L. G. Pobo

Gas phase reactions of iron clusters with hydrogen. I. Kinetics

The kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe6 to Fe68 have been investigated. It is found that reaction rate constants are a strong function of cluster size, varying by more than five orders of magnitude in this size range. The largest rate constants correspond to approximately 3% of a hard sphere cross section. Abrupt changes in the rate constant from one cluster to the next are seen. Qualitative temperature dependencies of cluster reactivity have been determined. The more reactive clusters show decreased reactivity with increased tempeature, while the least reactive clusters become more reactive. Strong isotope effects are seen only in the Fe10 to Fe14 size range. Mechanisms for the reactions of H2 and D2 with iron clusters are discussed in light of these observations.

Chemical probes of metal cluster structure: Reactions of iron clusters with hydrogen, ammonia, and water

Evidence is presented for structural changes in iron clusters in the Fe13 to Fe23 size range. Abrupt changes with cluster size are found for several chemical properties, including reactivity with hydrogen and binding energies of ammonia and water. These changes often come at the same cluster sizes, pointing to a common origin—fundamental changes in the structure of the bare iron clusters. In addition, changes in structure as a consequence of adsorbate binding are suggested. The experimental observations leading to these conclusions are detailed, and possible structures for clusters in this size range are proposed.

Reactions of iron clusters with hydrogen. II. Composition of the fully hydrogenated products

Reactions of iron clusters with an excess of hydrogen are found to yield fully hydrogenated products FenHm whose compositions remain fixed over a wide range of hydrogen pressures. For n=6 to 131, the observed m values are always even, have narrow ranges, and for many clusters are unique. Up to n=30, nearly stoichiometric 1:1 ratios of m to n are found. Above 30, cluster hydride compositions are consistent with a monolayer of chemisorbed hydrogen on the cluster surfaces. At sufficiently high hydrogen pressures additional hydrogens bind to the clusters, most likely as a second, physisorbed layer. The experimental results are discussed in terms of cluster structure and the relation to bulk iron behavior.

The reaction of iron clusters with ammonia. I. Compositions of the ammoniated products and their implications for cluster structure

Studies are described of the chemisorption of ammonia on isolated neutral iron clusters Fen for 2≤n≤165. Clusters are generated by laser vaporization in a continuous‐flow‐tube reactor, and reaction products are detected by laser‐ionization time‐of‐flight mass spectrometry. Ammonia is found to chemisorb nondissociatively on cluster surfaces on the 1 ms time scale of these experiments. Measurements of ammonia uptake provide information on adsorption kinetics and on the number and nature of the binding sites. The ammonia binding energy is found to decrease with increasing cluster coverage. For chemically saturated clusters, the ratio of adsorbed NH3 molecules to surface iron atoms is found to decrease with increasing cluster size, going from >1/3 for small clusters to 100. Ammonia chemisorption is accompanied by a large decrease in cluster ionization potentials, as much as 2 eV for saturated clusters. At sufficiently high exposure the beginning of the formation of a second, physisorbed layer of mo...

Reactions of iron clusters with hydrogen. III. Laser‐induced desorption of H2 by multiphoton absorption

Multiphoton absorption and ionization of hydrogenated (or deuterated) iron clusters generally leads to desorption of a specific number of H2 (or D2) molecules for a given cluster size and a given number of photons absorbed. Ionization via single photon absorption occurs without desorption. Experimental results demonstrate that the fragmentation pattern resulting from multiphoton absorption is independent of whether desorption precedes or follows ionization. From the number of desorbed molecules and the number and energy of absorbed photons an estimate of 1.3 eV for the desorption energy can be made without the necessity of modeling the desorption process. A simple statistical model of the process provides similar estimates of the desorption energies, and indicates that the energy has some dependence on cluster coverage.

The A←X transition in Cr2: Predissociation, isotope effects, and the 1–1 sequence band

Two‐color resonance‐enhanced ionization spectroscopy with mass analysis is performed on beams of Cr2 produced by laser vaporization and isentropic expansion cooling. For the A←X 0–0 band near 459.6 nm an extensive rotational spectrum is observed and its intensity profile is found to have a systematic modulation with J. This modulation, and its surprisingly strong dependence on isotopic composition, are interpreted in terms of a two‐step predissociation of the A state and suggest a potentially important new procedure for isotope enrichment in metal systems. An additional resonant feature is found ∼1 nm to the red of the origin band and is assigned to the 1–1 sequence band.

Metal‐deficient iron oxide clusters formed in the gas phase

Molecular beams of oxidized iron clusters are produced by pulsed laser evaporation of iron followed by reaction of the resulting iron clusters with O2 in a continuous flow reactor. Nonstoichiometry of the iron oxide clusters is found, the composition rapidly approaching Fe0.9O as cluster size increases. The relation of these results to the similar nonstoichiometries found in bulk FeO is discussed.

The reactions of iron clusters with water

The reactions of neutral iron clusters Fe7–27 with water are studied in a laser‐vaporization cluster source coupled to a continuous‐flow reactor. Reaction products are detected via laser ionization and time‐of‐flight mass spectrometry. The reactions of room‐temperature clusters with H2O show adsorbate decomposition and hydrogen desorption, as do the reactions with D2O at elevated temperatures. The room‐temperature reaction with D2O appears not to involve any decomposition, and is at equilibrium under the conditions of these experiments. The dependence of reaction extent on D2O pressure yields equilibrium constants for the addition of the first and second D2O molecules. The analysis is complicated by the presence of two‐photon ionization processes that are treated quantitatively with a rate‐equation model. This treatment also yields estimates for cluster photoabsorption cross sections, which are found to be approximately linear in cluster size, having a magnitude of 2.3×10−17 cm2 per iron atom. From the de...

The kinetics of reactions of nickel clusters with hydrogen and deuterium

The kinetics of reactions of nickel clusters with hydrogen and deuterium are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. The abslute rate constants for the addition of the first H2 (D2) molecule to nickel clusters Nin (n=7→36 for H2 andn=7→60 for D2) have been measured. Rate constants are found to be only weakly dependent onn forn≧14, showing a gradual increase with size that scales approximately withn(2/3), i.e., the cluster geometrical cross section. Reaction probabilities for clusters in this size range are approximately 0.6 for H2 and 0.3 for D2. Belown=14, there is a stronger dependence of reactivity on size, with Ni9 being far less reactive than any other cluster studied. These results are compared to bulk nickel studies, and a discussion of possible correlation of reactivity to cluster structure is presented.

The uptake of ammonia by iron clusters: A new procedure for the study of metal cluster chemistry

A new procedure is described for the study of chemical reactions of metal clusters with simple gas phase molecules. It consists of determining the average number of reagent molecules taken up by the clusters as a function of reagent pressure over a very wide range of pressures. When the results of such experiments are appropriately analyzed, they provide information on cluster reaction kinetics, thermodynamics, and the composition of reaction products. An illustration of this procedure for the reaction of iron clusters with ammonia is presented.