Effects of thermal energy deposition on material ejection in poly(methyl methacrylate)

The effects of absorption of 7.9 and 5.0 eV photons by the polymer poly(methyl methacrylate) are studied using molecular dynamics simulations. By rapidly depositing a critical amount of thermal energy in the surface region (greater than 0.03 eV Å⁻³), a pressure wave is formed which causes spallation of the substrate. If there is only one photon absorbed per monomer unit of the polymer, the 7.9 eV photons can supply sufficient energy density to initiate ejection.     

Simulation of sputtering and desorption of gold nanoclusters under bombardment with 180-eV/atom Au400 nanoclusters using the classical molecular dynamics method

Computer simulation of the interaction of an Au₄₀₀ nanocluster (the total energy E = 72 keV) with free spherical Au _N nanoclusters (6 and 12 nm in diameter) and Au₆₀₅₁ clusters deposited on the (111) surface of an Al substrate is performed by means of the classical molecular dynamics method. The distributions of the absorbed energy (ε) converted to one atom of the bombarded nanocluster and the sputtering yield are analyzed. It has been ascertained that the most probable values are either the small (ε ? ε_max = E/N) or the maximum possible (ε ～ ε_max) values of absorbed energy. The total sputtering yield and the absorbed energy decrease with increasing impact parameter. It has been demonstrated that, with a probability of ～10%, a direct impact can lead to ejection of the entire bombarded nanocluster from the substrate. This event occurs in the case where an incident cluster initiates the secondary emission of target-cluster atoms mainly in the direction of the substrate. As a result, the nonsputtered part of the target cluster acquires the momentum in the opposite direction. This recoil effect can be regarded as one of the possible mechanisms by which nanoclusters deposited on substrate surfaces desorb under ion and cluster bombardment.     

Mechanical modifications in dense polyethylene induced by energetic electron beams

Effects of radiation damage in ultra high molecular weight polyethylene are investigated by using energetic electron beams. Special attention is devoted to the mechanical characterization of the polymer, before and after the modification induced by 5 MeV electrons, as a function of the absorbed dose. Elastic modules, ultimate tensile and compressive strengths and roughness and hardness, have been measured in pristine and electron irradiated polymers. Infrared spectroscopy, scanning electron microscopy and differential scanning calorimetry are employed in order to investigate the microscopic modifications induced by the electron energy release to the polymeric chains. Results indicate that the polymer loses hydrogen and becomes rich in carbon content. Cross-linking effects are responsible for the higher mechanical resistance, fragility and hardness of the polymer submitted to a high absorbed dose.     

Vacuum heating of large atomic clusters by a superintense femtosecond laser pulse

The analytic approach of vacuum (Brunel) heating mechanism is generalized to the case of large atomic clusters irradiated by a superintense femtosecond laser pulse. The hydrodynamic cluster expansion is taken into account in this approach. Simple universal expressions are obtained for the absorbed laser energy by a cluster and for the radius of an expanding cluster. The absorption of laser energy and the cluster expansion are determined by only one dimensionless field parameter.     

Computational investigation into the mechanisms of UV ablation of poly(methyl methacrylate)

Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study UV ablation of poly(methyl methacrylate) (PMMA) at 157 nm. We discuss the onset of ablation, the formation and distribution of products in the plume and stress relaxation of the polymer matrix. Laser induced heating and bond-breaks are considered as ablation pathways. We show here that depending on the nature of energy deposition the evolution of ablation plume and yield composition can be quite different. If all of photon energy is converted to heat it can set off ablation via mechanical failure of the material in the heated region. Alternatively, if the photon energy goes towards breaking bonds first, it initiates chemical reactions, polymer unzipping and formation of gaseous products inside the substrate. The ejection of these molecules has a hollowing out effect on the substrate which can lead to ejection of larger chunks. No excessive pressure buildup due to creation of gaseous molecules or entrainment of larger polymer chunks is observed in this case.     

Effect of substrate temperature on MAPLE deposition of synthetic eumelanin films

Eumelanin is an important pigment almost ubiquitous in animals and plants exhibiting interesting charge transport capabilities. Its poor solubility in common solvents represents a severe limitation for preparing thin films. It was recently demonstrated that eumelanin films can be successfully deposited with the MAPLE (Matrix Assisted Pulsed Laser Evaporation) technique starting from a frozen water suspension, using infrared laser radiation. The low laser absorption of ice together with the high absorption of eumelanin suggests that the target ablation is due to laser energy absorbed by the eumelanin molecules, followed by thermal energy transfer, and ejection of ice/water/vapor containing undamaged eumelanin molecules and supramolecular structures.     

