Photofragment Imaging of Carbon Cluster Cations: Explosive Ring Rupture

Abstract

Carbon cluster cations (C n + ) produced by laser vaporization are mass selected and photodissociated at 355 nm. Multiphoton dissociation of smaller ions leads to the elimination of neutral C 3 , as in previous work, whereas larger clusters exhibit more varied fragmentation channels. Photofragment velocity-map imaging detects significant kinetic energy release (KER) in the various n − 3 cation fragments. Small cations (n = 6 or 7) with linear structures produce moderate KER, whereas larger cations (n = 10, 11, 12, 15, or 20) having monocyclic ring structures produce much higher KER values. Such high KER values are unanticipated, as optical excitation should produce a wide distribution of internal energies. These carbon clusters have a surprising ability to absorb multiple photons of ultraviolet radiation, achieving a state of extreme excitation prior to dissociation. The remarkable nonstatistical distribution of energy is apparently influenced by the significant ring strain that can be released upon photodissociation.

Probing Colossal Carbon Rings

“…Photofragmentation mass spectra obtained under these conditions are shown in Figure (b). The smaller clusters, up to normalC 22 + , tend to lose either a C 3 or C 5 unit, as observed in previous studies. ,, Rings larger than normalC 26 + also lose C 3 or C 5 units but additionally produce normalC 14 + , normalC 18 + , and normalC 22 + photofragments. These preferred larger cluster fragments would, if they were cyclic, have aromatic stability.…”

“…For the normalC 11 + – normalC 15 + clusters our calculations underestimate the experimental ionization energies reported in ref by between 4% and 17%. The CAM-B3LYP-D3(BJ)/cc-pVDZ method yields similar dissociation and ionization energies to previous calculations, which employed the B3LYP/def2-TZVPP method and the high level CCSD(T)/cc-pVQZ method . High level methods such as CCSD(T) are too expensive for the larger carbon clusters targeted in this study.…”

“…The smaller clusters, up to + C 22 , tend to lose either a C 3 or C 5 unit, as observed in previous studies. 27,30,31 Rings larger than where it can be seen that the lower energy fragmentation channels correspond to the preferred photofragments apparent in Figure 5(b). On energetic grounds, rings in the + C 28 − + C 36 size range can be expected to break up into two smaller rings, one neutral and one charged, consistent with the photofragment mass spectra shown in Figure 5(b).…”

“…We turn now to consider the photodestruction of cyclo[ n ]carbon radical cations. From previous investigations, it is known that charged fullerene carbon clusters decompose mainly through loss of C 2 molecules, whereas smaller clusters with n ≤ 20 tend to lose C 3 or C 5 molecules. ,,, In the intermediate size range 20 ≤ n ≤ 40 the photofragment distributions have been found to be sensitive to ion source conditions and laser power, suggesting that the cluster population contained different isomers, each with a distinct absorption cross section and photodissociation behavior . Here we focus on even carbon rings ( normalC 14 + to normalC 36 + ) and remove any structural ambiguity by selecting and probing only the ring isomer for each cluster size.…”

“…To learn more about the normalC 14 + – normalC 36 + rings and to complement the spectroscopic studies, we have also investigated their photofragmentation behavior. There have been several previous photodissociation studies of carbon cluster cations in the n = 14–20 range (presumably rings), demonstrating that they mainly lose neutral C 3 or C 5 photofragments. − Photodissociation studies for larger rings in the normalC 22 + – normalC 36 + range are potentially complicated by the presence of coexisting isomers–rings and bi-rings for normalC 22 + – normalC 30 + and rings, bi-rings and fullerenes for normalC 32 + – normalC 36 + . For example, Smalley and co-workers found that the photofragment distribution for normalC 36 + depended on light intensity and on ion source conditions, which was taken as evidence for coexisting fullerene isomers (losing C 2 units) and nonfullerene isomers (giving …”