rearrangement of fragments produced in the film by ionization. The temperature dependence is believed to be due to diffusion controlled migration in the film.

Electronic sputtering of low temperature molecular solids

Electronically stimulated sputtering of insulating molecular gas solids is a remarkably efficient process at excitation densities accessible by MeV light ions and keV electrons. This paper concentrates on the cases of CO and H2O (D2O). The approximately quadratic dependence of sputtering yield on the excitation density along individual particle tracks observed earlier for incident MeV ions has also been found for incident keV electrons in the case of CO. Coupled with time-of-flight energy spectra of ejected D2O from solid D2O, this behavior leads to a picture of rapid electronic relaxation with molecular repulsion involving pairs of molecular ions. We also report a dependence of CO sputtering yield on incident angle for MeV He ions which varies as (cosθ)−1.6, in qualitative support of the multiple ion picture. In addition to ejection of the principal molecular species of a solid film, electronic excitation of molecular solids even at very low temperatures leads to formation of new molecular species by bond disruption and fragment rearrangement. In the case of D2O the dominant new molecules are D2 and O2 whose ejection from the film is strongly thermally activated.

Ion-induced molecular ejection from D2O ice

Abstract Studies of molecules ejected from water ice by fast ions provide insight into the electronic relaxation processes and subsequent chemistry occurring in ice at very low temperatures. The ion-induced ejection of D 2 O, D 2 , and O 2 molecules from thin films of D 2 O ice has been measured as a function of the fluence of incident MeV ions at temperatures between 10 and 140 K. For a given beam current, the O 2 yield exhibits initial transients which are slow compared with the prompt ejection of D 2 O. We interpret these results as due to the build-up of O 2 in the films following fragmentation of D 2 O molecules by incident ions. The fragmens re-form into new molecular species which diffuse to and escape from the surface, aided by subsequent bombardment. The D 2 transient has a prompt component, which we postulate is due to rapid formation during electronic recombination near the surface. A slow component of the D 2 transient is also observed, which may arise through a two-step process similar to that of O 2 . Time-of-flight energy spectra of the ejected D 2 O molecules have also been measured. Incident ion masses and energies range from those for which nuclear elastic energy deposition dominates (50 keV argon) to those for which electronic energy deposition dominates (1.5 MeV helium). The spectra cannot be described by models typically employed for the sputtering of metals. For instance, the spectra do not have maxima characteristic of the sublimation energy of the solid. In addition, the sputtering yield in the high energy part of the ejection spectrum of D 2 O is too large to arise from nuclear elastic energy deposition. It must result instead from relatively energetic non-radiative relaxation of electronic excitation. For incident MeV ions that deposit their energy predominantly in electronic excitation, the low energy part of the D 2 O ejection spectrum is greatly enhanced, indicative of a weakly antibinding region formed along an incident particle track. Enhanced ion yields are also found in the collision cascade region which are attributed to electronic processes.

Sputtering of solid SO2

Abstract Solid SO2 has been bombarded by MeV and keV ions, and the transformation of the deposited energy studied by investigating sputtering from the surface. Measurements have been made of the total yield, and the mass and energy distributions of ejected atoms and molecules. These are examined in terms of existing sputtering models.

Sputtering of molecular gas solids by keV ions

Abstract The sputtering of D2O and CO solids by keV ions has been studied by measuring the absolute sputtering yield and the masses of ejected particles. The yield results for several ions on D2O indicate that for the case where the energy deposited in nuclear motion is comparable to that deposited electronically, the sputtering yield is still determined to a large extent by the electronic energy. The mass spectra ejected from D2O by 30 keV Kr+ ions have a behavior very much like those produced by MeV ions. The bombardment of CO by kr+ results in the rapid production of a dark residue, stable at room temperature, and the ejection of CO, CO2, O2, O, C2 and (CO)2.

