Diatomic clusters

The broken bonds (boldface=dissociated fragment) BDEs (boldface = recommended data reference in parentheses) Methods (reference in  

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Fig. 2.17 Portion of a pulsed-laser time-of-flight spectrum showing the formation of doubly charged Mo diatomic cluster ions in pulsed-laser stimulated field evaporation of Mo. From a multinomial expansion analysis, it is concluded that few Mo+ ions are formed. Although there are a total of only 438 ions in the entire spectrum, all the expected 15 mass lines due to an isotope mixing of the 7 Mo isotopes are present. A mass line, even when it contains only three ions such as that of Min = 100, is clearly identifiable.

Fig. 4.28 The atomic structure of the W (110) surface. Single adatoms sit at the lattice sites. Closely packed diatomic clusters have their centers of mass sitting in...

Atomic jumps in random walk diffusion of closely bound atomic clusters on the W (110) surface cannot be seen. A diatomic cluster always lines up in either one of the two (111) surface channel directions. But even in such cases, theoretical models of the atomic jumps can be proposed and can be compared with experimental results. For diffusion of diatomic clusters on the W (110) surface, a two-jump mechanism has been proposed by Bassett151 and by Cowan.152 Experimental studies are reported by Bassett and by Tsong Casanova.153 Bassett measured the probability of cluster orientation changes as a function of the mean square displacement, and compared the data with those derived with a Monte Carlo simulation based on the two-jump mechanism. The two results agree well only for very small displacements. Tsong Casanova, on the other hand, measured two-dimensional displacement distributions. They also introduced a correlation factor for these two atomic jumps, which resulted in an excellent agreement between their experimental and simulated results. We now discuss briefly this latter study. 

Fig. 4.32 Two-jump mechanism of diatomic cluster diffusion on the W (110) surface with the [110] and [001] intermediate bond configurations.

The time dependence of the probabilities of finding a diatomic cluster in configurations 0 and 1 after time t, starting from the same configuration, are respectively... 

The mean square displacements for the center of mass of the diatomic cluster in a series of walks starting from configurations 0 and 1 are, respectively, given by,... 

Relatively few direct comparisons have been made between results of Xoc calculations and those based on the more elaborate potentials. For many properties of simple systems (atomic ionization potentials , equilibrium spacings, vibrational frequencies and binding energies of first-row diatomics" , clusters of a simple metal, K" , the adsorption of O2 on Ag clusters ), the differences among the results provided by the various potentials are far from overwhelming. This is not surprising in view of the vast body of reasonable results which have been obtained with the Xa. method. 

Table 28.1.1 (continued) BDEs in Rare Gas Diatom Clusters... 

A characteristic of sputtering in SIMS is the formation of molecular ions. Several atoms can leave the surface almost simultaneously and can leave the surface as a bound diatomic cluster. The secondary ions can combine with residual vacuum species such as C, H, and O, with the primary beam ions, and with other ions from the sample. If clusters or molecular ions are detected, it does not mean that they were nearest neighbors in the analyzed surface [5]. 

Kamarchik and Mazziotti used clusters for which the interatomic interactions were approximated via Morse potentials to study the performance of a new method for unbiased structure optimization. They studied both monatomic and diatomic clusters. For the A B clusters, the parameters entering the Morse potential, eqn (2), are then different depending on whether A A, A B, or B B pairs are considered. In their study, Kamarchik and Mazziotti demonstrated that their approach was working for clusters with up to 12 atoms, which, however, is so small that no general conclusion about the performance can be made. 

Bartels, L, Meyer, G., and Rieder, K.H. (1997) Basic steps of lateral manipulation of single atoms and diatomic clusters with a scanning tunneling microscope tip. Phys. Rev. Lett., 79, 697-700. 

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