K. Wien

Fast heavy ion induced desorption

Abstract When solid material is irradiated by high energetic heavy ions, atomic, molecular and cluster particles are ejected from the surface with yields between 0.1 and 106 per impact depending on the stopping power of the projectiles and the properties of the samples. A review is given about the research of this phenomenon performed during the past 30 years. Firstly, the various theoretical concepts applied to heavy ion induced desorption are briefly listed, then the numerous experimental data are sorted and discussed. The article is restricted to irradiation with ions being heavier than 7Li and having an energy loss above 2 MeV/mg/cm2. The studies in context with the erosion of frozen gases are not presented. Also the early work about fission fragment induced desorption phenomena is mentioned only occasionally.

TOF-SIMS analysis of polymers

Abstract When solid polymers are irradiated with heavy ions, atomic and molecular particles are ejected from the uppermost layers of the surface. A technique to determine the mass spectrum of the charged fraction of these particles is time-of-flight secondary-ion-mass spectrometry, TOF-SIMS. The present article describes, how the mass spectra measured with polymers are generally structured and under which conditions the various types of secondary ions like cationized oligomers, fragment ions and “fingerprint” ions are observable. The mechanisms leading to formation and ejection of the ions are not well understood. At bombarding energies of 10 keV, they are mainly based on atomic collision processes, at 100 MeV on the electronic excitation of the solid in the vicinity of the nuclear track. Processes, which are capable to desorb large organic molecules, seem not to work with oligomers of similar mass. Mass spectrometry of “real world” polymers, i.e. thick samples, depends mostly on the low-mass fingerprint spectrum, which can be produced by keV MeV SIMS. Modern TOF-SIMS instruments are equipped with a pulsed ion gun and an energy focussing ion mirror. They provide high mass resolution ( m Δm ⋍ 10000 ) and high transmission (20–50%). Examples of applications are given, like the determination of mean molecular weights or investigations of radiation induced modifications of polymers.

MeV heavy ion induced desorption from insulating films as function of projectile velocity

Abstract Fast heavy ions beams from accelerators at Darmstadt and Orsay have been used to study secondary ion emission from surfaces of mineral and organic solids over a large projectile energy range. The projectile velocity dependence of the secondary ion yield shows a rather universal behaviour and it is clear that there is no direct relationship between the electronic stopping power and the yield excitation functions. Comparisons of theoretical models with experimental results are presented in the last section.

The charge state dependence of desorption. A way to study the energy transfer to the surface

Abstract Various experiments on the charge state dependence of secondary ion emission from insulators have been performed recently using the heavy ion accelerators in Orsay and Darmstadt. A survey of the results is presented with the intention of exhibiting phenomenological trends over a wide range of beam parameters. The yields of certain small fragment ions ejected from organic samples depend strongly on the initial charge of the incident ions but not on their atomic number. Large molecule-specific ions show a different behaviour. Their yields increase considerably with the atomic number when the initial charge state is kept constant. This phenomenon is interpreted in terms of a depth contribution to desorption. By means of an averaging procedure, we calculated a mean interaction depth, from which contributions to desorption reach the surface. For a coronen target, this mean interaction depth ranges from 12 to 160 A, when the target is irradiated by 1.16 MeV/n beams of Ne, Ar, and Kr having charge states from 5+ to 24+. The interaction depth, as defined in this work, increases linearly with the specific energy loss. An attempt was made to introduce the depth effect into the ion track model. The range of interaction predicted by these model calculations can not, however, reproduce the experimental observations.

Equilibrium charge state of fast heavy ions in solids measurements of post-ionizatton effects

Abstract Measurement of the H+ secondary ion yield from the surface of solid targets has been used to probe the charge state of fast heavy-ion projectiles at the target surface. Results have been obtained for ion beams of 32S, 40Ar and 84Kr at ≃ 1 MeV/u, and between 0.5 and 1.5 MeV/u for 127I. Increases in the charge state after passage through carbon and Au foils are clearly observed for 84Kr and 127I. The variation of charge state as a function of the thickness of matter traversed is compared with calculated values.

Some aspects of MeV heavy ion induced desorption from solid insulating surfaces

Abstract Various aspects of the secondary ion and electron emission from surfaces of solids bombarded by MeV ions are briefly presented. Two examples of applications in very different fields are given: the charge state variation of a fast ion traversing a solid and the desorption of very large molecules which can be desorbed intact from organic solid films.

