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Notes: Singly charged argon cluster ions produed by electron impact ionization of a neutral argon cluster beam are found to decay in the metastable time regime by a new mechanism leading to the ejection of "magic'' numbers of neutral argon atoms. The measured dependence of this new decay process on (i) the electron energy, (ii) cluster size and cluster properties, and (iii) the time since ion formation gives insight into the unique mechanism and the nature of this process. At a well-defined threshold energy of ca. 27 eV, the magic number loss mechanism occurs simultaneously with the well-known single monomer evaporation process which proceeds at all energies. Importantly, the new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.

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.

Notes: Cluster growth dynamics of vanadium oxide and titanium oxide clusters produced by laser ablation of vanadium and titanium metal in a He gas flow seeded with up to 2% O2 are studied by covariance mapping time-of-flight mass spectrometry. Covariance mapping enables the recognition of two different distribution components in the overall homogeneous mass spectra for both vanadium oxide and titanium oxide cluster systems. The oxygen-rich component Or shows small correlated fluctuations while the oxygen-poor component Op shows large correlated fluctuations. These two cluster distribution components are observed at low ablation laser powers and low expansion gas concentrations. Fluctuations of small vanadium oxide clusters (V2O, V2O2, and V2O3) and small titanium oxide clusters (Ti2O2 and Ti2O3) are covariance determining. The less fluctuating V2O3 and Ti2O3 clusters are "nuclei" for the oxygen-rich components Or. The more fluctuating V2O and Ti2O2 are "nuclei" for the oxygen poor components Op. Correlated fluctuations or covariances within each distribution component are constant. Covariances for the different distribution components are different. Studies of mass spectra and covariances as functions of ablation laser power and expansion gas concentration imply that V2O and Ti2O2 clusters are formed in different regions of the ablation plasma plume than V2O3 and Ti2O3. We suggest that V2O3 and Ti2O3 are formed in the hot and optically dense region near the ablated metal surface and that V2O and Ti2O2 are formed in the colder plasma region farther away from the ablated metal surface. Larger vanadium oxide and titanium oxide clusters grow from these small clusters by very specific pathways which involve only uptake of VO or VO2, and TiO2, respectively. © 1999 American Institute of Physics.

Notes: Neutral cluster growth and ionic cluster fragmentation are studied for toluene/water (TWn), aniline/argon (AnArn), and 4-fluorostyrene/argon (FSArn). Clusters are created in a supersonic expansion and ionized by both one-color and two-color (near threshold) resonance enhanced laser ionization. Toluene/water clusters are known to fragment subsequent to ionization by loss of water molecules or by proton transfer and loss of a benzyl radical. This system is selected to test the applicability of covariance mapping techniques to investigate the fragmentation behavior of singly charged cluster ions. To explore sensitivity of the parent ion/fragment ion correlation coefficient to cluster fragmentation, correlation coefficients are measured as a function of ionization photon energy as thresholds for the various fragmentation processes are scanned. For TW3+ parent ions, correlation coefficients correctly reflect switching between the benzyl radical loss and water loss fragmentation channels as the photon energy is increased. For T2Wn+ cluster ions, fragmentation contributes only about 20% to the correlation coefficient—the other 80% contribution is due to neutral cluster growth. The growth-dominated correlation coefficients scale approximately with the square root of the product of the two ion signal intensities and linearly with the ionization laser intensity, and therefore are not good relative measures of correlations between ions and signals of different intensities. A normalized covariance (covariance/product of signal intensities) is introduced to eliminate this dependence. The laser intensity [∼(signal product)1/2] independent component of the normalized covariance arises from ion correlation due to neutral cluster growth and the laser intensity dependent component of the normalized covariance arises from ion correlation due to cluster ion fragmentation. These findings are applied to study the cluster growth dynamics of AnArn and FSArn clusters. Covariance mapping shows that the broad intensity maxima in the mass spectrum of FSArn clusters are not caused by fragmentation but can be attributed to neutral cluster growth. The observed neutral cluster distribution appears to be a superposition of three broad, overlapping, log-normal-like distributions peaking around cluster sizes n=4, 8, 20. The difference between the overall shapes of the AnArn and FSArn mass distributions appears to be due to faster dimer and cluster growth kinetics for the FSArn cluster system. The growth kinetics for the latter two cluster systems can be fully explained and modeled by a simple closed form algebraic kinetic equation that depends on three parameters: dimer growth rate, overall cluster growth rate, and a cluster growth cross section that scales with cluster size. © 1998 American Institute of Physics.

Notes: We have obtained direct mass spectrometric evidence that fullerene ions C60z+ (z=1, 2, or 3) and C58z+(z=1,2) undergo unimolecular dissociation by sequential emission of twoC2 units, on a time scale of 10−5 s. Moreover, a comparison of experimental and theoretical breakdown graphs reveals that unimolecular formation of C56+ from the C60+ parent ion within a given observational time window is dominated by successive loss of C2; direct C4 loss does not contribute significantly. This conclusion is not affected by uncertainties in our knowledge of the energetics of C2 vs C4 loss. © 1997 American Institute of Physics.