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Notes: The chemical reactions of Si+n (n=10–65) with O2 have been investigated using selected ion drift tube techniques. The smaller clusters are etched by O2 to give Si+n−2 (and two SiO molecules) and the larger clusters chemisorb oxygen forming an SinO+2 adduct. The transition occurs between n=29 and 36 under the conditions employed. There are large variations in the reactivity of the smaller clusters: Si+13, Si+14, and Si+23 are particularly inert. The variations in reactivity are rapidly damped with increasing cluster size and for clusters with 40–65 atoms the reactivity is nearly independent of size. However, these large clusters are ∼102 times less reactive towards O2 than most bulk silicon surfaces. Studies of the temperature dependence of the reactions reveal that they proceed through a metastable precursor state which is probably molecular O2 physisorbed to the cluster surface. Variations in the size of the activation barrier for dissociative chemisorption account for the changes in reactivity with cluster size. However, the difference between the cluster and surface reactivities is not due to the size of the activation barrier, but could be accounted for by the presence of only a few reactive sites on the clusters.

Notes: The photodissociation of aluminum clusters, Al+n (n=7–17), has been studied over a broad energy range (1.88–6.99 eV). Measurements of the lifetimes of the photoexcited clusters are described. Dissociation energies have been determined by comparing the measured lifetimes with the predictions of a simple RRKM model. The dissociation energies show an overall increase with cluster size, but there are substantial oscillations around n=7–8 and n=13–15. Cluster cohesive energies are derived from these results and from previous measurements of the dissociation energies of the smaller clusters. The cohesive energies of the larger clusters (n〉6) are in good agreement with the predictions of a simple model based on the bulk cohesive energy and the cluster surface energy. However, the cohesive energies are substantially larger than the results of recent ab initio calculations. The photodissociation spectrum of Al+8 has been measured and shows a broad absorption feature with a maximum ∼470 nm.

Notes: A molecular beam of zinc–diethyl (ZnEt2) is photodissociated at 248 and 193 nm and the velocity distributions of the photofragments are measured by time-of-flight techniques. One and two photon processes are observed. The dominant one photon process at both wavelengths is the dissociation of (ZnEt2)2 to form two ZnEt2 monomers. The absence of secondary dissociation of the ZnEt2 photofragments at both excitation wavelengths and the small fraction of the available energy partitioned to product translation implicates dissociation to an excited electronic potential energy surface correlating to one electronically excited ZnEt@B|2 monomer. The mass spectrum of the ZnEt2 photofragments is the same as measured for "cold'' ZnEt2 monomers in the molecular beam, suggesting that the electronically excited ZnEt@B|2 monomers have fluoresced prior to ionization in the mass spectrometer. A small photodissociation signal of uncomplexed ZnEt2 is observed only at low expansion pressures. The sensitive dependence of this monomeric photodissociation signal to the Ar pressure of the adiabatic expansion suggests that ground state vibrational excitation is required for monomeric photodissociation at 248 nm.In contrast to the dimer single photon photodissocation channel, when ZnEt2 monomers are photodissociated, a significant fraction of the available energy appears as product translation. A qualitative molecular orbital analysis can explain the observed fast photoproduct velocity if dissociation occurs via a repulsive triplet state which correlates to electronic ground state products. The two photon process observed is assigned to single photon photodissociation of the electronically excited ZnEt@B|2 monomers produced in the dimer photodissociation step. The photofragment velocity distributions for the two photon channel can be quantitatively modeled by sequential ethyl eliminations on the ground state ZnEt2 and ZnEt potential energy surfaces. The product velocity distributions are consistent with a microcanonical energy distribution for both ethyl eliminations. Approximately 50% of the ethyl photofragments are created with sufficient vibrational energy to break the weak ethyl C–H bond (36 kcal/mol) forming ethylene. Implications of the ZnEt2 photodissociation mechanism for Zn film deposition using 248 and 193 nm excimer radiation are discussed.

Notes: A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 A(ring)2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193 nm can be quantitatively described as a statistical, unimolecular decomposition. At 248 nm, the measured product velocity distributions are qualitatively consistent with the mechanism deduced from the 193 nm results, but the energy available for translation at this wavelength is too small to extract quantitative producttranslational energy distributions which are required to independently test the applicability of the statistical dissociation model.

