Previous Issues

  ^{LEIF Newsletter in PDF format}

Coincident measurements of projectile energy loss and electron emission statistics for grazing incidence of slow ions on monocrystalline surfaces


(contributed from Institut für Allgemeine Physik, TU-Wien )

In recent years, the LEIF group at TU Wien in close cooperation with the group of Helmut Winter (Humboldt-Universitaet zu Berlin) has developed and demonstrated new techniques for investigation of slow ion-surface interactions. With these methods we now obtain increasingly more detailed information on the involved inelastic interaction processes. As a starting point, our previously developed detector for slow ion-induced electron emission statistics (ES) [1] was adapted for grazing incidence of primary ions [2]. Coincident measurements of given numbers of emitted electrons with scattered projectiles (cf. fig. 1) permit distinction between specularly and non-specularly scattered particles.

Depending on their angle of incidence and kinetic energy, the specularly scattered particles keep a well-defined distance above the target surface within the so-called selvedge [3]. Since potential electron emission (PE) caused by Auger electron transitions between projectile and target arises already before close surface contact, pertinent measurements with specularly reflected projectiles determine relative contributions from PE and kinetic electron emission (KE) to the total electron yield, as was demonstrated with atomically clean monocrystalline metal- [4] and insulator surfaces [5]. Coincident measurements of ES with the energy loss of grazingly scattered neutral atoms elucidated the mechanisms responsible for KE from LiF(001) [6] and Al(111) [7]. Fig. 2 shows results from similar measurements with multiply charged ions impinging on LiF(001) [8].

In summary, oblique ion impact on polycrystalline target surfaces produces electron yields which depend on experimental conditions (impact geometry, projectile- and electron diffusion in the target bulk) in a rather complex way. With the now available methods a much more detailed understanding of physical processes underlying slow particle-induced electron emission has become possible.

Fig. 2: Coincidence spectrum showing number of ejected electrons vs. scattered projectile energy loss for impact of 18 keV Ar³⁺ on clean LiF(001) under 3.8° with respect to the target surface [8]. From such plots, e.g., we can obtain the contribution to electron emission associated with no kinetic energy loss of the projectile (i.e. "pure" PE) by extrapolation along the red curve to zero energy loss. Alternatively, extrapolation to zero electron emission along the green curve results in purely excitonic projectile energy loss [6].

[1] G. Lakits, F. Aumayr and HP. Winter, Rev.Sci.Instrum. 60, 3151 (1989)
[2] C. Lemell, J. Stöckl, HP. Winter and F. Aumayr, Rev.Sci.Instrum. 70, 1653 (1999)
[3] H. Winter, Phys.Rep. 367, 287 (2002)
[4] C. Lemell et al., Phys.Rev.Lett. 80, 1965 (1998)
[5] J. Stöckl et al., Phys.Scr. T 92, 135 (2001)
[6] H. Winter et al., J.Phys. B:At.Mol.Opt.Phys. 35, 3315 (2002)
[7] S. Lederer et al., Phys.Rev. B 67, 121405(R) (2003)
[8] J. Stöckl, Ph.D. thesis, TU Wien (2003), and J. Stöckl, et al. to be published

This work was supported by Austrian FWF and carried out within Association EURATOM-OEAW.

Top

On the stability of multiply charged C₆₀ and C₇₀ ions

(contributed from Stockholm University,  University of Aarhus, and CIRIL/GANIL)

The C₆₀ fullerene has been used as a target in a vast number of collision experiments over the past decade. Creation of multiply charged C₆₀ ions and the investigation of the subsequent decay processes have been used to probe C₆₀ stabilities and dynamics. However, collisions involving higher fullerenes and clusters of fullerenes have been studied to a much smaller extent. The fragmentations of collisionally excited fullerenes depend on their internal energies, the mobility of the valence electrons and the couplings of electronic and vibrational degrees of freedom. One may ask what happens as the number of atoms, and thus the number of degrees of freedom over which the internal energy can be distributed increases, and how the response to the external perturbation thus changes_ This is one of the questions we try to address in this collaborative project through studies of collisions between slow highly charged ions and C₇₀ and C₆₀ targets. Here we report on measurements of target ionization and fragmentation in electron transfer processes [1]. This is the first such experimental study using a pure C₇₀ target, and we report on the so far highest charge state (10+) of an intact (on the ms time scale) C₇₀ ion produced in collisions with ions.

