Effective temperature in the impact surface region during 100 keV Xe+ implantation of copper bars

The temperature evolution of a copper bar during 100 keV Xe ions implantation has been experimentally recorded. The thermal behaviour of the implanted bar is quantitatively described by a simple model calculation. It is shown that the experimental results may be reproduced by considering a radiative energy dissipation from hightemperature surface regions intersected by ion impact. The quantities characterizing these thermal-spike regions like average temperature and lifetime are consistent with earlier thermodynamical estimations reported in the current literature.     

Reflection electron energy-loss spectra of the fullerenes C60 and C70

High purity polycrystalline samples of C₆₀ and C₇₀ were obtained and studied by the electron energy-loss spectroscopy in the reflection mode. The spectra were used for determination of the loss functions of fullerenes. Loss functions of the fullerenes were compared with those of graphite. It was established that the relative intensities of the peaks corresponding to (+)- and -plasmons depended on the primary electron energy, while the (+)-plasmon energies did not depend on the primary electron energy and were equal to 25.0 eV for C₆₀ and 24.8 eV for C₇₀. The conclusion on the space localization for plasma occilations in fullerenes was made on the base of the study of the energy dependent loss functions.     

Dynamics of cluster deposition on Ar surface

Using a combined quantum mechanical/classical method, we study the dynamics of deposition of small Na clusters on Ar(001) surface. We work out basic mechanisms by systematic variation of substrate activity, impact energy, cluster orientations, cluster sizes, and charges. The soft Ar material is found to serve as an extremely efficient shock absorber which provides cluster capture in a broad range of impact energies. Reflection is only observed in combination with destruction of the substrate. The kinetic energy of the impinging cluster is rapidly transfered at first impact. The distribution of the collision energy over the substrate proceeds very fast with velocity of sound. The full thermalization of ionic and atomic energies goes at a much slower pace with times of several ps. Charged clusters are found to have a much stronger interface interaction and thus get in significantly closer contact with the surface.     

Energy distributions of low energy H ions backscattered from graphite,stainless steel,and molybdenum

Energy distributions of H⁺ and H^– backscattered from graphite, stainless steel and molybdenum were measured for low incident beam energies (1501500 eV). The energy distributions of H^– resembled that of H⁺, while the intensity ratio of H^– to H⁺ varied from material to material and with the incident proton energy. A peak in the energy distribution moved to the cutoff energy with decreasing incident energy, which can be attributed to an increase in nuclear stopping cross section and a decrease in electronic stopping cross section. The ratio of the peak energy to the incident energy is related to the reduced energy of incident beam independent of target materials in the measured region (0.03<< 3.3).     

Influence of oxygen post-treatment on laser-induced damage of antireflection coatings prepared by electron-beam evaporation and ion beam assisted deposition

Antireflection coatings at the center wavelength of 1053 nm were prepared on BK7 glasses by electron-beam evaporation deposition (EBD) and ion beam assisted deposition (IBAD). Parts of the two kinds of samples were post-treated with oxygen plasma at the environment temperature after deposition. Absorption at 1064 nm was characterized based on surface thermal lensing (STL) technique. The laser-induced damage threshold (LIDT) was measured by a 1064-nm Nd:YAG laser with a pulse width of 38 ps. Leica-DMRXE Microscope was applied to gain damage morphologies of samples. The results revealed that oxygen post-treatment could lower the absorption and increase the damage thresholds for both kinds of as-grown samples. However, the improving effects are not the same.     

