K. P. Kerns

Chemistry and kinetics of size‐selected cobalt cluster cations at thermal energies. I. Reactions with CO

The chemistry and kinetics of size‐selected Co+n cluster‐ion (n=2–8) reactions with CO are studied using a selected ion drift tube affixed with a laser vaporization source operated under well‐defined thermal conditions. All reactions studied in the present work are found to be association reactions. Their absolute rate constants, which are determined quantitatively, are found to have a strong dependence on cluster size. Similar to the cases of reactions with many other reactants such as H2 and CH4, Co+4 and Co+5 display a higher reactivity toward the CO molecule than do clusters of neighboring size. The multiple‐collision conditions employed in the present work have enabled a determination of the maximum coordination number of CO molecules bound onto each Co+n cluster. It is found that the tetramer tends to bond 12 CO molecules, the pentamer 14 CO, hexamer 16 CO, and so on. The results are interpreted in terms of Lauher’s calculation and the polyhedral skeletal electron pair theory. All the measured maxim...

Generation of metal–carbon and metal–nitrogen clusters with a laser induced plasma technique

During the course of investigating dehydrogenation reactions induced by transition metals, we find that using a carrier gas containing hydrocarbons and ammonia instead of pure helium, in conjunction with a laser vaporization device, enables the facile production of metal–carbon and metal–nitrogen clusters in both the neutral and ionic forms. With only a change in the nature of the carrier gas, a variety of new classes of clusters can be produced.

Studies of reactions of small titanium oxide cluster cations toward oxygen at thermal energies

Abstract Reactions of small titanium oxide cluster cations with oxygen are studied using a selected-ion drift tube operated at thermal energies, in conjunction with a laser vaporization source. Clusters of most compositions are observed to react rapidly with oxygen in the drift tube reactor, but a few of them display very inert behavior. The studies of size-selected oxide clusters indicate that the inert clusters belong to two series, namely, (TiO 2 ) + n (n ⩾ 3) and TiO 3 (TiO 2 ) + m (m ⩾ 1). Interestingly, reactions of the oxygen-poor clusters with the oxygen molecule are seen to yield products containing fewer titanium atoms, while the clusters with higher oxygen-to-titanium ratios undergo attachment of one or two oxygen atoms. The reaction patterns are interpreted in terms of the difference in the bonding structure of the clusters.

Collision induced dissociation of titanium–carbon cluster cations

Titanium–carbon clusters are investigated by collision induced dissociation (CID) using our newly designed triple quadrupole mass spectrometer coupled with a laser vaporization source. Their fragmentation patterns are determined under various collision conditions. It is observed that the Met–Car Ti8C+12 mainly loses a neutral metal atom in the primary dissociation step and several metal atoms in sequential dissociation processes. The dissociation threshold of the Met–Car Ti8C+12 is estimated to be about 9 eV. In addition, several of the larger TixC+y cluster ions, including those containing nine or ten metal atoms, are found to fragment directly to Ti8C+12 during single step dissociation. Product channels for dissociation of Ti9C+12 establishes the ionization potential for Ti8C12 to be equal to, or less than the IP of the titanium atom.

Formation and stability of metallocarbohedrenes: TixMyC12(x + y = 8, M = Nb, Ta, Y, and Si)

Abstract Binary element metallocarbohedrenes, Ti x M y C + 12 (M = Nb, Ta, Y, and Si, x + y = 8) are produced using a laser-induced plasma source. The experimental findings enable the assessment of the results of several theoretical studies, providing indications of some shortcomings and pointingg out useful directions for future experimental work. Moreover, the findings provide further insight into the mechanisms responsible for the formation of metallocarbohedrenes. They show that the formation of σ-bonds between metal atoms and C 2 units are the key to the stability of metallocarbohedrenes, suggesting that late transitional metals should not lead to the formation of stable members of this class of clusters due to their rich d-electron character. The stability of these binary metallocarbohedrenes provides further evidence that these species do constitute another form of cluster, which have the potential of being the building blocks for useful new optical and catalytic materials.

The oxidation induced formation of metallocarbohedrene ions

Oxidation reactions of titanium metallocarbohedrenes (Met–Cars) and their derivatives are studied using a SIDT–LV (selected ion drift tube with laser vaporization source) and a triple quadrupole mass spectrometer, both operated at near thermal energies. A surprising finding is the selective and sole production of Ti8C+12 in the reactions of neutral metal–carbon species with oxygen. However, no Met–Car ions are formed when these neutral Met–Cars derivatives are interacted with other potentially reactive neutral molecules, including N2O and Cl2. The mechanisms for this oxidation induced ionization are discussed in light of oxidation induced excitation and thermionic emission.

Oxidation induced ionization and reactions of metal carbide clusters (Nb, Zr, V, Ta)

Abstract Following our recent report of the oxidation induced formation of Ti8C12+ (H.T. Deng, K.P. Kerns, and A.W. Castleman, Jr., J. Chem. Phys. 104 (1996) 4862), the oxidation induced ionization of niobium and zirconium carbide clusters are studied using a triple quadrupole mass spectrometer coupled with a laser induced plasma reaction source. It was found that reactions of both of these neutral carbide clusters with dioxygen leads to formation of carbide ions. The ion product distributions show that zirconium carbide clusters mainly take the form of Met—Car cations, but niobium carbide clusters favor a cubic-like crystalline pattern. Furthermore, reactions of mass-selected NbxCy+ with dioxygen result in a sequential loss of C2 units from NbxCy+, and leads to formation of Nbx+ and NbxC+ depending on y being an even or odd number. However, NbxCy+ shows comparably low reactivity towards nitrous oxide through a single oxygen abstraction mechanism. In comparison with the reaction products of VxCy+ with dioxygen, the complementary information obtained in the present study suggests that the C2 unit is a basic building block for formation of small early transition metal carbide clusters. The oxidation induced ionization mechanisms are also discussed in relation with the stability, ionization potentials, and structures of the clusters.

Dynamics of Cluster Reactions and Ionization: From Semiconductor to Hydrogen-Bonded/Van Der Waals Systems

Matter of nanoscale dimensions often displays properties unlike that of isolated gas-phase molecules, liquids or solids. Sometimes this is due to the bonding, structure and morphology, while in other cases it is because the small dimensions bring with it new phenomena arising from solvation effects or quantum confinement [1-4]. Work in our group is devoted to investigating the changing properties of matter at differing degrees of aggregation to provide a physical basis for interpreting the unique behavior of such systems. Studies deal with matter comprised of van der Waals (weakly bound) and hydrogen bonded (moderate interactions), as well as metallic and metal compound systems comprised of semiconductor-like materials. The general theme of our work is to elucidate the role of solvation phenomena and, where appropriate, to ascertain when discrete molecular properties begin to coalesce and display collective behavior characteristic of the condensed state [3-6].