ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The growth dynamics, stabilities, and structures of small zirconium oxide clusters (ZrnOm) are studied by covariance mapping time-of-flight mass spectrometry and density functional theory calculations. The zirconium oxide clusters are produced by laser ablation of zirconium metal into a helium gas flow seeded with up to 7% O2. The neutral (ZrnOm) cluster distribution is examined at high and low ionization laser intensities. At high ionization laser intensities (∼107 W/cm2) the observed mass spectra consist entirely of fragmented, nonstoichiometric clusters of the type [(ZrO2)n−1ZrO]+, while in case of lower laser intensities (∼0.2×107 W/cm2), cluster fragmentation is strongly reduced and predominantly stoichiometric clusters (ZrO2)n+ appear. Under such gentle conditions, (ZrO2)5+ is found to be much more abundant than its neighboring clusters (ZrO2)n+, n=1,2,4,6,7,8. The unusually high signal intensity of the Zr5O10+ ion is found to be due to the high stability of the (ZrO2)5 neutral cluster. Density functional theory calculations show a number of different conceivable isomer structures for this cluster and reveal the most likely growth pattern that involves the sequential uptake of ZrO2 units by a (ZrO2)4 cluster to yield (ZrO2)5 and (ZrO2)6. Based on a series of different density functional theory and Hartree–Fock theory calculations, and on kinetic modeling of the experimental results, isomer structures, growth mechanisms, and stability patterns for the neutral cluster distribution can be suggested. The (ZrO2)5 structure most stable at temperatures less than 3000 K is essentially a tetragonal pyramid with five zirconium atoms at the vertices, whereas an octahedral structure is the main building block of (ZrO2)6. Modeling of the covariance matrix over a wide range of ionization laser intensities suggests that (ZrO2)n neutral clusters absorb two photons of 193 nm radiation to ionize and then, for high laser intensity, the ion absorbs more photons to fragment. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1359177
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