Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Cluster size and velocity dependences of sputtering and secondary ion emission under gold cluster impact
Introduction
Large nonlinear enhancements of surface emission under cluster impact have been reviewed in recent articles on both sputtering [1] and secondary ion emission [2]. Most of the experimental works deal with small size clusters at low energy (keV range). Such studies are closely linked to the increasing use of cluster projectiles in Secondary Ion Mass Spectrometry, particularly in biological and biomedical mass spectrometry imaging [3], [4], where the accessible mass range is greatly enlarged. However the size and energy dependences of these effects are crucial information for the understanding of the physical processes involved.
The sputtering yields of gold and silver targets have been measured with cluster projectiles of different sizes (n = 1–9) at incident energies ranging from 20 keV/atom up to 5 MeV/atom [5], [6]. The yield variations with the cluster projectile velocity show maxima at velocity values below those of the maximum nuclear energy loss. A maximum sputtering yield was observed at incident energy around 250 keV/atom and 150 keV/atom for gold and silver, respectively. The energies of the maximum nuclear energy loss at the surface, calculated with the SRIM 2008 code [7] are ∼700 keV in gold and ∼500 keV in silver. For sputtering by Au+ atomic ions the maximum yield was found at roughly the same energy as the maximum of the nuclear energy loss at the surface. The large enhancement of gold sputtering yields under cluster impacts was studied by molecular dynamic simulations performed with clusters of various sizes at low energy (16 keV/atom) [8] and with clusters at 150 keV/atom [9]. In more recent simulations the effect of the electronic stopping on the sputtering yield and the duration of the sputtering process was demonstrated. [10], [11].
The negative secondary ion emission yields were measured with smaller size clusters (n = 1–4) for organic (amino acids and lipids) and CsI targets at energies between 4 keV/atom and 10 MeV/atom [12]. The emission yield of the (CsI)2I− cluster ions from the CsI target under impact of and clusters was observed to reach a maximum value at an incident energy of about 40–80 keV/atom, whereas the nuclear energy loss of Au+ ions in CsI is maximal at ∼500 keV.
These experiments evidenced that the maximum yields of emissions under gold cluster impact occur at energies below the energies of the maximum nuclear energy losses. Nevertheless, they did not allow us to assert that sputtering yields and ion emission yields are maximal at different energies, as sputtering and ion emission experiments were performed with different materials and different cluster size ranges. Up to now sputtering experiments with gold cluster beams have been performed with pure metal targets, and no data are available for insulating and compound materials. In addition published data on secondary ion emission yields under cluster impact were obtained with small size clusters (n ⩽ 4) and/or at low energy (keV range). The projectile size dependence (1 ⩽ n ⩽ 9) of the negative secondary ion emission yields from a gold surface measured at the energy of 150 keV/atom was discussed in a previous paper [13].
In this paper we report a comparative study of sputtering and ion emission from a metal, gold, and from an insulating compound, CsI, under the impact of the same gold cluster projectiles. CsI sputtering yields, and gold and CsI ion emission yields were measured using cluster beams (1 ⩽ n ⩽ 9) at energies between 30 and 350 keV/atom. The experimental results are presented and discussed including previously published data on gold sputtering [6].
Section snippets
Experimental
The gold cluster beams were delivered by the 2.5 MV Van de Graaff accelerator of the Institut de Physique Nucléaire de Lyon and the 15MV Tandem accelerator of the Institut de Physique Nucléaire d’Orsay which are equipped with liquid metal ion sources [14], [15].
CsI sputtering yields
The variations of the CsI sputtering yield as a function of the incident energy per atom of , and cluster projectiles are shown in Fig.1. The sputtering yield is expressed by the mean number of ejected atoms per cluster impact. As the changes in the quartz oscillation frequency are proportional to the removed mass, the sputtering yields may be calculated for any molecular combination of Cs and I. They were obtained here by assuming stoichiometric sputtering, i.e. a value of ∼259.8
Discussion
Our study of CsI sputtering under gold cluster impact shows a nonlinear increase of the sputtering yields with the gold cluster size with a maximum yield at energy lower than that of the maximum nuclear energy loss in CsI. We previously reported similar results for sputtering of metals like gold and silver [6]. Moreover, the maximum sputtering yields of the CsI and silver targets were found to occur at roughly the same energy (∼150 keV/at.). We note in this connection that the energy transfer in
Conclusion
The nonlinear enhancement of the CsI sputtering yield under gold cluster impact and the maximum yield found at impact energy lower than the energy of the maximum energy loss are evidences for an emission mechanism from collision spikes. Up to now such studies on sputtering by cluster impact mostly deal with metal sputtering. The energy dependences of the anion emission yields from CsI and gold reveal that the maximum anion yields occur at lower energy than the maximum sputtering yields. This
Acknowledgements
The authors thank Yves Champelovier from the IPNL accelerator group for his efficient technical support during the experiments and M. Pautrat for fruitful discussions during the article drafting.
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Present address: Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium.