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Mass Spectrometric Investigations of Binary Clusters:
Evidence for Endohedral Clusters with Enhanced Stability
Peter Lievens, Ewald Janssens, Sven Neukermans, Xin Wang, Nele Veldeman, Roger E. Silverans
Laboratorium voor Vaste-Stoffysica en Magnetisme, K.U.Leuven,
Celestijnenlaan 200D, B-3001 Leuven, Belgium
While clusters composed of rare gas atoms exhibit enhanced stabilities when they have
magic numbers enabling them to adopt high symmetry geometries ("shells of atoms"), the magic
numbers in alkali and other simple metal clusters are determined by the number of delocalized
valence electrons occupying a one-particle state in a (centro-)symmetric potential well ("shells
of electrons"). The quest for such electronically and geometrically closed shells, which goes
along with enhanced cluster stability, has been a leading thread in cluster research for many
years. One route has been the investigation of binary clusters, which allows tailoring both the
cluster geometry (number of atoms) and electronic properties (number of delocalized electrons)
independently.
We produce beams of binary clusters with a dual-target dual-laser vaporization cluster source,
and investigate the clusters in the gas phase with mass spectrometric and laser spectroscopic
techniques. In particular, size and composition dependent stability fluctuations are investigated
with photofragmentation and mass spectrometry, and size dependent ionization energies are
measured with threshold laser ionization spectroscopy.
Lately we focussed on investigations of two types of bimetallic systems: clusters consisting of
noble metals doped with transition metal atoms, and clusters of group IVa elements doped with
metal atoms [1-3]. In this contribution we will present evidence for the existence of combined
closures of shells of atoms and shells of electrons for specific binary cluster species.
Phenomenological interpretations of new electronic shell closures enhanced by size specific
geometry considerations will be given based on comparisons with density functional theory
calculations. In particular the formation of stable icosahedral 13 atom systems with the dopant
atom occupying a central position will be highlighted. Also, size dependent magnetic properties
of endohedral clusters with a magnetic atom occupying a central position will be discussed.
This work was supported by the Fund for Scientific Research – Flanders (FWO), the Flemish
Concerted Action (GOA) program, and the Belgian Interuniversity Poles of Attraction (IAP)
program. EJ and SN are Postdoctoral Researchers of the FWO.
References
[1] S. Neukermans, E. Janssens, H. Tanaka, R.E. Silverans, P. Lievens, Phys. Rev. Lett. 90 (2003)
#033401.
[2] E. Janssens, H. Tanaka, S. Neukermans, R.E. Silverans, P. Lievens, New J. Phys. 5 (2003) #46.
[3] S. Neukermans, E. Janssens, Z. Chen, R.E. Silverans, P.v.R. Schleyer, P. Lievens, Phys. Rev. Lett.
92 (2004) #163401.