Laser-filamentation-induced condensation and snow formation in a cloud chamber

Using 1 kHz, 9 mJ femtosecond laser pulses, we demonstrate laser-filamentation-induced spectacular snow formation in a cloud chamber. An intense updraft of warm moist air is generated owing to the continuous heating by the high-repetition filamentation. As it encounters the cold air above, water condensation and large-sized particles spread unevenly across the whole cloud chamber via convection and cyclone like action on a macroscopic scale. This indicates that high-repetition filamentation plays a significant role in macroscopic laser-induced water condensation and snow formation.     

Reflection from surfaces with a thin overlayer

An approach to treating experimental reflectometry and ellipsometry data for bulk samples covered by an overlayer is suggested. This approach can be used for measurement of optical constants of solids, characterization of overlayers, and probing the abruptness of the spatial distribution of a bulk dielectric function. Numerical simulation shows that in the soft-x-ray and extreme-UV ranges the method can be applied for overlayers up to 3-8 nm thick.     

Trail formation and condensation of trajectories in a direct walker model with feedback

A directed walker model with external memory is studied by numerical simulations and statistical approaches. The structure of the trail systems depends strongly on the microscopic realization of the feedback mechanism and on the general repulsive or attractive interaction between different paths. Especially, we find nonergodic behavior for kinetic attraction and an ergodic one for repulsive interaction. The strong attraction regime shows a pronounced condensation of trajectories to one common path.     

Kinetics of nanowire formation during vacuum condensation of metals on a crystal surface

The kinetics of nanowire formation on the steps of a crystalline dielectric substrate during vacuum condensation of metals from vapor has been investigated. The time of nanowire formation has been calculated at different temperatures and condensation rates.     

Kinetics of formation of discrete nanostructures during vacuum condensation from a single-component vapor

The kinetics of cluster formation during the vacuum condensation of thin films from a single-component vapor is investigated by numerically solving the system of kinetic equations. The size distributions of clusters containing from a few atoms to several hundred atoms are obtained. The regions of dominant nucleation on active centers (point defects of the crystal substrate) and random nucleation are determined in the “condensation rate-temperature” coordinates. It is demonstrated that the regions corresponding to the pseudolayer and three-dimensional (rough) growth mechanisms can be separated in the condensation rate-temperature coordinates. The inference is made that the experimentally observed bimodal size distributions of islands can be associated with the difference between the growth rates of clusters at the stage preceding the coalescence.     

Observing the formation of long-range order during Bose-Einstein condensation

We have experimentally investigated the formation of off-diagonal long-range order in a gas of ultracold atoms. A magnetically trapped atomic cloud prepared in a highly nonequilibrium state thermalizes and thereby crosses the Bose-Einstein condensation phase transition. The evolution of phase coherence between different regions of the sample is constantly monitored and information on the spatial first-order correlation function is obtained. We observe the growth of the spatial coherence and the formation of long-range order in real time and compare it to the growth of the atomic density. Moreover, we study the evolution of the momentum distribution during the nonequilibrium formation of the condensate.     

Laser-induced desorption of overlayer films off a heated metal substrate

The temperature-induced desorption of adsorbed overlayer films with thicknesses between 4 and 200 ML off a suddenly heated metal substrate is studied using molecular-dynamics simulation. We observe that the rapid heating vaporizes the surface-near part of the overlayer film. The initial heating-induced thermoelastic pressure and the vapor pressure in the vapor film drive the remaining film as a large relatively cold cluster away from the surface. In our simulations, the material present in the developing vapor film amounts to roughly 2 ML and is quite independent of the overlayer film thickness. For cluster thicknesses beyond 40 ML, the desorption time increases only little with film thickness, while the resulting cluster velocity decreases only slightly.     

A model for the dissolution behaviour of a graphite overlayer

A model based on a two-step first-order reaction mechanism is proposed to explain the dissolution behaviour of a graphite overlayer into the substrate. The two steps are (a) the decomposition of the graphite layer on and (b) the dissolution of the individual atoms into the substrate. The model was used to successfully explain some results from the literature.     

