D. M. Cox

Production and characterization of supersonic carbon cluster beams

Laser vaporization of a substrate within the throat of a pulsed nozzle is used to generate a supersonic beam of carbon clusters. The neutral cluster beam is probed downstream by UV laser photoionization with time‐of‐flight mass analysis of the resulting photoions. Using graphite as the substrate, carbon clusters Cn for n=1–190 have been produced having a distinctly bimodal cluster size distribution: (i) Both even and odd clusters for Cn, 1≤n≤30; and (ii) only even clusters C2n, 20≤n≤90. The nature of the bimodal distribution, and the intensity alterations in the observed C+n signals are interpreted on the basis of cluster formation and stability arguments. Ionizing laser power dependences taken at several different photon energies are used to roughly bracket the carbon cluster ionization potentials, and, at high laser intensity, to observe the onset of multiphoton fragmentation. By treating the graphite rod with KOH, a greatly altered carbon cluster distribution with mixed carbon/potassium clusters of for...

Photoionization spectra and electronic structure of small iron clusters

Laser photoionization spectra over the range 4.5–6.5 eV have been taken for iron clusters of from 2 to 25 atoms. From the observed ionization thresholds, the cluster ionization potentials are determined and trends in I.P. vs cluster size are examined. In the case of the iron dimer, a sharp, vertical threshold is seen which places the I.P. of Fe2 at 6.30±0.01 eV. Finally, SCF Xα scattered wave molecular orbital calculations have been carried out on the iron dimer and iron trimer. These results show the importance of ferromagnetic spin polarization in the electronic structure of Fe2 and Fe3.

Palladium clusters: H2, D2, N2, CH4, CD4, C2H4, and C2H6 reactivity and D2 saturation studies

In this paper both deuterium saturation measurements and gas‐phase kinetic measurements of chemisorption of H2, D2, N2, CD4, CH4, C2H4, and C2H6 on neutral palladium clusters are reported. Saturation studies with D2 show that small palladium clusters can bind up to three deuterium atoms per palladium atom in the cluster, in contast to H/M ratios near unity typically reported for metal surfaces. In addition, the small palladium clusters exhibit pronounced discontinuities in deuterium uptake which may be indicative of structural transformations or selective desorption of deuterium. From the kinetic studies we find that, in general, the rate constants for a given size cluster towards different reagents tend to order as D2, H2>N2>C2H4>CD4, CH4, C2H6. The shape of the reactivity pattern with the different reagents varies strongly with cluster size for clusters containing less than 25 atoms. Finally, an inverse hydrogen isotope effect is observed for both hydrogen and methane, i.e., the D2 and CD4 rate constant...

Gold clusters: reactions and deuterium uptake

We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n<15) gold cations react readily with D2, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Au10+ is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H2, CH4, but large ones not reactive, odd electron anions reactive towards O2) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH3 and H.

CO chemisorption on free gas phase metal clusters

Pulsed fast flow reactor techniques have been used to study the reactivity of CO toward clusters of many different transition metals; V, Fe, Co, Ni, Cu, Nb, Mo, Ru, Pd, W, Ir, and Pt as well as Al, for clusters containing up to 14 atoms. Clusters are produced by pulsed laser vaporization of metal substrates, injected into the reactor, formed into a molecular beam, and detected by photoionization time of flight mass spectrometry. Our results show that CO is readily chemisorbed on most transition metal clusters containing five or more metal atoms, and that the reactivity for larger clusters varies by a factor of 2 or 3, depending on both cluster size and metal type. Depending on the metal, certain atoms, dimers, trimers, and tetramers exhibit little evidence of reactivity toward CO. This observation is explained in terms of a competition between unimolecular decomposition and collisional stabilization, and leads to a prediction of the ordering of the metal–CO bond strengths.

C60La: a deflated soccer ball?

A recent paper reported very exciting results; the production and detection of C/sub n/La complexes, n = 44, 46, 48,...76, with n = 60 the dominant species. For the dominant C/sub n/La species, C/sub 60/La, the metal atom was postulated, on the basis of the experimental photoionization mass spectroscopy data, to be strongly bound within a spheroidal carbon shell possessing unusually high stability. In this paper we present further experimental evidence which when added to the information presented by HOZLCKTS and Kroto et al. (KHOCS) seriously challenges this enticing conclusion.

Carbon clusters revisited: The ‘‘special’’ behavior of C60 and large carbon clusters

We have performed a series of experiments in which the conditions necessary for synthesis and detection of carbon clusters have been systematically investigated. The effects of ionizing laser intensity and frequency, vaporization and condensation conditions, and graphite substrate composition (pure and metal loaded) on the carbon cluster ion signals show that ‘‘special’’ conditions are needed to observe locally enhanced ion signals on particular clusters (e.g., a large C+60 signal). We present evidence for two sources of carbon clusters, one source which produces mostly smaller (n 32) carbon clusters. In addition we find that several, not just one, metal atom can be attached to C60. Lastly we propose that large variations in ion signals may be explained in terms of the electronic properties of the carbon clusters.

Aluminum clusters: Magnetic properties

We report the first measurement of the magnetic moments of gas phase aluminum clusters ranging in size from 2 to 25 atoms. Aluminum clusters are produced by pulsed laser vaporization of an aluminum rod inside the throat of a high pressure pulsed nozzle. The highly collimated cluster beam is passed through a Stern–Gerlach magnet and the deflected beam is analyzed by spatially resolved photoionization time‐of‐flight mass spectrometry. Aluminum clusters less than nine atoms in size are found to have magnetic moments generally consistent with those predicted from spin and orbital moments of the ground electronic states. As expected, a general trend toward reduced magnetic moment per atom with increasing cluster size is observed.

Dependence of metal cluster reaction kinetics on charge state. I. Reaction of neutral (Nbx) and ionic (Nb+x, Nb−x) niobium clusters with D2

The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo‐first‐order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state...

Electron binding and chemical inertness of specific Nbx clusters

Photoionization threshold measurements on niobium clusters Nbx x=4–29, reveal an unprecedentedly strong dependence on x, the number of atoms in the cluster. Major maxima in threshold energies occurring at x=8, 10, and 16 correspond to those clusters recently shown to be unreactive toward molecular deuterium.