Formation of C H Ablation of Graphite, Coal or C Particles Suspended in Selected Solvents - Polyynes: Synthesis, Properties, and Applications - page 144

selective signals of C 2n H 2 (n 2-5) and C 10 at longer delay times. On the

basis of laser power and delay time dependence between vaporization pulse
and ionization pulse, the selective formation of C 2n D 2 (n

¼

2-5) polyynes at
long delay time was attributed to the fragmentation of larger carbon clusters
C m (m>30) by collisions with thermally decomposed hydrogen atoms
produced by intense vaporization pulses, and to survival from heavy
collisions due to the chemically inert nature of polyynes. In our experiments
for the laser ablation of C 60 , besides C 2x carbon clusters, large C 60 2x clus-
ters must be produced after C 2 -loss processes. Thus, similar fragmentation
reactions between large carbon clusters C m (m>30) and H, as proposed for
the formation of C 2n D 2 polyynes from graphite in the gas phase [24], may
also contribute to the formation of C 2n H 2 (n

¼

¼ 4-8) polyynes from C 60 in

solutions.

C 2x 2x ð > 30 Þ þ H ! C y H þ C ð 2x y Þ fragmentation of large clusters ð 7 : 6 Þ

C y H þ H þ M ! C y H 2 þ M ð 7 : 7 Þ

The relative abundance of C 2n H 2 polyynes from C 60 in hexane is larger
than that in methanol at 355 and 532 nm laser irradiation. Bond dissociation
energies of C-H in hexane and C-H and O-H bond in methanol are

4.2,
4.5, and 4.1 eV, respectively [25]. The dissociation energy of C-H bond in
hexane is smaller than that in methanol. Number ratios of H atom in hexane
and methanol in the same volume were calculated to be 1.1:1.0, respectively,
using their known densities, molecular weights, and numbers of hydrogen
atoms in molecules [25]. Thus, the number ratio of hydrogen atom in hexane
is slightly larger than that in methanol. Based upon the above facts, one
possible reason for larger relative abundance of C 2n H 2 from C 60 in hexane
is larger H concentration in hexane than that in methanol. Another reason
is slower H-abstraction reactions of C 2n radicals with methanol than those
with hexane. Therefore, C 2n radicals polymerize to form large carbon ring
clusters, and they finally grow to graphite-like carbon.
It is known that laser ablation of graphite in the gas phase give C atoms
and C m (m 2) clusters with a wide m distribution [26,27]. This is strongly

contrasted with the case of C 60 , where simple C 2 -loss processes are domi-
nated. C 2n H 2 polyynes from graphite are expected to be formed via follow-
ing polymerization and recombination reactions.

ð Þþ ð Þþ !

C n þ m þ

ð : Þ

C n n 1 C m m 1 M M 7 8

C n þ m þ H þ M ! C n þ m H þ M ð 7 : 9 Þ

C n þ m H þ H þ M ! C n þ m H 2 þ M ð 7 : 10 Þ

C n þ m H þ C l H þ M ! C n þ m þ l H 2 þ M ð 7 : 11 Þ