The generation of magic number silica clusters [(SiO 2 ) n O 2 H 3 ]with n ) 4 and 8 by XeCl laser (308 nm) ablation of porous siliceous materials is reported. The production of magic cluster [(SiO 2 ) 4 O 2 H 3 ]can be enhanced by sample selection and experimental optimization so that it becomes the most prominent species in silica clusters. To study the structure of the magic cluster [(SiO 2 ) 4 O 2 H 3 ] -, we performed structural optimization for the neutral bare cluster (SiO 2 ) 4 , the neutral complex cluster (SiO 2 ) 4 O 2 H 4 , and the anionic cluster [(SiO 2 ) 4 O 2 H 3 ]at the HF/6-31G** level. It was found that the ground state of the bare silica tetramer has a linear chain structure whereas a pseudotetrahedral cage-like structural isomer of S 4 symmetry is most stable for the complex cluster (SiO 2 ) 4 O 2 H 4 . The stabilization of the three-dimensional (3D) structure can be attributed to the active participation of the O 2 H 4 group in chemical bonding during cluster formation. Our theoretical calculation and bonding analysis indicate that the magic number anionic cluster [(SiO 2 ) 4 O 2 H 3 ]might also take a pseudotetrahedral structure similar to (but with a different symmetry) that of the neutral precursor (SiO 2 ) 4 O 2 H 4 as the ground state in which the valence, coordination, and bonding characteristics of all the constituent atoms are nearly fully satisfied.

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With a genetic algorithm, the structure of adatom clusters is optimized and studied systematically on different metal fcc ͑111͒ surfaces modeled by the embedded atom method. It is found that the various lowestenergy structures on the different surfaces can be explained in terms of the relative interaction range and the adatom-substrate interaction. The longer the relative interaction range becomes, the sharper or more extended the shape of the lowest-energy structure tends to be, in which the number of next-nearest-neighbor bonds decreases. When the compensation effect from the adatom-substrate interaction becomes strong, the configuration with the smaller number of nearest-neighbor bonds could be the lowest-energy structure; it also induces the shape of the lowest-energy structure as being sharp or extended. In this case, another interesting phenomenon is that the plane of the cluster is obviously bent.

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The magic number silica clusters [(SiO(2))(n)O(2)H(3)](-) with n = 4 and 8 have been observed in the XeCl excimer laser (308 nm) ablation of various porous siliceous materials. The structural origin of the magic number clusters has been studied by the density functional theoretical calculation at the B3LYP/6-31G** level, with a genetic algorithm as a supplementary tool for global structure searching. The DFT results of the first magic number cluster are parallel to the corresponding Hartree-Fock results previously reported with only small differences in the structural parameters. Theoretical calculation predicts that the first magic number cluster (SiO(2))(4)O(2)H(4) and its anion [(SiO(2))(4)O(2)H(3)](-) will most probably take pseudotetrahedral cage-like structures. To study the structural properties of the second magic number cluster, geometries of the bare cluster (SiO(2))(8), the neutral complex cluster (SiO(2))(8)O(2)H(4), and the anionic cluster [(SiO(2))(8)O(2)H(3)](-) are fully optimized at the B3LYP/6-31G** level, and the corresponding vibrational frequencies are calculated. The DFT calculations predict that the ground state of the bare silica octamer (SiO(2))(8) has a linear chain structure, whereas the second magic number complex cluster (SiO(2))(8)O(2)H(4) and its anion [(SiO(2))(8)O(2)H(3)](-) are most probably a mixture of cubic cage-like structural isomers with an O atom inside the cage and several quasi-bicage isomers with high intercage interactions. The stabilization of these structures can also be attributed to the active participation of the group of atoms 2O and 4H (3H for the anion) in chemical bonding during cluster formation. Our theoretical calculation gives preliminary structural interpretation of the presence of the first and second magic number clusters and the absence of higher magic numbers.