Effect of γ-irradiation on optical and chemical properties of CR-39 polymer

CR-39 polymer samples were irradiated with γ-irradiation up to dose ranging from 500 to 2000 kGy. The virgin and γ-irradiated polymer samples were investigated using UV–visible spectroscopy and Fourier transform infrared (FTIR) spectroscopy. In the present work, the Urbach energy was calculated using the Urbach edge method. Also, the direct and indirect energy band gaps in virgin and γ-irradiated CR-39 polymer samples were calculated. The values of indirect energy band gap were found to be lower than the corresponding values of direct energy band gap. The decrease in the optical energy band gap with increasing γ-irradiation dose was discussed on the basis of γ-irradiation-induced modifications in CR-39 polymer. The correlation between optical energy band gap and the number of carbon atoms in a cluster with modified Tauc's equation was also discussed. The FTIR spectra show considerable changes due to γ-irradiation, indicating that the detector is not chemically stable.     

Effect of Ar bombardment on the electrical and optical properties of low-density polyethylene films

The influence of low-energy Ar ion beam irradiation on both electrical and optical properties of low-density polyethylene (LDPE) films is presented. The polymer films were bombarded with 320 keV Ar ions with fuences up to 1×10¹⁵ cm^?2. Electrical properties of LDPE films were measured and the effect of ion bombardment on the DC conductivity, dielectric constant and loss was studied. Optically, the energy gap, the Urbach’s energy and the number of carbon atoms in a cluster were estimated for all polymer samples using the UV–Vis spectrophotometry technique. The obtained results showed slight enhancement in the conductivity and dielectric parameters due to the increase in ion fluence. Meanwhile, the energy gap and the Urbach’s energy values showed significant decrease by increasing the Ar ion fluence. It was found that the ion bombardment induced chain scission in the polymer chain causing some carbonization. An increase in the number of carbon atoms per cluster was also observed.     

乙醇-水团簇分子发射荧光的机理研究

利用含时密度泛函理论分别计算了乙醇分子、水分子、以及乙醇和水形成的新团簇分子的基态和激发态能级,得出了各自吸收光能量和光波长的理论极限值;利用分析化学的理论分析了链式的乙醇-水团簇分子的特殊结构,得到了基态团簇分子间形成碰撞复合物的结论;利用量子力学的态叠加原理和量子化学的轨道理论,并结合Hckel近似对该复合物的能级和对应波函数进行了计算和推导,从而解释了乙醇-水溶液能吸收较长波长的光并发射荧光的机理.     

A comparative study of the desorption efficiencies in the UV irradiation of molecular van der Waals films above and below the ablation threshold

In the irradiation of thick films of aromatics (C₆H₅Cl and C₆H₅CH₃ enriched with dopants of varying volatilities), the attainment of the threshold is shown to result in qualitatively different ejection characteristics. In particular, the comparison of the desorption efficiencies either of species premixed in the film or of photoproducts formed by the irradiation shows that below the threshold only highly volatile species desorb. In contrast, above the threshold, even highly involatile species are found to be ejected efficiently. The efficient ejection of these species cannot be accounted for by a change in the absorbed energy. Instead, the operation of a non-thermal ejection mechanism is strongly indicated. The results are consistent with the delineation drawn by molecular dynamics simulations [12] for surface vaporization at fluences below the ablation threshold and ejection as a result of pressure buildup above it.     

Caloric curves of small fragmenting clusters

The kinetic energy release distribution of neutral atoms emitted from photoexcited clusters Sr(+)(n) with n=4-15, has been obtained by time-of-flight velocity dispersion. The deduced temperature is plotted as a function of the excitation energy. For small sizes n<7 a general increase is observed. For cluster sizes larger than n=9, the deduced caloric curves first increase, and then show evidence of a plateau regime as excitation energy increases. This limiting temperature in neutral atom ejection is consistent with a bound cluster-vapor phase transition in a microcanonical system.     

Diffusion and Growth of Metal Clusters in Nanocomposites: A Kinetic Monte Carlo Study

Noble metals that are deposited on a polymer surface exhibit surface diffusion and diffusion into the bulk. At the same time the metal atoms tend to form clusters because their cohesive energy is about two orders of magnitude higher than the cohesive energy of polymers. To selfconsistently simulate these coupled processes, we present in this paper a Kinetic Monte Carlo approach. Using a simple model with diffusion coefficients taken as input parameters allows us to perform a systematic study of the behavior of a large ensemble of metal atoms on a polymer surface eventually leading to polymer nanocomposites. Special emphasis is placed on the cluster growth, cluster size distribution and the penetration of clusters into the substrate. We also study the influence of surface defects and analyze how the properties of the resulting material can be controlled by variation of the deposition rate (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dispersion of nanoparticles: From organic solvents to polymer solutions

This work is devoted to a systematic study of nanoparticle dispersion by ultrasonication in different solutions: from organic solvents to polymer solutions. The cluster size of nanoparticles at different concentrations in both organic solvents and polymer solutions were directly characterized by Dynamic Light Scattering to study the effect of solid concentration, surfactant and polymer on the dispersion. It reveals that in stabilized suspensions, the smallest attainable size or aggregate size of nanoparticles is independent of solvent type and solid content over the tested range. Furthermore, nanoparticles in simple solvent and in polymer solutions had the similar evolution of cluster size and almost the same final size, which could be very helpful to optimize the dispersion of nanofillers in polymer solutions and nanocomposites. It is also shown that, with appropriate sonication amplitudes, the dispersion procedure developed for very dilute suspensions could be transferred to higher concentration suspensions or even to polymer suspensions.     