Ion-induced chemistry in condensed gas solids

Abstract Solids of H2O, D2O, CO2, and SO2 have been bombarded by MeV and keV ions, and the sputtering of these materials investigated by measuring the total yield and the mass and energy spectra of ejected particles. One observes the results of considerable chemical activity in these low temperature solids. With MeV ions incident on D2O and CO2, the production of D2, O2 and CO, O2 respectively are found, while keV ions on SO2 produce SO, SO3 and O2.

Ejection of sodium from sodium sulfide by the sputtering of the surface of Io

Measurements have been made of the sputtering yields, the mass spectra of ejected molecules, and ejection rates for various kiloelectronvolt ions incident on sodium sulfide (Na2S). The sputtering yields were small compared to those measured earlier for the more volatile sulfur (

Electronic excitation of condensed CO : sputtering and chemical change

8) and SO2 due to the strong ionic bonding in the solid. The mechanism of sputtering for the corotating sulfur and oxygen ions in Jupiters magnetosphere is due to a cascade of quasi-elastic collisions initiated by the incident ion. The mass spectrum indicated that sodium is ejected predominantly as a molecule with a lesser amount ejected as atomic sodium. Making several assumptions it seems unlikely that the sputtering of Na2S by magnetospheric ions can maintain the observed neutral cloud densities. Instead, the sodium probably exists as a larger polysulfide for which we show that the sputtering yield should be greater.

Sputtering of sulfur by kiloelectronvolt ions - Application to the magnetospheric plasma interaction with Io

Abstract We have bombarded condensed CO “ice” films with keV and MeV ions at low temperatures to investigate the physical sputtering mechanisms and chemical modifications of the ice. The sputtering yield measured for keV and MeV H + and He + ions indicates that the yield has a quadratic dependence on the electronic stopping power of the ions. Energy spectra of CO molecules sputtered by bombardment with 53 keV He + and 34 keV Ar + ions are very similar and show a collision-cascade-like behavior. We explore the origin of this quadratic dependence in terms of several models proposed for sputtering by electronic excitation and conclude that the quadratic dependence is intrinsic to the electronic relaxation process for CO. RBS measurements of the residue produced from sputtering various thicknesses of the initial CO film (∼ (3–25) × 10 17 CO/cm 2 ) indicate the production of the residue is approximately proportional to the initial thickness, accounts for ∼ 2% of the initial mass, and has a stoichiometric composition of approximately C 3 O. The production and composition of the residue is discussed in terms of existing mechanisms for the radiolysis of gas phase CO.

Charged Particle Modification of Ices in the Saturnian and Jovian Systems

Abstract Measurements of total yields, temperature dependences, mass spectra, and energy spectra of molecules sputtered from condensed sulfur (S 8 ) at low temperatures by keV ions are reported and results are given for Jovian plasma ion bombardment of Io. A change in the reflectance of the sulfur, which can be removed by annealing, is produced by the most penetrating ions and may be connected with the darker, colder polar regions on Io. The measured sputtering yields are much lower than those estimated earlier for room temperature sulfur films but are comparable to previous measurements of keV ion sputtering of SO 2 at low temperatures. The corrected mass spectrum indicates that ≈66% of the total yield corresponds to S 2 ejection while only 5 and 16% correspond to S and S 3 , respectively. Therefore, if ions reach the surface of Io its atmosphere will have a non-negligible sulfur component of primarily S 2 . The ejection of S and S 2 is temperature independent for temperatures characteristic of most of the surface of Io. The energy spectrum for S has an approximate 1/ E 2 dependence at high ejection energies, whereas S 2 and S 3 fall off more rapidly. Assuming 50% coverage of both sulfur and SO 2 and a thin atmosphere (e.g., nightside and polar region) the direct sputter injection of sulfur atoms and molecules into the Jovian plasma torus and the indirect injection due to coronal processes are estimated. These injection rates for sulfur are compared to those for SO 2 showing that injection from sulfur deposits contributes 13% to the total mass injection rate of ∼2–3 × 10 29 amu/sec.