Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Energy loss and fragmentation of 3 keV ions at grazing scattering from a KCl(0 0 1)
Introduction
The interactions of ions with surfaces have been extensively studied for the past two decades. Considerable progress has been achieved in the understanding of ion–surface interactions, such as charge exchange, energy loss, secondary particle emission, using grazing angle scattering of ions from surfaces [1]. Compared to atomic ions, however, the interaction of molecular/cluster ions with surfaces was rarely studied. The interesting aspect of the cluster–surface interaction is the internal degrees of freedom. The internal excitations may play an important role during ion–surface scattering. When a cluster ion impinges on the surface, however, the cluster ion easily shatter into fragment ions. As a result, it is difficult to observe the role of the internal excitations in the ion–surface interaction.
Regarding the fragility of the cluster ions, the Buckminster fullerene ion is unusually stable against surface impacts [2], [3]. Monte Carlo simulations for C60 impact on a structureless potential wall showed that there is a threshold impact energy of ∼150 eV for fragmentation of C60 [4]. This threshold energy corresponds to the grazing angle of incidence θi = 7° for 10 keV , indicating that keV ions can be reflected from a surface without fragmentation under grazing incidence. A recent study, however, showed that the fragmentation of occurs via delayed C2 emission when keV ions are incident on a clean and flat Al(0 0 1) surface at θi = 1–3° [5]. It was shown that the kinetic energy for the motion along the surface normal (normal energy) is efficiently transferred to internal excitations of , and the internal excitations cause the delayed C2 emission. We have also observed fragmentation of when 3 keV ions are incident on a KCl(0 0 1) surface at θi = 1–5° [6]. In this case, however, we did not observe dissipation of the normal energy. The possible source of the internal excitations is therefore the kinetic energy for the motion parallel to the surface (parallel energy), although the mechanism of energy transfer from the parallel energy to the internal excitations was not clarified. In the present paper, we extend our previous study and discuss the energy transfer from the parallel motion to the internal excitations during grazing angle scattering of 3 keV ions from KCl(0 0 1).
Section snippets
Experimental details
A single crystal of KCl was cleaved in air and mounted on a five axis precision goniometer in an ultra high vacuum chamber (base pressure 2 × 10−10 Torr). The surface of KCl(0 0 1) was heated at 300 °C for several hours to prepare a clean surface [7] and kept at 250 °C during the measurements to prevent surface charging [8]. Powder of C60 was evaporated in a small oven installed in a 10 GHz ECR ion source. The ions extracted from the ion source were mass separated by a double focusing 90° sector
Charge state distribution
Fig. 1 shows an example of the scattering angle distribution of the reflected particles when 3 keV ions are incident on KCl(0 0 1). There are three well defined peaks. The sharp peak on the left hand side is the residual incident beam. The reflected particles are separated into two peaks corresponding to and by means of the electric field plates. The most striking feature seen in this figure is the negligibly small fraction of [9]. Fig. 2 shows the observed fraction as a
Conclusion
We have observed the charge state, angular and energy distributions of reflected particles from a clean KCl(0 0 1) surface under grazing angle incidence of 3 keV ions. The observed charge state distribution shows that is dominant irrespective of the incident charge state. This can be understood from the electronic structures of KCl and C60 ions. Both resonant and Auger neutralization processes are prohibited for above KCl(0 0 1) while effective resonant neutralization of is
Acknowledgement
This work was supported in part by Center of Excellence for Research and Education on Complex Functional Mechanical Systems (COE program) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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