Dynamics of laser-induced channel formation in water and influence of pulse duration on the ablation of biotissue under water with pulsed erbium-laser radiation

The ability to use fiber-delivered erbium-laser radiation for non-contact arthroscopic meniscectomy in a liquid environment was studied. The laser radiation is transmitted through a water-vapor channel created by the leading part of the laser pulse. The dynamics of the channel formation around a submerged fiber tip was investigated with time-resolved flash photography. Strong pressure transients with amplitudes up to a few hundreds of bars measured with a needle hydrophone were found to accompany the channel formation process. Additional pressure transients in the range of kbars were observed after the laser pulse associated with the collapse of the vapor channel. Transmission measurements revealed that the duration the laser-induced channel stays open, and therefore the energy transmittable through it, is substantially determined by the laser pulse duration. The optimum pulse duration was found to be in the range between 250 and 350 µS. This was confirmed by histological evaluations of the laser incisions in meniscus: Increasing the pulse duration from 300 to 800 µs leads to a decrease in the crater depth from 1600 to 300 µm. A comparison of the histological examination after laser treatment through air and through water gave information on the influence of the vapor channel on the ablation efficiency, the cutting quality and the induced thermal damage in the adjacent tissue. The study shows that the erbium laser combined with an adequate fiber delivery system represents an effective surgical instrument liable to become increasingly accepted in orthopedic surgery.     

Sub-wavelength surface structures on silicon irradiated by femtosecond laser pulses at 1300 and 2100 nm wavelengths

We present periodic ripples and arrays of protrusions formed on the surface of silicon after irradiation by low-fluence linearly polarized femtosecond laser pulses. Laser-induced periodic surface structures (LIPSS) are observed for irradiation at center wavelengths of 800, ∼ 1300, and ∼ 2100 nm, with the structure periods somewhat less than the incident wavelengths in air. Additionally, we observe structures with spatial periods substantially less than the incident laser wavelengths. These sub-wavelength periodic structures form only when the photon energy is less than the silicon bandgap energy. We discuss a number of factors which may contribute to the generation of this surface morphology.     

C 1s core photoemission of C60 and C48N12

We make a theoretical study of the shake-up of the 1s photoemission of C₆₀. The method takes into account the N-body reactions of the π and σ electrons which appear during the formation of the photoemission hole on one carbon atom. We analyze the origin of the satellite in the spectra due to transitions between N-body states. Our calculation shows that the satellite spectra is essentially given by N-body transitions which involve the creation of one or two electron-hole pairs. The method has been applied also to C₄₈N₁₂. The situation is more complex. The spectra of the two most stable species have been investigated. Moreover the influence on the spectra of the position of the hole created on the carbon atom in C₄₈N₁₂ has been examined (all the carbon positions are not equivalent for some isomers).     

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N)∼N^-τ with exponents different for small, up to ∼15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution. Received: 13 October 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-434/982-5660, E-mail: lz2n@virginia.edu     

Morphology and composition on Al surface irradiated by femtosecond laser pulses

We study the surface chemicals and structures of aluminum plates irradiated by scanning femtosecond laser pulses in air for a wide range of laser fluence from 0.38 to 33.6 J/cm². X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate clearly that crystalline anorthic Al(OH)₃ is formed under femtosecond laser pulse irradiation. Besides aluminum hydroxide, crystalline Al₂O₃ is also found in the samples irradiated at high laser fluence. Field emission scanning electron microscopy demonstrates that the surfaces of the samples irradiated with low laser fluence are colloidal-like and that nanoparticles with a few nanometers in size are embedded in glue-like substances. For high laser fluence irradiated samples, the surfaces are highly porous and covered by nanoparticles with uniform size of less than 20 nm.     

Laser etching of transparent materials at a backside surface adsorbed layer

The laser etching using a surface adsorbed layer (LESAL) is a new method for precise etching of transparent materials with pulsed UV-laser beams. The influence of the processing parameters to the etch rate and the surface roughness for etching of fused silica, quartz, sapphire, and magnesium fluoride (MgF₂) is investigated. Low etch rates of 1 nm/pulse and low roughness of about 1 nm rms were found for fused silica and quartz. This is an indication that different structural modifications of the material do not affect the etching significantly as long as the physical properties are not changed. MgF₂ and sapphire feature a principal different etch behavior with a higher etch rate and a higher roughness. Both incubation effects as well as the temperature dependence of the etch rate can be interpreted by the formation of a modified near surface region due to the laser irradiation. At repetition rates up to 100 Hz, no changes of the etch rate have been observed at moderate laser fluences.     

