Volume 110, Issue 23, 15 June 1999
 SURFACES, INTERFACES, AND MATERIALS


Coherent acoustic mode oscillation and damping in silver nanoparticles
View Description Hide DescriptionUsing a femtosecond pumpprobe technique, the fundamental mechanical radial mode of silvernanoparticles is coherently excited and probed via its interaction with the electron gas. The mechanical oscillations are launched by an indirect displacive process and are detected via the induced modulation of the surface plasmonresonance frequency. The measured fundamental radial mode period and damping time are found to be proportional to the nanoparticle radius in the range of 3–15 nm, in agreement with theoretical predictions.

Nucleation in periodically driven electrochemical systems
View Description Hide DescriptionWe calculate both the exponent and the prefactor in the nucleation rate of a periodically driven system. Nucleation dynamics is described by the Fokker–Planck equation for the probability distribution of the nuclei over their size. This distribution is found using the concept of the most probable (optimal) nucleation path. The results apply in a broad range of driving force amplitudes, from weak to moderately strong forces where the nucleation rate is changed exponentially strongly, and also in the broad range of the driving frequencies, from lowfrequency driving, where the system follows the force adiabatically, to highfrequency nonadiabatic driving. For strong driving forces, the time dependence of the nucleation rate changes from strongly nonsinusoidal to a weak with the increasing frequency of driving. The response of the nucleation rate to the driving force is described in terms of logarithmic susceptibility (LS), which can be obtained from the optimal nucleation path in the absence of the driving. LS is a smooth function of frequency, and therefore even a driving force with comparatively high frequency can change the modulation rate exponentially strongly. LS and the Faraday current are calculated for simple models of electrochemical systems, where the ac driving is produced by modulation of the electrode potential. We also suggest how to find LS from measurements of the average nucleation rate.

Homogeneous nucleation of supersaturated cesium vapor
View Description Hide DescriptionThe homogeneous nucleation of supersaturated cesium vapor was investigated in a thermal diffusion cloud chamber operating in both the upward and the downward mode. In the upward operating mode, critical supersaturations were measured in the temperature range, 446–492 K. By operating the chamber in the downward mode, it was possible to circumvent experimental difficulties which arise at low pressures (due to phoretic effects) for the measurements made in the upward mode. Our previously published measurements on cesium were obtained in the upward mode in the temperature range, 421–554 K. These earlier measurements agree perfectly (to within their scatter) with the measurements presented here in the temperature range where they overlap, i.e., 421–492 K. The use of the downward mode enabled the extension of the temperature range of the measurements by 132 K towards lower temperatures. This makes cesium the substance whose homogeneous nucleation has been measured, in a thermal diffusion cloud chamber, over the largest range of temperatures, i.e., 289–554 K. The measured critical supersaturations were compared to the predictions of the Internally Consistent version of Classical Nucleation Theory and to the predictions of the Classical Theory.

Unidirectional diffusion of methane in
View Description Hide DescriptionTracer diffusion of methane molecules in the unidirectional channels of has been studied by means of molecular dynamics simulations. A onedimensional hopandcross model is introduced and is shown to be able to reproduce the molecular dynamics results accurately and we profit by extensive speedup in computational time. After elimination of system size effects by using the new model, two regimes can be recognized: a shorttime regime where the mean square displacement is proportional to and a longtime regime where the proportionality is linear.

Surfaceinduced layer formation in polyelectrolytes
View Description Hide DescriptionWe analyze, by means of a random phase approximation (RPA) calculation, the conditions under which a mixture of oppositely weakly charged polyelectrolytes can microsegregate in the neighborhood of a charged surface creating a layered structure. A number of stable layers can be formed if the surface is sufficiently strongly charged even at temperatures at which the bulk of the mixture is homogeneous. The wavelength of the induced concentration fluctuation is proportional to near the microphase transition, where f is the charge density of the polyelectrolytes.

Simulation of kinetic oscillations in surface reactions on reconstructing surfaces
View Description Hide DescriptionA recently introduced lattice gas model [Kuzovkov et al., J. Chem. Phys. 108, 5571 (1998)] is used for the microscopic description of surface reactions on reconstructingsurfaces. The model can easily be adapted to different surface reaction systems, e.g., the CO+NO, and reactions. In addition many reconstructingsingle crystalsurfaces such as Pt(100), Pt(110), Rh(100), and Rh(110) can be simulated because only those properties of the different surface phases are considered which carry the essential physics. Changes in the coordination number are neglected for the different surface phases. In the present study the CO oxidation on Pt single crystal catalysts is investigated for illustration. The model takes CO diffusion and surface reconstruction into account. Very interesting phenomena are observed besides the kinetic oscillations in the particle densities: formation of mesoscopic patterns and synchronization. Only few parameters, as there are the CO gas phase concentration y, the CO diffusion constant D, the CO desorption constant k, and the surface phase propagation velocity V are sufficient to obtain a realistic model showing regular [Pt(110)] and irregular [Pt(100)] oscillations as well as the presence and absence of spatiotemporal patterns, respectively, as observed in experiments. In addition the model gives the critical CO coverage for the surface reconstruction as a result of the model. The small number of parameters renders it possible to study the whole parameter regime and to associate the observed phenomena with these parameters.

