D. J. Trevor

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.

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.

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.

Advances in research on clusters of transition metal atoms

Abstract This article reviews research on the physical and chemical properties of clusters of transition metal atoms, with special emphasis on recent advances made through the new molecular beam and flow techniques. Following our introductory section describing motivation for the study of metal clusters, we outline the major synthetic techniques: Matrix isolation or suspension and free-jet or cold flow condensation of laser-vaporization plasmas. Before discussing the new experiments based on these methods, we review physical and chemical models of transition metal clusters. These models are adaptations of bulk or molecular descriptions of metals or metal containing compounds, and are concerned with the prediction of electronic structure and elementary excitation in clusters, magnetic order and structural rigidity, or chemical reactivity and perturbations. Within this framework we survey recent experimental measurements and their interpretations. Highlighted examples include: (1) the precision measurement of rovibronic or magnetic properties of dimers and trimers as tests of computational electronic structure approximations, (2) measurements of electron binding energies of Fe and Ni clusters as a probe of the molecular versus bulk behavior of these systems, (3) the current status of experiments on the optical properties of small metallic particles, (4) magnetic moment measurements on Fe clusters as critical tests of the molecular theory of metallic and magnetic behavior, (5) discussion of possible effects of cluster melting, and the melting of Au clusters, (6) the facile size-selected reactions of free Fe and Ni clusters with H 2 , 0 2 and CO, and (7) the evidence for new chemistry in the adsorption of hydrocarbons on free Pt. Ir and Ru clusters.

Effect of hydrogen chemisorption on the photoionization threshold of isolated transition metal clusters

Abstract Large increases in the photoionization threshold energies of small V x , NB x , and Fe x clusters ( x = 3–25) induced by chemisorption of H 2 have been observed using photoionization time-of-flight mass spectrometry of a molecular beam. These shifts exhibit a definite dependence both on the number of atoms constituting the bare metal cluster and on the number of chemisorbed hydrogens, and are particularly large (≳ 0.8 eV) for multiple-H 2 chemisorption on small clusters. A simple frontier orbital model for the chemisorption process predicts the direction of adsorbate-induced shifts in cluster ionization threshold for both H 2 and NH 3 as adsorbates.

The Chemistry and Physics of Molecular Surfaces

This article reviews the results of several recent experiments performed in our laboratory designed to elucidate the fundamental chemical and physical properties of clusters of both transition metals and other refractory elements containing from one to several hundred atoms. The gas-phase reactivity of clusters towards a variety of reagents is explored using a fast-flow reactor system. Strong cluster size-dependent variations in reactivity are observed, especially for the case of hydrogen chemisorption. Measurement of cluster photoionization thresholds (IPs) provides a sensitive probe of the evolution of cluster electronic structure as a function of the number of constituent atoms.Cluster ionization potentials are observed to exhibit fluctuations about the smooth global falloff predicted by the classical drop model, indicating the non-bulk-like behavior of small clusters. Measurement of shifts in IP induced by chemisorption of different reagents provides insight into the nature of adsorbate-cluster bonding. The formation and properties of bare and metal-doped carbon clusters are explored, with particular emphasis on elucidating the photophysics and photochemistry of the postulated ultrastable larger clusters. The results suggest that further work is required to prove soccer ball-like structures for C50, C60, etc. Finally, infrared multiple-photon dissociation (IR-MPD) is demonstrated to be a viable technique for obtaining infrared spectra of absorbate-cluster complexes. This technique is an important new tool for obtaining information about the molecularity of gas-phase reactions beyond that currently available from mass spectrometric analysis. As an illustration of the method, IR-MPD spectra of methanol chemisorbed on small iron clusters are obtained.

Infrared spectroscopy of unsupported metal cluster complexes using multiple photon dissociation

Premiere observation de la dissociation induite par un laser IR des complexes agregat moleculaire ― adsorbat dans un faisceau moleculaire. Cas du systeme Fe x (CH 3 OH) y

Ionization threshold energies for metal clusters

We have measured the ionization threshold energies as a function of cluster size for V, Nb, and Fe clusters. The metal clusters are produced by laser vaporization of a metal substrate inside the throat of a pulsed nozzle. The clusters are detected by photoionization TOF mass spectrometry. Using tunable UV lasers, the ionization thresholds are measured as a function of cluster size. In addition, ionization thresholds for clusters with dissociatively chemisorbed hydrogen are found to increase significantly over that of the corresponding bare cluster.

Measurements of magnetic moments of metal atom clusters (abstract)

We report the first measurements of the magnetic moments of isolated metal atom clusters. In particular we have measured the magnetic moments of iron atom clusters ranging in size from 2 to 15 atoms and for aluminum atom clusters Al2 and Al3 and their corresponding mono‐oxides Al2O, Al3O. We produce a supersonic beam of metal atom clusters by pulsed laser vaporization of a metal substrate within the narrow throat of a pulsed nozzle. The clusters are passed through a Stern–Gerlach magnet and the spatial profile of the magnetic deflection pattern is measured using UV laser photoionization combined with time‐of‐flight mass analysis of the resulting photoions. The magnetic moments of the iron clusters are found to increase linearly with the number of atoms in the cluster consistent with a net spin per atom of about 3.0 μB compared to the bulk value of 2.2 μB. These observations are consistent with theoretical predictions of the magnetic moments of iron clusters of 4, 9, and 15 atoms. For Al2 we observe Stern–...