Margie L. Homer

Impact-Induced Cleaving and Melting of Alkali-Halide Nanocrystals

Impact of nanocrystalline alkali-halide clusters against solid surfaces causes them to fission exclusively into low surface-energy fragments. In time-of-flight scattering experiments, this process appears at an impact energy so low that it must result from a single-step cleavage of the nanocrystal along low surface-energy cleavage planes. At higher energies (more than 1 electron volt per atom), a crossover occurs to an entirely different behavior—evaporative cascades that proceed irrespective of the structureenergetic properties of the fragments. These cascades, and the approximately linear scaling of the crossover energy with cluster size, are characteristic of impact-induced transformation of the cluster to a molten state. Collision with the high-rigidity surface of silicon gives a substantially greater cleavage probability than the soft basal-plane surface of graphite.

Generation and photoionization of cold Nan clusters; n to 200

Abstract Featureless abundance distributions are found for sodium atomic clusters in a cold He flowstream by laser ablation, demonstrating that the “magic number” ledges observed previously from oven-beam experiments are caused by high cluster temperatures. The utility of the cold pulsed beams indicated here by experiments on photoionization and photofragmentation properties, including ionization potential estimates for n as large as 137. Electronic and chemical measurements on cold alkali metal clusters in beams or flowing gases will permit critical tests of the electronic shell model.

Demetallization of alkali/alkali-halide clusters: Impact-induced fragmentation of cubic Na14F+12

Abstract The impact of the cubic excess-electron cluster Na 14 F + 12 against a graphite surface at low energies (0–20 eV) results selectively in the formation of Na 13 F + 12 . This fragmentation channel is interpreted as Na (0) desorption from the energized F-center cluster, confirming the prediction of such a low-energy channel [U. Landman, D. Scharf and J. Jortner, Phys. Rev. Letters 54 (1985) 1860] and validating the use of Na-desorption energies in calculating the abundance variations in neutral, excess-electron clusters [E.C. Honea, M.L. Homer and R.L. Whetten., Phys. Rev. Letters 63 (1989) 394]. The resemblance of the fragmentation patterns of Na 14 F + 12 to that of Na 13 F + 12 at higher energy impact (25–60 eV) indicates that the structure of the nascent Na 13 F + 12 generated by impact-heating is the same as that formed in the cold jet.

Ionization energies and stabilities of Na n ,n<25: shell structure from measurements on cold clusters

The stability patterns found in alkali-atomic clusters and their explanation in terms of electronic and structural factors have been controversial for some time. Generation of very cold Nan clusters in a novel source and use of a special photoion normalization method resolve the remaining questions by allowing precise determination of photoionization thresholds. This is demonstrated here for several sizes in the 7<n<26 range, where in two crucial cases the interpretations of earlier ionization threshold measurements on oven-beam clusters [M. Kappes, M. Schär, P. Radi, and E. Schumacher, J. Chem. Phys.84, 1863 (1986)] disagreed with the explanation of the observed stability pattern. Combining the new values with the charged-cluster fragmentation energies of Bréchignac et al. [C. Bréchignac, P. Cahuzac, J. Leygnier, and J. Weiner, J. Chem. Phys.90, 1492 (1989)] yields neutral cluster fragmentation energies that successfully account for the famous “magic-number” ledges [W.D. Knight, K. Clemenger, W.A. de Heer, W.A. Saunders, M.Y. Chou, and M.L. Cohen, Phys. Rev. Lett.52, 2141 (1984)]. Our measurements offer decisive support for the applicability of the spherical/spheroidal electronic shell-model to smaller Nan clusters, even in their low-temperature form.

Excess-electron and excess-hole states of charged alkali halide clusters

Abstract Charged alkali halide clusters from a He-cooled laser vaporization source have been used to investigate two distinct cluster states corresponding to the excess-electron and excess-hole states of the crystal. The production method is UV-laser vaporization of an alkali metal rod into a halogen-containing He flow stream, resulting in variable cluster composition and cooling sufficient to stabilize weakly bound forms. Detection of charged clusters is accomplished without subsequent ionization by pulsed-field time-of-flight mass spectrometry of the skimmed cluster beam. Three types of positively charged sodium fluoride cluster are observed, each corresponding to a distinct physical situation: NanF+n-1 (purely ionic form), Nann+1F+n-1 (excess-electron form), and NanF+n (excess-hole form). The purely ionic clusters exhibit an abundance pattern similar to that observed in sputtering and fragmentation experiments and are explained by the stability of completed cubic microlattice structures. The excess-electron clusters, in contrast, exhibit very strong abundance maxima at n = 13 and 22, corresponding to the all-odd series (2n + 1 = jxkxl;j,k,l odd). Their high relative stability is explained by the ease of Na(0) loss except when the excess electron localizes in a lattice site to complete a cuboid structure. These may correspond to the internal F-center state predicted earlier. A localized electron model incorporating structural simulation results as account for the observed pattern. The excess-hole clusters, which had been proposed as intermediates in the ionization-induced fragmentation of neutral AHCs, exhibit a smaller variation in stability, indicating that the hole might not be well localized.

Adsorption reactions of alkali-halide nanocrystals: identification of an important surface defect

The adsorption reactions of alkali-halide clusters have been investigated on a size-selected basis using flow-reactor methods. The reactivity of larger sodium-fluoride clusters [NanFn−1]+ toward polar molecules NH3 and H2O shows a distinctive pattern as a function ofn at ambient temperature. Comparison with computed structures shows that aparticular kind of defect greatly facilitates the initial adsorption process. This defect can be formed by removal of an ion-pair from the face of an otherwise perfect nanocrystal, to create a basket-like opening for the adsorbed molecule. It is shown that this kind of defect occurs as a most stable low-temperature structure only to a certain size, after which a less reactive defect takes its place. The implications of these findings for the adsorption reactivity of the surfaces of ionic solids are briefly discussed.

Picosecond resonant two-photon ionization of cold sodium clusters

Large sodium clusters, up ton=21, generated by a low-temperature modification of the laser ablation gas jet source were ionized with two photons of visible radiation from an amplified picosecond dye-laser system.