A secondorder effect causing the layer structure of arsenic
View Description Hide DescriptionThe role of secondorder perturbations in the interlayer interactions of gray arsenic is discussed. A hypothetical structural change from the germanium (or diamond) to the arsenic structures is considered using a twolayer model in terms of secondorder Jahn–Teller distortions. We show from the concept of transition density or transition force that the secondorder term originating from mixing of the pseudodegenerate bands near the Fermi level leads to the wellknown layer structure of arsenic. The interesting structure of arsenic is a consequence of orbital interactions near the Fermi level.

Calculation of reduced partial cross sections of molecules photodesorbing from a cold crystal surface with internal vibrations: Inclusion of curvecrossing effects
View Description Hide DescriptionA Gaussian wave packet/path integral (GWD/PI) method is used to compute final internal state distributions for a molecule photodesorbing from the surface of a zerotemperature crystal with internal vibrations in the situation where nonadiabatic coupling between two excited statepotential surfaces is significant. The internal state distributions of the desorbed molecule are influenced by vast numbers of internal vibrational state transitions in the crystal which are not resolved in the calculation (or in experiment). A correlation function technique, introduced previously for the case of direct photodissociation on a single excited potential surface, is generalized to systems where two or more excited potential surfaces are nonadiabatically coupled. The accuracy of the method is successfully tested on a twodimensional model for which numerically exact results can be computed. The method is then applied to a collinear model of a diatomic molecule photodesorbing from a chain of atoms coupled by Hooke’s law springs. While exact results cannot be obtained in this case, sum rule checks suggest that the results of the GWD/PI are of acceptable accuracy (fractional error of several percent). It is found that for the class of problems under study, which feature nonadiabatic coupling that decays to zero along the photodesorption coordinate, only a few paths through the electronic state space have significant weight. This suggests that the method can be utilized to treat more complicated problems.

Macroscopic and mesoscopic characterization of a bistable reaction system: CO oxidation on Pt(111) surface
View Description Hide DescriptionThe catalyticoxidation of CO by oxygen on a platinum (111) singlecrystal surface in a gasflow reactor follows the Langmuir–Hinshelwood reaction mechanism. It exhibits two macroscopic stable steady states (low reactivity: COcovered surface; high reactivity: Ocovered surface), as determined by mass spectrometry. Unlike other Pt and Pd surface orientations no temporal and spatiotemporal oscillations are formed. Accordingly, can be considered as one of the least complicated heterogeneous reactionsystems. We measured both the macroscopic and mesoscopic reaction behavior by mass spectrometry and photoelectron emission microscopy(PEEM), respectively, and explored especially the region of the phase transition between low and high reactivity. We followed the ratedependent width of an observed hysteresis in the reactivity and the kinetics of nucleation and growth of individual oxygen and CO islands using the PEEM technique. We were able to adjust conditions of the external control parameters which totally inhibited the motion of the reaction/diffusion front. By systematic variation of these conditions we could pinpoint a whole region of external control parameters in which the reaction/diffusion front does not move. Parallel model calculations suggest that the front is actually pinned by surface defects. In summary, our experiments and simulation reveal the existence of an “experimental” bistable region inside the “computed” bistable region of the reactivity diagram (Sshaped curve) leading to a novel dollar ($)shaped curve.

Oxygen adsorption on Si(100)2×1 via trappingmediated and direct mechanisms
View Description Hide DescriptionWe present the results from a molecular beam study of the initial adsorption probability of on Si(100)2×1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trappingmediated mechanism is dominant, exhibiting a strong increase in with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are welldescribed by a simple precursor model, which gives a difference in activation barrier heights of and a ratio of preexponentials Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in with kinetic energy indicates that a direct chemisorption mechanism is active, with the observed energy scaling proportional to There is also an unusual increase in with surface temperature, with only a weak increase below 600 K, and a stronger increase above 600 K. The direct mechanism trends are discussed in terms of a possible molecular ion intermediate with thermally activated charge transfer. The molecular beam measurements are also used in calculating the reactivity of a thermalized gas with a clean surface. The precursor model is combined with a tworegion fit of the direct adsorption data to predict chemisorption probabilities as a function of the incident conditions. These functions are then weighted by a MaxwellBoltzmann distribution of incident angles and energies to calculate the adsorption probability for a thermal gas. These calculations indicate that the predominant mechanism depends strongly on temperature, with trappingmediated chemisorption accounting for all of the adsorption at low temperatures, and direct adsorption slowly taking over at higher temperatures.
