Desorption and sputtering from solids induced by polyatomic projectiles

Abstract

I Introduction E ects induced by cluster ions colliding with solids have been studied for many y ears. There is actually a renewal of interest due to new experimental possibilities. Thin lm deposition 1 shallow ion implantation 2 , surface smoothing and polishing 3 with cluster beams have been developed in several laboratories 4 . Enhanced sputtering with clusters by comparison with single atoms has also been demonstrated long ago by Andersen and Bay 5 with Te and Te 2 projectiles and later on by Beuhler and Friedmann 6 using large water clusters. The secondary emission yield from cluster impact is larger than the sum of the yields of individual atoms. The non linear e ects in the emission yield were also measured with Sb, Sb 2 , S b 3 at 30 keV atom 6 and Bi, Bi 2 at 45 keV atom 7 b o m barding metallic surfaces as Au and Ag. A review on non linear effects in sputtering was recently presented by Andersen 8 . A yield enhancement w as also found for secondary ion emission. This was rst observed with dimers of Ne 2 and Ar 2 9 and with small hydrogen cluster H n 10 . Large non linear e ects in the emission of molecular ions from solid surfaces were an important nding 11, 12 . These authors emphasized the role of cluster projectiles as SF 6 for analytical applications in secondary ion mass spectrometry SIMS , by comparison with single atomic ions of Cs + or Xe + . In order to investigate the mechanisms of secondary emission of neutrals and ions clusters and molecular ions by polyatomic projectiles and clusters, experiments have been performed at Orsay with projectiles over a large energy range from keV to MeV per atom and various types of targets metals, insulating material as thin or thick organic layers . Cluster ion sources and accelerators were used. Most of the experiments were made in the event by e v ent mode at the limit of single impacts on the target. Experiments with keV polyatomic projectiles were also performed at Texas A&M 13 . Actually cluster beams of SF 5 and or CO 2 are being used in SIMS on commercial instruments. The eld of mass spectrometry of organic molecules may gain considerably from these developments. II Secondary ion emission with gold cluster projectilesPositive gold cluster ions Au n with 1 n 10 can be easily produced with a liquid metal ion source 14 . A double pulsation technique allows to select a given mass and therefore a single cluster can be chosen as a projectile 14 . The source with its beam line and deection plates was mounted on a reaction chamber that contains short distance time-of-ight mass spectrometers for secondary ion measurements. The ion emission yield is de ned as the ratio between the number of emitted secondary ions and the number of incident projectiles see ref. 14 . A gold target was bombarded with gold cluster Au n n = 1-5 in the energy range 5 to 60 keV. A non linear e ect was observed in the emission of single negative atomic gold ions, induced by impact of Au 1 to Au 5 . More spectacular is the emission of gold cluster ions from a solid gold target. It was s...

Cluster Size Dependence and Yield Linearity in Cluster Bombardment Simulations of Benzene

“…The question we still have not addressed is why our data and the work of Anders et al indicate that the yield is independent of cluster size while other studies ,,, show a clear dependence on cluster size. The MEDF model provides insight into why the increase in yields might be appear to be independent of projectile type/size when, in reality, there is still a dependence.…”

“…They proposed a model that indicates the yield for any given cluster is proportional to both the cluster size and the total energy of the cluster . Simulations of cluster bombardment of water ice by Russo et al produced a linear increase in yield for both C 60 and Au 3 projectiles; however, the increase for C 60 bombardment was much more rapid than observed for Au 3 bombardment, which was very similar to the results of Le Beyec et al for SIMS detection of a phenylalanine target using C 60 + and Au 4 + ion beams . Anders et al also saw a linear trend for multiple cluster sizes falling on the same line when plotted on a log−log scale for Ar projectiles containing 1−10 000 atoms bombarding a solid argon sample.…”

“…21 Simulations of cluster bombardment of water ice by Russo et al produced a linear increase in yield for both C 60 and Au 3 projectiles; however, the increase for C 60 bombardment was much more rapid than observed for Au 3 bombardment, 10 which was very similar to the results of Le Beyec et al for SIMS detection of a phenylalanine target using C 60 + and Au 4 + ion beams. 24 Anders et al also saw a linear trend for multiple cluster sizes falling on the same line when plotted on a log-log scale for Ar projectiles containing 1-10 000 atoms bombarding a solid argon sample. They hypothesized that the velocity of the cluster is a greater factor in determining ejection yield than the cluster size.…”