The projectile ions were provided by the 14.5 GHz ECR ion source at the Manne Siegbahn Laboratory, Stockholm University. At the experimental set-up the projectiles collide with a collimated effusive fullerene jet and are analysed with respect to scattering angles and the numbers of stabilized electrons. The target fragment distributions are analysed by time-of-flight spectrometry - while dissociation energies are deduced from the corresponding recoil detector images, see [1-3] for details. For these experiment a new oven was designed allowing fast target changes and avoiding any contamination from different fullerene powders.

Comparisons between the C₇₀ and C₆₀ mass spectra produced under identical conditions strongly indicate that C₇₀ ions are inherently more stable than C₆₀ ions. The most important evidence for this is the observation of a prominent peak of intact C₇₀¹⁰⁺ ion following 69 keV Xe²³⁺-C₇₀ collisions, while the C₆₀¹⁰⁺-peak is insignificant using the same production method. Further the apparent production cross section of C₇₀⁹⁺ is larger than that of C₆₀⁹⁺ in collisions with Xe²³⁺ regardless of the number of stabilized electrons (s = 1,2 or 3). The intensity distributions in the fragmentation spectra for C₆₀ and C₇₀ are, however, to large extents similar indicating that excitations and decay processes are similar for these two fullerenes. Figure 1 shows mass spectra for 24 keV Xe⁸⁺ collisions with C₇₀ (left), and C₆₀ (right).

Figure 1: Mass spectra from 24 keV Xe⁸⁺-C₇₀ collisions (left) and 24 keV Xe⁸⁺-C₆₀ collisions (right). The upper spectra are taken in coincidence with one electron stabilized on the projectiles (s=1), and the lower with two stabilized electrons (s=2). Position distributions on the recoil detector of the C₆₀³⁺ ions are shown as insets. The dimensions of the detector images are 16´16 mm2, which are only smaller parts of the whole detector area (50 mm in diameter).

Our measured Kinetic Energy Releases (KER's) for asymmetric fission (C²⁺ emission) are found to be similar for the decays of C₇₀^r+ and C₆₀^r+ ions. A simple electrostatic fragmentation model [2,4] in which the fragments are treated as conducting spheres reproduces this behavior. The present experimental KER's are found to be close to previous experimental results for C₆₀ ions using slow highly charged ions, while important differences remain in comparison with some results using electron impact ionization and the MIKE-technique [5,6].

Fission barrier heights for C₇₀ ions, which are deduced from the KER's, are found to be similar to the ones for C₆₀ ions. The observed larger stability for C₇₀^r+ ions (as seen by increased intensity of the higher charge states of C₇₀ ions) is thus rationalized as due to its larger number of internal degrees of freedom (as compared to C₆₀) on which the internal excitation energy may be distributed. Estimates of the internal energies for C₇₀¹⁰⁺ and C₆₀⁹⁺ yield results around 30 eV, and assuming the same internal energies (30 eV) for C₆₀¹⁰⁺ and C₇₀¹¹⁺ we arrive at lifetimes in the ns-range, which readily explains why these ions are not observed in the present experiment in spite of their (estimated) finite fission barriers.

Future experiments aim to study even larger fullerenes and also fullerene derivatives.

More detailed information is available by contacting us:

[1] J. Jensen et al., Phys. Rev. A, accepted for publication (2004).
[2] H. Cederquist et al., Phys. Rev. A 67, 062719 (2003).
[3] J. Jensen et al., NIM B 205, 643 (2003).
[4] H. Zettergren et al., Eur. Phys. J. D (2004), in press.
[5] S. Matt et al., Hyperfine Interactions 99, 175 (1996).
[6] G. Senn et al., J. Chem. Phys. 108, 990 (1998).

Top