Carbon clusters clustering mechanisms

A striking observation that lacks a satisfactory explanation is the existence of magic numbers, i.e. the fact that in a distribution of clusters some species with a certain number of carbon atoms are much more abundant than others. The exact clustering mechanisms are not completely understood, and, as noted by Rohifing et al.(IO), the origin of the observed distribution of clusters may depend upon instrumental factors. Accounting for this fact, however, there still seems to be a preference for clusters with certain numbers of atoms which cannot be explained solely as due to the experimental conditions. 

Since two mechanisms are possible for the competition between association and reaction, detailed ab initio calculations of the potential surface are even more necessary in theoretical determinations of the rates of association channels. More experimental work is also needed it is possible that as a larger number of competitive systems is studied, our understanding of the competition will increase. Critical systems for interstellar modeling include the association/reactive channels for C+ and bare carbon clusters, as well as for hydrocarbon ions and H2. 

The mechanism for the formation of complex hydrocarbons through fullerenes is loosely taken from Helden et al.119 and Hunter et al.,120 and is depicted in Figure 2. As in the work of Thaddeus,117 linear carbon clusters grow via carbon insertion and radiative association reactions, although in this case a large number of additional reactions involving neutral atoms such as C, O, and H and neutral molecules such as H2 are also included. Reactions with H and H2 serve to produce... 

The other key factor for removing atomic carbon from the surface can be achieved by isolating atomic carbon adsorption. The isolated atomic carbon adsorption is easier to remove from surfaces than carbon clusters. By examining the mechanism of the removing process, the appropriate alloy will reduce reaction barriers in the key process, as illustrated in Figure 2.32(b). 

An argument against mechanisms utilizing particular intermediate size clusters can be based on the fact that a wide variety of intermediate size clusters are observed in carbon cluster distributions which ultimately will produce larger clusters. Thus it is hard to account for the high yields of Cff when these other intermediates pathways, which presumably lead to other products, are present. 

As argu in the earlier TTOF study, loss of C2 from a carbon cluster is hard to understand unless that cluster is a closed, edgeless carbon cage. Otherwise it should lose the much more stable C3. The linear chain and monocyclic ring clusters in the 2-30 carbon atom range are known to lose C3, and graphitic sheets should also lose C3. But for closed cages a concerted C2 loss mechanism... 

In order to find a clue to the mechanism of the formation of the hydrocarbons stated above, we have studied the dependence of the TOF mass spectral pattern upon the power of the ablation laser. As a result, it will be proposed that the hydrocarbons are formed by reactions between large carbon clusters C (n 10) and hydrogen atoms which are produced from the buffer hydrogen gas thermally dissociated in the intense field of the ablation laser. 

The mechanism of formation of C60 and other fullerenes is a challenging and controversial area of active research. Detailed understanding of the factors which determine the structures and stabilities of smaller carbon clusters is of obvious importance in this regard. As a final example, we give some results of applying DFT to these types of systems. 

The mechanism of nanotube formation in chemical vapor deposition features characteristics rather distinct from those found for the synthesis by arc discharge or laser ablation. Contrary to the latter, a solution of small carbon clusters in and subsequent diffusion through catalyst particles play a minor role in the deposition from the gas phase. The employed hydrocarbons decompose directly on the surface of the catalytic particle. The carbon, therefore, becomes immediately available for nanotube growth. 

Figure 7. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a DLC interlayer. (I) Formation of carbon clusters on substrate surface and change in bonding structure from sp to sp. (II) Conversion of sp sp bonding.

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