Notes: The results of extensive studies of the chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) with ammonia are described. At room temperature all clusters react at close to the collision rate and collisional annealing of the clusters does not influence their reactivity. At temperatures slightly above room temperature (∼400 K) it is possible to establish an equilibrium. Binding energies of ammonia to the silicon clusters of ∼1 eV were determined from measurements of the equilibrium constants as a function of temperature. These small binding energies indicate that molecular adsorption occurs at close to room temperature. Saturation experiments reveal that ammonia only binds molecularly to a small number of sites on the clusters. In contrast, on bulk silicon surfaces at room temperature, rapid dissociative chemisorption occurs until all the surface dangling bonds are saturated. At temperatures above ∼470 K another process, probably dissociative chemisorption, becomes important. Absolute rate constants were measured for clusters with 30–70 atoms at a temperature of 700 K where the dissociative chemisorption process dominates. The sticking probabilities at this temperature are between 10−3 and 10−5, two to four orders of magnitude smaller than on bulk silicon at 700 K.

Notes: The chemical reactions of Si+n (n=10–50) with ammonia have been studied using injected ion drift tube techniques at thermal energies (296–414 K), and low energy ion beam techniques, at a center of mass collision energy of ∼0.2 eV. Virtually all of the products arise from the adsorption of one or more ammonia molecules on to the parent cluster ion. In the drift tube experiments all clusters (except those with 11, 13, 14, 19, 22, and 23 atoms) were found to react with ammonia at close to the collision rate at room temperature. The reaction rates decrease with increasing temperature. Thermally activated desorption of ammonia from the products contributes to the negative temperature dependence. This observation suggests that unlike bulk silicon surfaces, which are known to adsorb NH3 dissociatively (and desorb H2 at ∼800 K), the silicon clusters may not be able to dissociate ammonia (at least on the time scale of our experiments). For clusters with 30–50 atoms, total cross sections for adduct formation were measured at collision energies of ∼0.2 eV. The cross sections are close to the hard sphere values and increase slowly with cluster size. In contrast to the results of Smalley and co-workers, obtained using Fourier transform ion cyclotron resonance, we do not find Si+33, Si+39, and Si+45 to be particularly unreactive. Several possible explanations for the large differences in the reactivities of these clusters (as measured by the two different experimental techniques) are discussed.

Notes: A crossed laser-molecular beam study of the photodissociation mechanism of Fe(CO)5 has been performed at 193 nm where time-of-flight measurements of the primary iron containing photofragments have been recorded under collision free conditions. The center-of-mass velocity distributions derived from the TOF data by the method of moments show that Fe(CO)2 accounts for 〉99% of all photoproducts formed after absorption of one photon. The only mechanism which quantitatively reproduces the measured velocity distributions is a sequence of three, uncorrelated, statistical CO eliminations. At high photon flux, a second photon can be absorbed by the Fe(CO)2 photofragment which decomposes by an uncorrelated sequential elimination of the remaining two CO ligands.

Notes: The chemical reactions of size selected Si+n (n=10–65) with D2O have been studied using injected ion drift tube techniques between temperatures of 258 and 404 K. The only products detected were a series of Sin(D2O)+m adducts. Large variations in reactivity were observed for the smaller clusters (n〈40) that diminish with increasing cluster size. Si+11, Si+13, Si+14, Si+19, and Si+23 are particularly unreactive compared to their neighbors. At room temperature the larger clusters (n〉40) are a factor of ∼10–1000 (depending on the bulk surface) less reactive towards water than bulk silicon. The reaction rates for all clusters exhibit an unusually strong negative temperature dependence but are independent of the buffer gas pressure. These results suggest that the reaction mechanism probably involves two steps. In the first step, a weakly bound molecularly adsorbed Si+n⋅⋅⋅D2O adduct is produced. The second step involves rearrangement to give a more strongly bound (and probably dissociatively adsorbed) SinD2O+ product. It appears that the reaction rates for some of the smaller clusters show a faster than linear dependence on D2O pressure. One possible explanation for this unusual observation is that a second D2O molecule solvates the transition state and significantly lowers the activation barrier for dissociative adsorption.