A time-resolved total internal reflection aqueous fluorescence (TIRAF) microscope for the investigation of cell adhesion dynamics

To investigate dynamic processes of cell adhesion on smooth and structured surfaces we developed a time-resolved TIRAF microscope with high spatial and temporal resolution. A lateral image resolution, close to the resolution limit of 223 nm, is reached using a variable microchannel with maximum height of 210 μm for cell cultivation in combination with a ×60 oil immersion objective of numerical aperture 1.4. Using Rhodamine Green as aqueous volume marker, no significant cell membrane damage occurs within 100 min under continuous irradiation in our experimental conditions. In time-discrete experiments with L929 cells over a period of 11 h we observed interactions of adhesion areas with the surface to which the cell adhered. Short-term measurements of a defined area of a single cell containing only a few adhesion contacts showed interactions between single adhesion clusters and also putative formation of new clusters. Stationary TIRAF experiments with cells growing on structured surfaces or between microelectrodes are possible without producing additional reflections. We conclude from our first results that time-resolved TIRAF microscopy is a powerful technique for studying the mechanism of cell motion, especially under a variety of artificial conditions used to influence cell migration. Received: 21 December 1998 / Accepted: 6 January 1999 / Published online: 7 April 1999     

Formation of gold nanowires through self-assembly during scanning force microscopy

Gold films with a nominal thickness of 5–40 monolayers were grown on dielectric substrates and imaged by scanning force microscopy (SFM). The films originally consisted of well-separated or densely packed clusters. During imaging in contact mode, the morphology of the films changed drastically. At low coverage, i.e. Θ<10 monolayers, the well-known stripes originating from mobile clusters, eventually accumulated into larger aggregates, were observed. In contrast, at larger coverage, highly ordered structures consisting of one-dimensional wires evolved during scanning. They often were parallel with equal separation, i.e. well-defined periodicity, over distances of several μm. Typically, the wires were 5–10 nm high and 50–100 nm wide. Investigations of Au films prepared at varying temperature on different dielectric substrates allow us to suggest a self-assembling mechanism for wire formation in which gold is periodically collected by the SFM tip and redeposited as soon as a critical amount is reached. Received: 22 February 1999 / Accepted: 2 March 1999 / Published online: 7 April 1999     

Evanescent wave scattering by aggregates of clusters – application to optical near-field microscopy

Extended Mie-theory is used to investigate scattering and extinction of evanescent waves by aggregates of clusters. In an application to apertureless near-field optical microscopy involving total internal reflection at the surface substrate–air, the variation of the scattered power is calculated when a silicon particle is scanned across single clusters or aggregates of clusters in the evanescent field. Metallic, dielectric, and semiconducting particles are taken into consideration, and the dependence on sizes, materials, and the wavelength is discussed. Received: 27 August 1999 / Revised version: 8 November 1999 / Published online: 1 March 2000     

Structural characterization of pumice-supported silver-palladium metal clusters by means of XAFS and AWAXS

Silver-palladium bimetallic clusters were synthesized on pumice by the reduction of aqueous solution of metal salts with different Pd:Ag ratios. Used as bimetallic catalysts, in hydrogenation reactions, in situations where molecules can undergo several different reactions, they eliminate undesired reactions and maximize the desired ones. To characterize the structure of these bimetallic catalysts and control if the two metallic elements do mix or not to each other to form an alloy, the structure of the Ag-Pd bimetallic clusters was investigated by means of X-ray-absorption fine-structure spectroscopy (XAFS) and anomalous wide angle X-ray scattering (AWAXS) experiments performed at the European Synchrotron Radiation Facility (ESRF) using the GILDA and the BM16 beamlines. A correlation between Ag and Pd was found but not a clear evidence of alloying. Received: 21 December 1998 / Received in final form: 19 March 1999     

Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.?Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T _l=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T _c=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below T _c as well as the two-dimensional decay above T _c.?In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers. Received: 25 March 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

The electronic structure and the band gap of nano-sized Si particles: competition between quantum confinement and surface reconstruction

