Structural transition in(C60)nclusters

Abstract

The structure of (C 60 ) n clusters (nϭ1-150) was investigated by heating them to definite temperatures in a helium bath ͑heating cell͒ with subsequent characterization in a time-of-flight mass spectrometer. The intensity anomalies in the resulting mass spectra reflect particularly stable configurations that resisted evaporation. However, the set of enhanced peaks ͑magic numbers͒ turned out to be temperature dependent. Clusters heated to a moderate temperature of 490 K yielded mass spectra indicating the presence of icosahedral structures that can be characterized by a set of unusually strong peaks, namely nϭ13, 19, 23, 26, 35, 39, 43, 46, 49, 55, 55 ϩ3m, 116, 125, 131, 137, and 147 (mϭ1-14). Based on constant-temperature molecular dynamics simulations it could be concluded that these icosahedra represent metastable structures that had not yet attained thermal equilibrium on the experimental time scale (t s ϭ0.5 ms). Only when the (C 60 ) n clusters were heated to appreciably higher temperatures they could reach thermodynamical equilibrium, manifested by a thermally induced structural transition into close-packed structures, decahedral structures and structures based on the 98-Leary tetrahedron. Thus, mass spectra of clusters heated to 600 K demonstrate the new set of magic numbers, nϭ38, 48, 58, 61, 64, 68, 71, 75, 77, 84, 89, 91, 96, and 98. Additionally, the experimental arrangement used allows both charged and neutral (C 60 ) n clusters to be studied. Thereby, it was possible to show that the structure of large (C 60 ) n clusters is insensitive to their charge state (Ϫ/n/ϩ), disproving a hypothesis that has been discussed in the literature for several years. The heating cell was also used to study the thermally induced dissociation in the fullerene dimers (C 60 ) 2 , (C 70 ) 2 , and (C 60 ) 2 ϩ . The dissociation energies were determined to be 0.275 eV, 0.313 eV, and 0.37 eV, respectively (Ϯ0.08 eV).

Magic alkali-fullerene compound clusters of extreme thermal stability

“…The balance between temperature and annealing time can be exploited to perform a wealth of new experiments probing the PES, as was demonstrated on large clusters of C 60 molecules. 12 It has previously been pointed out that experimentally observed magic peaks deviate from those predicted by electronic and geometrical shell filling arguments, and it has been suggested that entropy might be responsible for this. [17][18][19] Here, we present the first evidence from ab initio calculations that entropy is indeed important in determining absolute cluster stability.…”

“…Details of the experiment are published elsewhere. 12,15 Using this setup the thermal stability of small fullerene clusters was investigated. The peak intensity of ͑C 60 ͒ m clusters in the mass spectra was recorded in the temperature range between T = 150 K to 500 K ͑Ref.…”

“…Equilibration is usually strived for by heating the clusters in the beam, e.g., by photon absorption. [10][11][12] The heating promotes structural ordering of initially disordered clusters, and at the same time causes evaporation of atoms or molecules, reducing the cluster size. 12 As clusters of particularly stable structures resist this evaporation process more strongly, they can thus be identified in the mass spectrum.…”

“…[10][11][12] The heating promotes structural ordering of initially disordered clusters, and at the same time causes evaporation of atoms or molecules, reducing the cluster size. 12 As clusters of particularly stable structures resist this evaporation process more strongly, they can thus be identified in the mass spectrum. In the present report we demonstrate by using a novel cluster heating method, that C 60 -alkali and -alkaline-earth compound clusters with incompletely filled shells are the most stable structures.…”

A combined heating cooling stage for cluster thermalization in the gas phase

“…For instance, well-defined cluster temperatures are necessary for the determination of the heat capacity and melting point of free sodium clusters, [6][7][8] the activation energies and evaporation rate for the evaporation of weakly bound sodium clusters, 9,10 as well as the study of the decay behavior of fullerene dimers. 11,12 Mainly two approaches are commonly used to vary the cluster temperature experimentally. On the one hand, the cluster can be heated by the absorption of laser light.…”