Photophysics of buckminsterfullerene and other carbon cluster ions

Abstract

The laser-induced fragmentation behavior of positive carbon cluster ions has been investigated by tandem time-of-flight techniques for the jet-cooled clusters up to 80 atoms in size. Two distinct photophysical regimes were found. The first applies to clusters with 34 atoms or more, all of which dissociate to produce even numbered fragments. Large even clusters fragment by the loss of the high energy species C2, odd ones lose a C atom. The second regime applies to clusters composed of 31 or less atoms, all of which fragment by the loss of C3. These two regimes are sharply separated by C+32 which fragments to produce small cluster ions in the 10–19 atom size range. Fragmentation of the large clusters occurs on a microsecond or faster time scale only at very high levels of excitation (>12.8 eV). These photophysical results are interpreted as consequences of the large even clusters having edgeless, spheroidal cage structures while the small ones have linear chain or ring structures.

Synthesis of C60 from Small Carbon Clusters

“…There do exist closed, fullerene-type solutions for all even-numbered clusters in this size range (with the possible exception of C 22 ). However, two properties characterize even the smallest fullerenes: (1) Even-numbered clus ters are much more stable than odd-numbered clusters (2,43), and (2) the pri mary photofragmentation pathway is loss of C 2 (9). None of the clusters in the C 2 j-C 31 size range exhibit either property.…”

“…None of the clusters in the C 2 j-C 31 size range exhibit either property. Indeed, the primary photofragmen tation pathway for these species is loss of C 3 , which is also the primary photoproduct of the monocyclic rings and the linear chains (9,44). However, the An = 4 intensity alternation, prevalent for the C 1Q -C 20 size range, weakens throughout this region.…”

“…They appear to consist of an unstable transition region between the monocyclic rings and the fullerene-type intermediates, which dominate the C 32 -C 69 size range. The odd-numbered clusters in the C 3^-C 69 size range are unstable toward loss of an atom (9), and are probably quite close to fullerene structures. Unlike even-numbered C n , no closed solution contains only pentagons and hexagons for the odd-numbered clusters, however.…”

“…Here, the major driving force for chemical reaction is the minimiza tion of adjacent pentagons (release of strain), and and are, again, the first stable products. O'Brien et al (9) have proposed closed-shell mechanisms to explain the loss of C 4 , C 6 , etc., observed in fullerene photofragmentation ( Figure 9). These mechanisms cannot be switched to explain fullerene growth because, taken in reverse, they tend to maximize the number of adjacent pentagons.…”

“…This finding is consistent with the calculation that low-frequency bending modes lead to anomalously high vibrational state densities. For C 9 , at an excitation energy of only 2000 cm -1 , equation 4 indicates that the vibra tional state density is 10 8 cm !…”