Energy loss and fragmentation of 3 keV C60+ ions at grazing scattering from a KCl(0 0 1)

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Abstract

Charge state, angular and energy distributions of reflected projectiles are measured when 3 keV C60+,2+ ions are scattered from a clean and flat KCl(0 0 1) surface under grazing incidence. The dominant charge state is found to be C60+ irrespective of the incident charge state as is expected from the electronic structure of KCl and C60 ions. The observed angular distribution has a well defined peak at a specular angle, indicating that the normal energy is not dissipated during the grazing angle scattering. In spite of no dissipation of the normal energy we observe the fragmentation of the scattered C60+ ions. The energy transferred from the parallel energy to the internal excitations was estimated from the observed fragment distribution. The transferred energy changes from almost 0 to ∼15 eV when the angle of incidence changes from 1° to 6°, which is less than 10% of the observed energy loss of the C60+ ions.

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 C60+ 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 C60+, indicating that keV C60+ ions can be reflected from a surface without fragmentation under grazing incidence. A recent study, however, showed that the fragmentation of C60+ occurs via delayed C2 emission when keV C60+ 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 C60+, and the internal excitations cause the delayed C2 emission. We have also observed fragmentation of C60+ when 3 keV C60+ 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 C60+ 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 C60+ 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 C60+ and C600 by means of the electric field plates. The most striking feature seen in this figure is the negligibly small fraction of C600 [9]. Fig. 2 shows the observed C60+ 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 C60+,2+ ions. The observed charge state distribution shows that C60+ 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 C60+ above KCl(0 0 1) while effective resonant neutralization of C602+ 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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