I. M. L. Billas

Cage substitution in metal–fullerene clusters

Photofragmentation mass spectra of metal–fullerene clusters C60Mx and C70Mx (M={Fe, Co, Ni, Rh}; x=0,…,30) reveal the existence of a reaction channel which yields clusters having the composition C59−2nM and C69−2nM (n=0,…,10). Enhanced abundances of clusters with 44, 50, and 60 atoms, as well as the presence of clusters containing almost exclusively an even number of atoms, indicate that one carbon atom of the fullerene cage is replaced by a transition metal atom. Additional tandem time-of-flight (TOF) experiments on mass selected C59M and C69M indicate that the initial fragmentation step of this new kind of substitutionally doped fullerenes is the loss of a neutral MC molecule. Measurements on preselected C70Rh3 and C70Ir2 were performed in order to monitor in detail their laser induced transformation into C69Rh and C69Ir.

Bonding character of bimetallic clusters AunXm (X=Al, In, Cs)

Bimetallic cluster ions of composition AunXm+ (X=Al,In,Cs) have been studied using time-of-flight mass spectrometry. The mass spectra of gold–aluminum clusters exhibit electronic shell effects for arbitrary composition. Differences in the sequence of shell closings for gold-rich and aluminum-rich clusters can be explained in terms of the differing free electron densities of the two materials. Spectra of gold-indium clusters indicate the formation of electronic shells only for gold-rich species. Among clusters with a higher indium content, the series In+(InAu)n is found to have enhanced stability. This indicates an appreciable charge transfer from gold to indium atoms. Similar spectra are found for the system gold-cesium, where Cs+(CsAu)n are most stable.

First principles calculations of Si doped fullerenes: Structural and electronic localization properties in C59Si and C58Si2

Si-doped heterofullerenes C59Si and C58Si2, obtained from C60 by replacing one and two C atoms with Si atoms, are investigated via first principles calculations. Static geometry optimizations show that structural deformations occur in the vicinity of the dopant atoms and give rise to Si–C bonds significantly larger than the ordinary C–C bonds of the fullerene cage. In the case of C58Si2, the lowest energy isomer has two Si atoms located at distances corresponding to third nearest neighbors. The electronic structure of these heterofullerenes, although globally close to that of C60, is characterized by a strong localization of both the HOMO’s and the LUMO’s on the Si sites. Charge transfer occurs from the dopant atoms to the nearest neighbor C atoms, contributing to the formation of polar Si–C bonds. A detailed analysis of the charge localization, based on the electron localization function and maximally localized Wannier function approaches, reveals that the bonding of Si in the fullerene cage consists of ...

Experimental and computational studies of heterofullerenes

Abstract External doping of fullerenes C 60 and c 70 with heteroatoms is performed in a low pressure condensation cell, through the mixing of a vapor of doping material with a vapor made of preformed fullerenes. Mass spectrometric investigations of the resulting cluster distributions of C 60 M x and C 70 M x clusters are carried out for M = Fe, Co, Ni, Rh, Ir and Si. Further laser induced photofragmentation of these C 60 M x and C 70 M x precursor clusters leads to the production of in-cage doped heterofullerenes with composition C 59−2n M and C 69−n M. We complement our study with ab-initio calculations of the geometrical and electronic structure of C 59 Si and C 58 Si 2 , obtained within density functional theory by relaxing selfconsistently the structures to the local minima.

Fullerenes coated with sulfur and phosphorous molecules

Clusters composed of a single fullerene molecule and several phosphorous or sulfur molecules produced in a low pressure inert gas aggregation cell are investigated by vacuum-UV photoionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra indicate that the molecules form a layer on the fullerene surface.

First principles calculations of iron-doped heterofullerenes

Abstract We present results of first principles calculations of the geometric and electronic structure of the networked iron fullerene C59Fe. This heterofullerene is obtained from C60 by replacing a C atom with a Fe atom and relaxing self-consistently the structure to the local minimum. The C59Fe molecule has a closed-cage structure reminiscent of that of C60, but locally deformed in the vicinity of the Fe dopant atom. On the other hand, in-cage doping with iron leads to an electronic structure considerably modified with respect to that of C60. The Kohn–Sham energy diagram of this new iron coordination compound is presented and features characterizing the Fe–C bonding in this system are discussed.

Polymerized C60 clusters

Clusters produced by co-evaporization of C60 and graphite in a low pressure inert gas condensation cell are characterized by photoionization time-of-flight mass spectrometry. The recorded mass spectra suggest that fullerene molecules can be linked by carbon atoms to form polymers C60(CC60)n−1. The dimer C121 is especially abundant, however we find evidence for polymers containing up to n=7 C60 molecules. Photo-fragmentation measurements of preselected clusters confirm the proposed multicage structure. At moderate fragmentation laser intensities the polymers are found to dissociate into the constituent monomers. At high laser intensities a second minor C2-loss dissociation channel can be observed, indicating the presence of large single cage fullerenes formed by coalescence of the C60 molecules.

Photoabsorption and ionization energies of nonstoichiometric CsI clusters: Metallization of a salt

Cs and CsI vapors were mixed to produce clusters having compositions varying from pure metal to the ionic salt. Ionization potential measurements on these clusters were performed using photoionization and a time-of-flight mass spectrometer. Results are reported for two metallization sequences, Cs14In for n=1–13 and CsmIn for m+n=26, 27 and m>n. The ionization potentials show a good qualitative agreement with recent computations and experiments carried out on nonstoichiometric NaF. Photoabsorption spectra are presented for CsmIn+ clusters for m+n=26, 27. The spectra were obtained by heating mass selected clusters in a beam by means of photoabsorption to induce evaporation of atoms. The resulting mass loss was observed in a time-of-flight mass spectrometer. The spectra of metal rich clusters with m−n>8 are very similar, showing one broad absorption peak around 1.42 eV, the energy of the plasmon resonance of pure Cs clusters of this size. CsmIn+ clusters with less than 7 excess valence electrons show a clear...

Metal Clusters and Atomic Nuclei

It is not obvious that metal clusters should behave like atomic nuclei — but they do. Of course the energy and distance scales are quite different. But aside from this, the properties of these two forms of condensed matter are amazingly similar. The shell model developed by nuclear physicists describes very nicely the electronic properties of alkali metal clusters. The giant dipole resonances in the excitation spectra of nuclei have their analogue in the plasmon resonances of metal clusters. Finally, the droplet model describing the fission of unstable nuclei can be successively applied to the fragmentation of highly charged metal clusters. The similarity between clusters and nuclei is not accidental. Both systems consist of fermions moving, nearly freely, in a confined space.

Fission and dipole resonances in metal clusters

It is not obvious that metal clusters should behave like atomic nuclei—but they do. Of course the energy and distance scales are quite different. But aside from this, the properties of these two forms of condensed matter are amazingly similar. The shell model developed by nuclear physicists describes very nicely the electronic properties of alkali metal clusters. The giant dipole resonances in the excitation spectra of nuclei have their analogue in the plasmon resonances of metal clusters. Finally, the droplet model describing the fission of unstable nuclei can be successively applied to the fragmentation of highly charged metal clusters. The similarity between clusters and nuclei is not accidental. Both systems consist of fermions moving, nearly freely, in a confined space.