J. Schou

Sputtering of frozen gases

Abstract Particle-induced erosion of frozen gases takes place as beam-induced evaporation as well as sputtering. At sufficiently low temperatures electronic or knock-on sputtering is the dominant mechanism for particle ejection. Knock-on sputtering may largely be compared to ordinary sputtering of metals, although the yields are much higher for ices than for metals because of the low surface binding energy. Electronic sputtering exhibits large differences from the solid rare gases to the solid diatomic homonuclear gases or the solid heteronuclear molecular gases.

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.

Erosion of rare gas solids by electron bombardment

Abstract Ar and Xe solids have been bombarded by 0.5 keV electrons. Mass spectrometry and time of flight measurements were made of the ejected neutral particles. The former measurements indicate that only atoms are emitted from the surface in contrast to sputtering by ions in which also clusters are found. The time of flight distribution of Ar atoms shows features distinctly different from those for Xe. The results are explained by creation of excitations, their transport towards the surface and subsequent decay of molecular excitons to a repulsive state.

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.

Energy distributions from electron-sputtered solid nitrogen

Abstract Solid N2 has been bombarded by 0.5 keV electrons. Mass spectrometry and time of flight measurements were performed for the sputtered neutral particles. The dominant component is molecular N2, but also N-atoms and a small number of N3- and N4-molecules were observed. The energy distribution of N2 and N indicates that the sputtering is caused by non-radiative transitions that release up to more than 6 eV. The important non-radiative transitions in solid N2 are discussed.

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.