Energies and yields of secondary ions ejected from alkali halides by a 0.18–6.0 MeV nitrogen beam

Abstract Nuclear and electronic sputtering was studied for the alkali halides LiF, CsI, NaF, NaCl, NaBr and NaI being vacuum evaporated on thin Al-foils and irradiated by 14 N ions having an energy loss of 20–190 eV/A. A double-grid time-of-flight technique was used to determine the yields and the axial energy distributions of the alkali and halide ions ejected from the sample surface. In case of LiF, the most carefully investigated compound, the dependence of ion yields on the energy loss and the shape of the energy distributions clearly mark the onset of collisional sputtering towards low beam energies. Apart from F − , for all other investigated alkali and halide ions electronic sputtering is the dominant ejection process. Above 1 MeV bombarding energy, their mean axial energies were found to range from 0.26 to 2.0 eV, their energy distributions fall off with E − n , n having values between 3.0 and 4.5. In cases, where the nuclear energy loss is relatively low, the ion yields scale with about the second power of the electronic energy loss; the mean axial energies and the yields change only little or not with the charge state of the incident ions. These observations lead to the general conclusion, that electronic sputtering from alkali halides is established by cooperative processes and that probably electron-hole production in deep layers contribute essentially to formation and ejection of ions at the surface. The results are discussed in terms of exciton production and migration.

Energy distribution of H+ ions desorbed from metal surfaces by MeV ion impact

Abstract Energy distributions of desorbed H+ ions from different solid surfaces (Au, Ni, Ti, Nb, V, Ta, C) under 252Cf fission fragment bombardment are reported. The surfaces were exposed to distinct hydrogen containing gases (CH4, H2 or D2O) throughout the measurements. Energy distributions were obtained by the double-grid time-of-flight method. Experimental results show that H+ energy distributions are rather insensitive to the substrates and gases investigated. A model based on desorption induced by secondary electrons escaping the ion track is developed. Energy distributions of emitted H+ ions have been calculated and compared with the present experimental data. Published by Elsevier Science B.V.

Heavy particle induced ion emission from Langmuir-Blodgett films: dependence on the charge state and the angle of incidence

Abstract At the Orsay tandem accelerator beams of 32S and 127I at energies of 1 MeV u were used to investigate secondary ion emission from Langmuir-Blodgett films deposited on Au. The yields of negative parent ions, dimer and trimer ions were measured for various superposed fatty acid layers as function of primary ion charge state and angle of incidence. By means of TOF mass spectrometry the ejection of intact molecular ions was observed from layers as deep as 200 A underneath the surface. For grazing primary ion trajectories, the charge state dependence of yields are almost independent of the charge state of the primary ions (PI). The results are in fair agreement with theoretical calculations based on crater formation due to a shock wave expanding from the highly energized zone along the nuclear track.

Cluster emission and chemical reactions in oxygen and nitrogen ices induced by fast heavy-ion impact

Two ices, O2 and a mixture of O2 and N2, are bombarded by 252Cf fission fragments (FF) (approximately 65 MeV at target surface); the emitted positive and negative secondary ions are analyzed by time-of-flight mass spectrometry (TOF-SIMS). These studies shall enlighten sputtering from planetary and interstellar ices. Three temperature regions in the 28-42-K range are analyzed: (1) before N2 sublimation, in which hybrid chemical species are formed, (2) before O2 sublimation, in which the TOF mass spectrum is dominated by low-mass (O2)p cluster ions and (3) after O2 sublimation, in which (N2)p or (O2)p cluster ions are practically inexistent. In the first region, four hybrid ion series are observed: NOn-1+, N2On-2(+/-), and N4On-4(-). In the second region, two positive and negative ion series are identified: (O2)pO(+/-) and (O2)pO2(+/-). Their yield distributions are fitted by the sum of two decreasing exponentials, whose decay constants are the same for all series. It is observed that the cluster ion desorption from solid oxygen is very similar to that of other frozen gases, but its yield distribution oscillates with a three- or six-atom periodicity, suggesting O3 or 3O2 units in the cluster structure, respectively.