The electronic structure and especially the band gap of Si_n clusters (n=3–45 atoms) is studied by photoelectron spectroscopy. Contrary to expectations of quantum confinement, almost all clusters studied here have a band gap smaller than that of crystalline Si or even display a continuous (metallic) density of states. We attribute this to covalent bond formation analogous to the reconstructions observed on single-crystal surfaces. Additionally, for Si₃₀ and Si₃₃ a gap size of 0.6 eV (0.4 eV) is observed, supporting the prediction of stable, spherically symmetric structures of these particular clusters. Received: 18 November 1999 / Accepted: 24 November 1999 / Published online: 5 April 2000     

Modelling gold clusters with an empirical many-body potential

Molecular Dynamics Simulated Annealing has been used to probe the structure of small Au clusters consisting of between 2 and 40 atoms. The interatomic interactions within these clusters are described using an empirical Murrell-Mottram many-body potential energy function. Four distinct structural motifs are present in the structures of the predicted global minima, based on octahedra, decahedra, icosahedra and hexagonal prisms. Received 30 September 1999 and Received in final form 23 March 2000     

Percolation with constraints in the highly polarizable oxide KTaO<Subscript>3</Subscript>:Li

Experimental data on the temperature and electric field dependence of the Second Harmonic Generation Intensity in the solid solution KTaO₃:Li is analyzed in the framework of a new percolation-type approach which considers the history of cluster formation averaged over a large number of samples. In order to take into account that two cluster dipole moments turn when joining clusters together, a constraint is imposed on the cluster growth. This constraint leads to cluster-size saturation just after the percolation threshold. Owing to this circumstance, the connected cluster coexists with free clusters in some range of the impurity concentration and temperature. There is qualitative agreement between the computational results and experiments on Second Harmonic Generation intensities. The algorithm was written in C++ which allowed realistic computations to be performed on a standard PC. Received 26 August 1999 and Received in final form 27 December 1999     

Clusters on surfaces: high-resolution spectroscopy at low temperatures

With controlled growth in nanometer-sized pits we produced silver and gold clusters on a graphite surface. We give a summary of the preparation method and discuss the scanning tunneling imaging and the crystalline orientation of the clusters. The electronic structure of the clusters was studied by an in-situ combination of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). For both techniques we obtained an energy resolution in the range of 10 meV employing low sample temperatures. Dynamic final-state effects together with averaging over a cluster-size distribution result in characteristic spectral shapes in UPS, which can be understood referring to STS data taken on individual clusters. Finally, directions for future experiments are pointed out. Received: 13 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Ultrafast relaxation dynamics of electronic excitations in noble-metal clusters

We investigate non-equilibrium relaxation processes in optically excited large gold and silver clusters. Time-resolved pump-probe experiments and model calculations show that optical excitation of the clusters by femtosecond laser pulses results in a heating of the electron system, which is followed by electron cooling via phonon emission. The electron heating leads to an enhanced damping of the surface-plasmon resonance in the clusters. This enhanced damping is caused by an enhancement of the Landau damping and electron scattering rates at high electron temperatures. Furthermore, we find that the rate of electron cooling in the clusters changes with electron temperature; this is a consequence of the temperature-dependent specific heat of the conduction electrons. Finally, pump-probe experiments on ellipsoidal silver clusters show that the thermal expansion of the heated clusters triggers mechanical vibrations at the acoustic eigenfrequencies of the clusters. Received: 6 December 1999 / Published online: 7 August 2000     

Modeling and recording characteristics of a dye-doped polymer bilayer film

The optical, thermal, and structural behavior of a laser-irradiated dye-in-polymer bilayer film consisting of an elastic and a plastic layer was investigated by a computer modeling and recording experiment. The results of numerical calculations describe absorption of optical energy in the recording layers, the heat flow, and the resultant bump formation throughout the multilayer. Recording performance of the material is strongly affected by the dye concentration and the thickness of the plastic layer. The calculated and measured properties of the media are compared, and the relationship between the dye concentration, thickness of plastic layer, and thermal, structural, and recording performance are described. Received: 9 March 1999 / Revised version: 10 May 1999 / Published online: 29 July 1999     

Magnetization of Gd cluster: Anomalous thermal behavior

Theoretical studies of the temperature (T) dependence of magnetization of Gd₁₃ clusters have been carried out within a classical Heisenberg model using Monte-Carlo simulations. It is shown that for a broad range of values of , defined as the ratio between competing ferro and anti-ferro magnetic couplings, the cluster magnetization increases with T in the low T region, as seen in experiment. The clusters are also shown to exhibit a wide distribution of moments at a given T, which broadens significantly with increasing T. It is suggested that this may affect the observed magnetic behavior of magnetic clusters in Stern-Gerlach experiments. Received 29 May 1999 and Received in final form 5 September 1999     