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     

Disordering and annealing of a Si surface under low-energy Si bombardment

Abstract

We simulated the bombardment of Si(100)(2 × 1) by Si atoms using molecular dynamics. The kinetic energies of the projectiles were 100 and 50 eV. To model the Si–Si-interactions the empirical potential of Stillinger and Weber with the two body part of the potential splined to the universal potential was used. A geometric criterion based on the Lindemann radius was defined to study damage in the Si target. We observed clusters of disordered Si atoms in the target induced by the bombardment. Large clusters of about 50 atoms are formed in the beginning of the bombardment; they shrink and decay into smaller clusters until and equilibrium cluster size of about 10 atoms is reached. Upon annealing at elevated temperature the disordered zones dissolve into point defects.     

Backside etching of fused silica with Nd:YAG laser

The laser-induced backside etching of fused silica with gallium as highly absorbing backside absorber using pulsed infrared Nd:YAG laser radiation is demonstrated for the first time. The influence of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography was studied. The comparable high threshold fluences of about 3 and 7 J/cm² for 18 and 73 ns pulses, respectively, are caused by the high reflectivity of the fused silica-gallium interface and the high thermal conductivity of gallium. For the 18 and 73 ns long pulses the etch rate rises almost linearly with the laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. Incubation processes are almost absent because etching is already observed with the first laser pulse at all etch conditions and the etch rate is constant up to 30 pulses.The etched grooves are Gaussian-curved and show well-defined edges and a smooth bottom. The roughness measured by interference microscopy was 1.5 nm rms at an etch depth of 0.6 μm. The laser-induced backside etching with gallium is a promising approach for the industrial application of the backside etching technique with IR Nd:YAG laser.     

The influence of laser-induced surface modifications on the backside etching process

Spectroscopic measurements in the UV/VIS region show reduced transmission through laser-induced backside wet etching (LIBWE) of fused silica. Absorption coefficients of up to 10⁵ cm⁻¹ were calculated from the transmission measurements for a solid surface layer of about 50 nm. The temperatures near the interface caused by laser pulse absorption, which were analytically calculated using a new thermal model considering interface and liquid volume absorption, can reach 10⁴ K at typical laser fluences. The high absorption coefficients and the extreme temperatures give evidence for an ablation-like process that is involved in the LIBWE process causing the etching of the modified near-surface region. The confinement of the ablation/etching process to the modified near-surface material region can account for the low etch rates observed in comparison to front-side ablation.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Plasma and surface modeling of the deposition of hydrogenated carbon films from low-pressure methane plasmas

The elementary mechanisms are described which determine the plasma and surface processes during the plasma-enhanced chemical vapour deposition of hydrogenated carbon films from methane. Corresponding model calculations are reviewed and critically discussed in comparison to experimental results. A realistic modeling requires the simultaneous and self-consistent treatment of plasma and surface effects. Several experimental data sets on plasma parameters and the growth and the composition of the films have been reproduced successfully. However, a broader experimental data base is needed for more critical tests of the models. The reliability of the modeling, in particular of the surface effects, is still limited due to the poor availability of elementary data.     

Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC

We report observation of nanostructures formed on thin TiN and DLC films that were irradiated by 800- and 267-nm, femtosecond (fs) Ti:sapphire laser pulses at an energy fluence slightly above the ablation threshold. On the ablated thin-film surfaces, the linearly polarized fs pulses produce arrays of fine periodic structures that are almost oriented to the direction perpendicular to the laser polarization, while the circularly polarized light forms fine-dot structures. The size of these surface structures is 1/10–1/5 of the laser wavelength and decreases with a decrease in the laser wavelength. Received: 3 September 2002 / Accepted: 4 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-778/62-3306, E-mail: yasuma@fukui-nct.ac.jp