Atomic, Diatomic, and Cluster Species

It is now fairly well established that atomic and few-atom cluster arrays can be generated and trapped in weakly interacting matrices (92), and subsequently scrutinized by various forms of spectroscopy. Up to this time, IR-Raman-UV-visible absorption and emission-esr-MCD-EXAFS-Mossbauer methods have been successfully applied to matrix-cluster samples. It is self-evident that an understanding of the methods of generating and identifying these species is a prerequisite for 

Solutions were obtained, either analytically or numerically, on a computer. The quenched-reaction, kinetic model considered that the nucleation sequence of reactions evolves to some time (the quenching time) and then promptly halts. Both kinetic models yield a result having the same general form as the statistical model, namely. 

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Tig. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

Observed and Calculated Optical Spectra for Discandium and Dititanium (107) 

Vanadium atom depositions were further studied in alkane matrices (109) in an effort to observe the influence of other low-temperature, matrix environments on the optical spectra and clustering properties of metal atoms. Thus, vanadium atoms were deposited with a series of normal, branched, and cyclic alkanes over a wide range of temperature. The atomic spectra were somewhat broadened compared to those in argon, but the matrix-induced, frequency shifts from gas-phase values were smaller. As shown in Fig. 3, these shifts decrease with in- 

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Most of the electronic properties of molecules and clusters depend on their shape and size. Even though there are varied definitions for shape and size of a species, a widely used definition is the MED topological one. Boyd has used the MED contour to discuss the relative sizes of atoms [49]. On the other hand for molecules, Bader et al. [32] proposed the surface of constant electron density (0.002 a.u.) to describe shape and size of diatomic molecules. Here in the present paper, for justifying the variation in polarizability and to quantify the delocalization of the valence electrons in these clusters, the volume enclosed within the 0.0001 a.u. MED isosurface is quantified. The MED contours at 0.01,0.001, and 0.0001 a.u. of the Lij, Li4, Na2, and Na4 are depicted in Eigure 11.8. The respective volume enclosed within the 0.0001 a.u. MED isosurfaces of lithium and sodium clusters with sizes ranging from 2 through 40 is reported in Table 11.7. 

Of diatomic species, the best-studied molecule is CO, where ab initio cluster-type calculations of Qco are widely used (143). Still, one of the most accurate self-consistent field-configuration interaction (SCF-CI) allelectron calculations by Bagus et al. of the cluster Ni5-CO to mimic CO/Ni(100) gave Qco = 13 kcal/mol (143d), to be compared with the experimental value of 30 kcal/mol (and the BOC-MP estimate of 27 kcal/ mol). A much simpler and rather popular technique is the atom superposi-... 

The electron affinities of clusters behave in a similar manner. This fact, undoubtedly, has a role to play in the chemistry exhibited by nanometals that has been reported in the literature recently. For example, it has been shown that Au atoms (Gold is a noble metal in the bulk state) supported on a TiOa substrate shows a marked size effect in their ability to oxidize the diatomic gas CO to CO2 via a mechanism involving O2 dissociative chemisorption and CO adsorption (Valden et al., 1998). Small Ni particles have also been found to dissociate CO (Doering et al., 1982). Smaller nanoparticles of Ag can dissociate molecular oxygen to atomic oxygen at low temperatures, whereas in the bulk state, the species adsorbed on the Ag surface is O2 (Rao et al., 1992). 

Reversible Equilibria. As noted in Table I, there are many reversible equilibria available in alkali metal vapors to accomodate external stresses and perturbations. The liquid t vapor equilibrium is of fundamental importance in connection with heat pipes (1) and also in many applications, e.g. lithium in fusion (25). Equilibria involving clustering (beginning with atom-atom recombination to the diatom (3, 4)) are just beginning to be understood quantitatively. Equilibria involving ionization are of major importance for the readily ionized alkali metal species (18, 19, 38). A final equilibrium which has apparently been much less studied (1) is the "Mott" metal-insulator transition which occurs for all alkali metal vapors at high density (52, 53). 

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]. 

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