Adiabatic following in standing-wave diffraction of atoms

We report experiments on the diffraction of atoms from a standing light wave in the channeling regime, characterized by long interaction time and large potential height. The observed far-field diffraction patterns depend specifically on the way in which the potential is switched on and off. For fast switching, the evolution is non-adiabatic and many diffraction orders are populated. For slow switching, however, the evolution is adiabatic and the number of populated diffraction orders decreases dramatically. The experiments are performed in two different setups employing rubidium and argon atoms, respectively. In one of the setups, we study the dependence of the diffraction pattern on the interaction time, in the other setup that on the incidence angle. Received: 30 November 1998 / Revised version: 5 July 1999 / Published online: 8 September 1999     

Fluorescence quantum yield of rhodamine 101 in the presence of absorption saturation

Using the photothermal method we measure the fluorescence quantum yield of a rhodamine 101 solution in ethanol for different values of the pump-field fluence. Our experiments reveal a depletion of the fluorescence quantum yield as the pump fluence increases. To explain the observed fluorescence quenching, a dependence of the nonradiative relaxation rates on the field fluence is proposed. Predictions of the model are in good agreement with the experiments. Received: 16 November 1999 / Revised version: 5 July 2000 / Published online: 13 September 2000     

Resonance and composition effects on the Raman scattering from silver-gold alloy clusters

Low-frequency Raman scattering experiments have been performed on thin films consisting of pure gold or gold-silver alloy clusters embedded in alumina matrix. It is clearly shown that the quadrupolar vibrational modes are observed by Raman scattering because of the effect of resonance with the excitation of the electronic surface dipolar plasmon. This is due to the strong coupling between the collective electronic dipolar excitation and the quadrupolar vibrational modes. This effect of resonance does not exist with the core electron excitations. The mixing of the conduction electron dipolar excitation (surface plasmon) with the core electrons leads to the quenching of the resonant Raman scattering. Received 16 November 2000     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Incubation of laser ablation in fused silica with 5-fs pulses

The threshold fluences for laser-induced damage of fused silica with single 5-fs pulses from a Ti:sapphire laser system were determined by extrapolating the ablated volume to zero. These thresholds are about 4 times as high as the values previously obtained from multi-shot experiments. This result is interpreted in terms of an irreversible modification of the original material below the single-shot threshold (incubation). Received: 14 June 1999 / Accepted: 24 June 1999 / Published online: 8 September 1999     

Deposition and diffusion of size-selected (Ag400+) clusters on a stepped graphite surface

400 clusters on a stepped graphite surface by a combination of scanning electron microscopy experiments and computer simulations (molecular dynamics and Monte Carlo methods). We find that the shape of the clusters is only partially deformed by the impact with the surface, moreover the clusters do not create surface defects upon landing, and so are able to diffuse freely over the surface. Many clusters are found to become trapped at surface steps, where their mobility is reduced by the higher binding energy. Exploring the 1-D diffusion of clusters along the steps reveals the low mobility for larger islands, as well as the importance of defects on the step (for example kinks), which trap the mobile clusters. Received: 9 April 1998/Accepted: 25 August 1998     

A variational description of the ground state structure in random satisfiability problems

A variational approach to finite connectivity spin-glass-like models is developed and applied to describe the structure of optimal solutions in random satisfiability problems. Our variational scheme accurately reproduces the known replica symmetric results and also allows for the inclusion of replica symmetry breaking effects. For the 3-SAT problem, we find two transitions as the ratio of logical clauses per Boolean variables increases. At the first one , a non-trivial organization of the solution space in geometrically separated clusters emerges. The multiplicity of these clusters as well as the typical distances between different solutions are calculated. At the second threshold , satisfying assignments disappear and a finite fraction of variables are overconstrained and take the same values in all optimal (though unsatisfying) assignments. These values have to be compared to , obtained from numerical experiments on small instances. Within the present variational approach, the SAT-UNSAT transition naturally appears as a mixture of a first and a second order transition. For the mixed 2+p-SAT with p<2/5, the behavior is as expected much simpler: a unique smooth transition from SAT to UNSAT takes place at . Received 12 July 1999