Effects of initial humidity and temperature on laser-filamentation-induced condensation and snow formation

We investigated the effects of initial humidity and temperature on the formation of water condensation and various snowflake patterns by firing high-repetition filaments of femtosecond Ti-sapphire laser pulses at 9 mJ/50 fs per pulse into a cloud chamber. While adjusting initial temperature and humidity distributions near the filaments, snowflakes and ice particles with various shapes and sizes were observed on the bottom cold plate. In addition, significant differences were found in the weights of the laser-induced snow below the filament center. From those data, initial conditions of low temperature (<?15 °C) and high humidity (relative to water RH_w > (90 ± 11) % and to ice RH_i > (120 ± 15) %) were found being important to assist an efficient laser-filamentation-induced condensation and snow formation.     

Island formation in metal halide intercalation compouds

Acceptor action, non-stoichiometry, and island formation of metal halide intercalants in graphite are explained by the tendency toward full six-fold halogen coordination at the boundary of the intercalant. The Fermi level is pinned on the acceptor level split-off from the halogen p-derived valence band.     

Steps as adatom sources for surface chemistry: oxygen overlayer formation on Ag(110)

Surface chemical reactions often require a ready supply of substrate atoms to occur. In principle, steps serve as an efficient source of these atoms, provided that detachment rates from the step edges are sufficiently large. In this paper, we characterize atomic detachment rates from steps on clean Ag(110) by examining step fluctuations. We show that these rates are sufficient to supply atoms to form the added-row reconstruction of oxidized Ag(110) when the oxygen partial pressure is low. For high oxygen pressures, however, we find that step detachment rates are slow compared with oxidation rates, and the step source of Ag is supplemented by vacancy-island generation on the terraces. These results are compared to those obtained for the similar O/Cu(110) and O/Ni(110) systems.     

NEXAFS and SEXAFS studies of chemisorbed molecules

Near-edge x-ray absorption fine structure (NEXAFS) and surface extended x-ray absorption fine structure (SEXAFS) spectroscopies and their application to the determination of the adsorption geometry and bonding of low-Z molecules on surfaces are discussed. NEXAFS is characterized by intramolecular resonances and probes the internal structure of the molecule (intramolecular bond lengths and possibly bond angles) as well as its orientation relative to the surface. SEXAFS provides information about the adsorption site and the molecule-substrate distances. After demonstrating the full power of SEXAFS in the analysis of oxygen adsorption on Cu(110) and on Ag(110) an example is given of a complete structure determination for the formate species (HCO₂) on Cu(110) using NEXAFS and SEXAFS.     

Field emission through atoms chemisorbed on a metal surface

An exact general expression for the field emission current is derived under two alternative forms in the frame of a new formalism of tunnelling developed in preceding articles. This formalism is applied to study qualitatively the TED (total energy distribution) in the presence of a chemisorbed atom. It is shown in particular that the overlap between the atomic electronic state and the continuum of states of the vacuum induces, in the high-field regime, a non-negligible variation of the position and width of the resonance peak with the electric field.     

On the structure of a water molecule chemisorbed on a metal surface

A model using the electrostatic image force and the harmonic restoring force has been adapted to find the equilibrium structure of a chemisorbed water molecule on a metal surface. An estimate of the maximum possible increase in the HOH interbond angle upon chemisorption due to image forces is given.     

Laser irradiation of chemisorbed oxygen on Si(111): Electronic states and clump formation of SiO2

The laser annealed Si(111) 1×1 surface with chemisorbed oxygen at submonolayer coverages and its irradiation with a ruby laser has been studied with ultraviolet photoelectron spectroscopy and high-resolution electron-energy-loss spectroscopy. The surface oxide which forms directly upon O₂ exposure is found to be similar to that which forms on the Si(111) cleaved 2×1 and the 7×7 reconstructed surfaces. Ruby-laser irradiation converts this surface oxide at submonolayer coverages into clumps of silicon dioxide and regions of clean silicon. Both surface oxides show electronic transitions in the visible and ultraviolet energy region which may be related to known network and point defects in vitreous and crystalline silicon dioxide.