H. Sørensen

Electronic sputtering of solid nitrogen and oxygen by keV Electrons

Sputtering of solid N2 and O2 has been performed with electrons in the keV regime by means of a quartz microbalance technique. Good agreement is found between the sputtering yields obtained with this and the emissivity-change method. O2 sputters more efficiently than N2, although these solids are very similar in their physical properties. The yields are almost proportional to the electronic stopping power of the primary electrons. Different models for electronic sputtering of solid condensed gases are discussed and compared with the results. For low excitation densities predictions are attempted on the basis of a simple collision-cascade model where the low-energy cascades are generated by kinetic energy release from electronic deexcitations.

The measurement of electron-induced erosion of condensed gases: Experimental methods

Abstract Two experimental methods for measuring the erosion yield of condensed gases are described. One, the frequency-change method, utilizes a quartz-crystal microbalance operating at liquid-helium temperature. The other, the emissivity-change method is based on the strongly varying electron emission as a function of the condensed-gas film thickness. Satisfactory results have been obtained for both methods for solid Ne and D 2 at electron energies up to 3 keV, and the mutual agreement is good as well. Accurate measurements are affected critically by the beam conditions, particularly if the erosion yield depends on the film thickness. The erosion yield has been measured for dominant electron sputtering of solid Ne ( ≈ 28 Ne-atoms/electron) as well as for beam-induced evaporation at 2 keV. In the latter case a clear lateral broadening of the erosion spot is observed.

Charged particle erosion of solid rare gases and dilute rare gas alloys

We present the first experimental results on electron-induced erosion of solid neon. The measurements are interpreted qualitatively within a new model invoking excitation transport by free excitons and their subsequent decay at the surface. The model accounts for the magnitude of the observed yield and the energy dependence. A theoretically predicted decrease in the erosion yield due to doping with a heavier rare gas, in casu argon, has been observed experimentally. The strong influence of very small amounts of different types of impurities makes sample purity a crucial problem in investigations of the erosion of solid rare gases.

Sputtering of solid nitrogen and oxygen by keV hydrogen ions

Abstract Electronic sputtering of solid nitrogen and oxygen by keV hydrogen ions has been studied at two low-temperature setups. The yield of the sputtered particles has been determined in the energy regime 4–10 keV for H+, H 2+ and H3+ ions. The yield for oxygen is more than a factor of two larger than that for nitrogen. The energy distributions of the sputtered N2 and O2 molecules were measured for hydrogen ions in this energy regime as well. The yields from both solids turn out to depend on the sum of the stopping power of all atoms in the ion. The yield increases as a quadratic function of the stopping power for oxygen, but slightly slower for nitrogen. The energy distributions do not exhibit strong features, but are similar to those published earlier for electron sputtering.

SPUTTERING OF SOLID NEON BY KEV HYDROGEN-IONS

Abstract Sputtering of solid Ne with the hydrogen ions H + 1 , H + 2 and H + 3 in the energy range 1–10 keV/atom has been studied by means of a quartz microbalance technique. No enhancement in the yield per atom for molecular ions was found. The results for hydrogen ions are compared with data for keV electrons. The thickness dependence of the yield is almost the same for the two types of bombarding particles. The energy dependence as well as the absolute magnitude of the yield are discussed on the basis of mobile electronic excitations.

Sputtering of Volatile Solids from Nonoverlapping Subspikes

Elastic low-energy spikes can be produced in volatile materials such as condensed gases even for primary particles with comparatively low nuclear stopping power. The sputtering yield from solid neon bombarded by (5 ÷ 10) keV He+-ions has been measured. Model calculations demonstrate that nonoverlapping subspikes are responsible for particle ejection from this volatile solid.

Thickness dependence of the sputtering yield from solid deuterium by light keV ions

Abstract Measurements of the thickness dependence of the sputtering yield from solid deuterium bombarded by 1–10 keV/amu hydrogen ions are reported. For film thicknesses larger than 2 ×1018 D2/cm2 the yield is largely independent of the film thickness. A strong enhancement of the yield is observed for thin films. The results for different ion energies demonstrate convincingly that this enhancement is a result of the interaction between the primary ion and the metallic substrate rather than a beam-independent structural interface effect.

Sputtering yields and energy distributions from nonoverlapping subspikes in ion bombarded volatile solids

Abstract The yields and energy distributions of particles sputtered from rare gas solids bombarded by keV ions have been studied. The experimental yields are typically much larger than those predicted from linear collision cascade theory, but the dependence on the nuclear stopping power is similar. This discrepancy is explained by a sputtering model based on nonoverlapping elastic subspikes. The predictions from this model agree well with the experimental results for helium ions incident on solid neon and argon as well as nitrogen ions on solid xenon. The analysis of the collision regime demonstrates that the subspikes do not overlap at the primary energies considered.

Sputtering of frozen gases by molecular hydrogen ions

Abstract Measurements of the energy dependence of the electronic sputtering yield from solid nitrogen and deuterium bombarded by 4.5–10 keV atomic and molecular hydrogen ions are reported. The yield depends on the total energy deposition at the surface of the solid by the molecule. The behaviour in this energy range agrees well with that observed for electronic sputtering of water ice for energies at the high-energy side of the stopping power maximum.

Stopping of 1–2 keV/amu hydrogen ions in solid N2

Abstract Thin films of N 2 on a substrate of solid Xe were bombarded with 1–2 keV/amu H 1 + , H 2 + , and H 3 + ions, and the energy spectra for those positive ions backscattered through 135° were measured. The minimum energy loss of particles scattered from the Xe-surface was found to vary linearly with film thickness up to typically ∼ 150 A , where multiple collision effects become important. It is argued that stopping powers for solid N 2 may be extracted from these measurements. Existing stopping powers for gaseous N 2 are about twice as large as our results.