Joanna M. Hunter

Annealing c60+: synthesis of fullerenes and large carbon rings.

Laser vaporization of graphite generates C60+ cluster ions that are fullerenes and a mixture of roughly planar polycyclic polyyne ring isomers. Experimental studies of the annealing of the non-fullerene C60+ ions indicate that they can be converted (in the gas phase) into the fullerene and an isomer that appears to be a large monocyclic ring. Some fragmentation is associated with conversion to the fullerene geometry, but the majority of the non-fullerene C60+ isomers are cleanly converted into an intact fullerene. The emergence of the monocyclic ring (as the clusters are annealed) suggests that this is a relatively stable non-spheroidal form of these all carbon molecules. The estimated activation energies for the observed structural interconversions are relatively low, suggesting that these processes may play an important role in the synthesis of spheroidal fullerenes.

Electronic and geometric structure in silver clusters

Abstract One-photon ionization mass spectra were recorded for silver clusters (generated by pulsed laser vaporization) throughout the 4.92 to 5.96 eV energy range. Results include: (i) the observation of electronic structure effects to Ag x , x ⩽440 and (ii) the determination of vertical ionization potentials for most x ⩽100 and x =138. Data are discussed in terms of various model calculations incorporating particle-size-dependent electronic/geometric structure and compared to analogous measurements of cold sodium and copper clusters.

Annealing and dissociation of carbon rings

Laser vaporization of graphite generates carbon clusters which possess a number of different geometric structures ranging from planar ring systems to fullerenes. We have employed a multicollision excitation scheme combined with injected ion drift tube techniques to examine the dissociation and annealing of carbon cluster ions. In this paper, we focus on clusters containing 30–50 atoms. Nonfullerene cluster ions in this size range can be annealed in the gas phase to just two dominant isomers—the fullerene and an isomer which appears to be a large monocyclic ring. Fragmentation by loss of C2 units is associated with annealing into the fullerene. This process occurs at low injection energies and rapidly becomes more important with increasing cluster size. However, relatively few clusters follow this path in the size range examined here (less than 3% for C40+). Most of the nonfullerene isomers anneal into a large monocyclic ring which (for clusters in the size range examined here) does not appear to convert i...

Small carbon rings: dissociation, isomerization, and a simple model based on strain

Abstract Injected ion drift tube techniques have been used to examine the properties of carbon rings with 10–36 atoms. Previous studies have shown that the monocyclic ring is the dominant isomer for annealed clusters with 10–36 atoms, while for unannealed clusters a bicyclic ring first appears at n ≈ 22 and becomes the dominant isomer for n > 30. A detailed study of the annealing of the bicyclic ring has been performed for clusters with 24–36 atoms. Activation energies for the bicyclic to monocyclic transition are around 2.4eV and show a slight systematic decrease with increasing cluster size. The fragmentation of carbon cluster ions containing 6–30 atoms has been studied and dissociation energies estimated. A simple model for the strain energy of the carbon rings accounts for many of their physical properties, including isomerization to monocyclic rings, their fragmentation patterns, and dissociation energies. From simple estimates of their stabilities, planar graphite fragments should be lower in energy than the monocyclic rings for medium sized clusters ( n > 30), but little, if any, of this isomer is observed even after annealing. The low abundance of the graphite fragments is attributed to the large activation energy (induced by strain energy) for their formation from monocyclic rings.

Isomerization of pure carbon cluster ions: From rings to fullerenes

Laser vaporization of graphite generates medium size carbon cluster ion (30–100 atoms) which are either fullerenes or a mixture of polyyne ring isomers. Studies of the isomerization of the polyyne ring isomers have been performed using injected ion drift tube techniques. These experiments reveal the existence of giant carbon rings (monocyclic rings containing up to 90 atoms) and show that for some clusters sizes it is possible to convert the polycylic polyyne rings into spheroidal fullerenes. The mechanism of this remarkable structural transformation is discussed.