U. Zimmermann

Evidence for a size‐dependent melting of sodium clusters

Geometric shell structure in the mass spectra of sodium clusters was found to disappear as the clusters were heated. The exact temperature at which the shells disappeared was dependent on the size of the clusters. These observations are interpreted as evidence for a size‐dependent melting. Clusters containing 1000 atoms appear to melt at 288 K, clusters containing 10 000 atoms at 303 K. Both values lie well below the bulk melting temperature of 371 K.

Metal coated fullerene molecules and clusters

Coevaporation of C60 and an alkali metal in a gas aggregation cell yields a distribution of clusters with composition (C60)nMx with 0≤x<150, n=1,2,3, and M=Li, Na, and K. For singly ionized clusters the mass peaks are strong for odd values of x but only after reaching the composition C60M7+. It is suggested that the onset of even–odd alternation marks the end of electron transfer between metal and C60 and the beginning of metal–metal bonding.

Metal-coated fullerenes

Clusters of C60 and C70 coated with alkali or alkaline earth metals are investigated using photoionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra of clusters with composition C60Mx and C70Mx (x = 0… 500; M ϵ| Ca, Sr, Ba) seem to be caused by the completion of distinct metal layers around a central fullerene molecule. The first layer around C60 or C70 contains 32 or 37 atoms, respectively, equal to the number of carbon rings constituting the fullerene cage. Unlike the alkaline earth metal-coated fullerenes, the electronic rather than the geometric configuration seems to be the factor determining the stability of clusters with composition (C60)nMx and (C70)nMx, M ϵ| Li, Na,K,Rb, Cs. The units C60M6 and C70M6 are found to be particularly stable building blocks of the clusters. At higher alkali metal coverage, metal-metal bonding and an electronic shell structure appear. An exception was found for C60Li12, which is very stable independently of charge. Semiempirical quantum chemical calculations support that the geometric arrangement of atoms is responsible for the stability in this case.

Producing and detecting very large clusters

The cluster source we use, a low pressure, rare gas condensation cell, is capable of producing clusters containing more than 45 000 atoms or having masses exceeding 2 500 000 amu. Details of this source and the dependence of the cluster size distribution on adjustable working parameters (oven temperature, inert gas pressure, inert gas type) are discussed in this report. Measurements of the mass-dependent velocity distributions of the clusters emitted by the source are presented and compared to a simple model calculation. The clusters are mass-analyzed with a time-of-flight mass spectrometer and detected by a multi-channel plate. The dependence of the detectability of large clusters on the acceleration voltage is investigated.

Quantum chemical study of lithium–C60 clusters

Semiempirical quantum chemical calculations at the modified‐neglect‐of‐diatomic‐overlap self‐consistent‐field level are performed for differently charged clusters of composition LixC60 with x=0...14. The ground state energies of various isomers are calculated to find the most stable configuration for each cluster stoichiometry. The energies required to remove a Li‐atom from these configurations are calculated to determine abundance spectra of distributions of heated LixC60 clusters. These spectra show intensity anomalies at x=6+n (where n=0...+2 is the cluster charge), interpreted to be of electronic origin, and at x=12, interpreted to be of geometric origin. Identical anomalies are observed in experimentally obtained mass spectra of LixC60 clusters.

Fission of highly charged alkali metal clusters

Highly charged metal clusters can be easily produced using the technique of multi-step photoionization. The critical sizes at which fission occurs are reported here for alkali metal clusters having charges as high as +7. An interpretation within the framework of the liquid droplet model reveals that the fission process is strongly asymmetric and thermally activated.

Geometrical shell structure of clusters

Periodic oscillations in the mass spectra of large clusters often indicate the existence of geometric shell structure. There is, in fact a unique relation between the period of the oscillations and the geometry of the cluster. Applying this rule to the data available for aluminum clusters in the size range of 200–15 000 atoms shows that these clusters probably have an octahedral shape.

Detection of large fullerene clusters

Abstract Large clusters are notoriously hard to detect. Clusters of fullerene molecules containing up to 10000 carbon atoms pose a special problem in that they tend to bounce elastically off the detector. High post acceleration and multiple charging can be used to overcome the problem of detection. Intensity anomalies in the mass spectra of (C 60 ) n and (C 70 ) n indicate that these clusters have a shell structure with icosahedral symmetry.

Clusters of fullerene molecules and metal atoms

Intensity anomalies in the mass spectra of (C60)n, (C60)Mx and (C60)nMx clusters can be used to identify particularly stable fullerene and fulleride units. Small, n < 147, fullerene clusters appear to have icosahedral symmetry. Multiple layers of alkaline earth metals can be deposited on single C60 molecules. The first four layers of metal are well described in terms of a periodically reoccurring icosahedral motif. The stability of alkali metal coated C60, on the other hand, is better described by the completion of shells of electrons rather than shells of atoms. Finally, data are presented for shrink-wrapping fullerenes around alkali halide molecules.

Characterization of a Source of Large Clusters

A cluster source, based on vapor condensation in a low pressure, inert gas cell, is capable of producing clusters containing more than 45 000 atoms or having masses exceeding 2 500 000 amu. Details of this source and the dependence of the cluster size distribution on adjustable working parameters (oven temperature, inert gas pressure, inert gas type) are discussed in section 2 of this report.