Comparison of surface damage under the dry and wet impact: Molecular dynamics simulation

Molecular dynamic simulation was applied in analyzing the difference of surface damage during the impact of a large silica cluster on a crystal silicon substrate in dry and wet condition, respectively. The simulation results show that the damage of silicon substrate under the dry impact is more severe than that under the wet impact. A reason is that the water film buffers the impact of the incident cluster at the init stage under the wet impact. The more important reason is that the water film might be solidified into a continuous thin film at the cluster deformation stage. So, the considerable great impact energy of the cluster will be absorbed by the water film and transformed into thermal dissipation of energy between the simulation ensemble and the water film.     

Laser-induced ablation dynamics and flight of thin polymer films

We investigated the ejection dynamics of triazene polymer layers in the thickness range of 40 nm to 600 nm upon nanosecond laser ablation at a wavelength of 532 nm. The ablation is due to laser-induced thermal degradation of a small part of the polymer in contact with the silicon substrate. The subsequent dynamics of the flying polymer layer are measured with sub-nanosecond time resolution. The evaluation of the initial velocity for different film thicknesses gives insight into the energy transfer process during the acceleration of the films.     

Inverse-bremsstrahlung absorption of an intense laser field in cluster plasma

Inverse-bremsstrahlung absorption of an intense laser field in cluster plasma is considered in the Born approximation with allowance made for electron interaction with the entire subsystem of clustered ions. The electromagnetic power absorbed in plasma is calculated for linearly and circularly polarized laser radiations. It is shown that plasma “clustering” can give rise to much more effective absorption of electromagnetic energy as a result of collisions. The collective effects (the action of the overall field of clustered ions on an electron) dominate over the elementary processes (electron scattering by individual ions in the cluster) in the course of inverse bremsstrahlung.     

Legendre transformation of a self-interacting polymer chain

The end-to-end distribution function for a polymer chain with self-interaction is derived from a functional having the significance of a generator for connected diagrams which consist of vertices representing clusters of monomers in contact, and lines representing free polymer sections. In spite of the dependence of the vertex weights on the multiplicities of the contacts and the presence of an extra weight-factor peculiar to the polymer chain, the generator for connected diagrams is shown to be expressible in terms of a similar generator for irreducible diagrams through a reduction procedure that is analogous to the transformation from connected to irreducible cluster diagrams in classical gas theory. The relation between the generators for connected and irreducible diagrams can be cast into the form of a functional Legendre transformation by considering them as characteristic functionals of new activity and density functions, that embody the multiplicity dependent cluster activity and densities figuring as vertex weights in the connected and irreducible diagrams respectively. This Legendre transformation can be reduced to a much simpler form by imposing an upper bound to the allowed contact multiplicity of the interacting chain.     

用Xα-DV方法研究CO,NO在Ⅷ族过渡金属表面的化学吸附(Ⅰ)——CO吸附在Rh(111)面的电子结构

本文用自洽Hartree-Fock-Slater分子丛方法计算了CO在Rh(111)面上(θ≤1/3)的电子结构。计算了分子丛的总能量、基态能级随吸附高度的变化。从总能量曲线确定的最佳键长为1.85?与实验值1.95±0.1?符合得较好。相应的吸附能为0.98eV比实验值1.3eV略小。在以上最佳键长处计算了总态密度,考虑终态和弛豫效应后与UPS实验结果符合更好。通过CO分子接近表面时各分子轨道能量本征值的变化,讨论了各轨道的成键、反键特征。通过Mulliken总数分析和用CO分子波函数展开总波函数的系数分析着重讨论了CO分子被过渡金属Rh吸附前后的电荷转移。这种电荷转移导致被吸附CO分子的活化。 关键词：     

Mechanisms of small clusters production by short and ultra-short laser ablation

The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD-DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings.     

Electron beam induced modification of poly(ethylene terephthalate) films

Electron beam processing of poly(ethylene terephthalate) (PET) films is found to promote significant changes in the melting heat, intrinsic viscosity and polymer film-liquid (water, isooctane and toluene) boundary surface tension. These properties are featured with several maximums depending on the absorbed dose and correlating with the modification of PET surface functionality. Studies using adsorption of acid-base indicators and IR-spectroscopy revealed that the increase of PET surface hydrophilicity is determined by the oxidation of methylene and methyne groups. Electron beam treatment of PET films on the surface of N-vinylpyrrolidone aqueous solution provided graft copolymerization with this comonomer at optimum process parameters (energy 700 keV, current 1 mA, absorbed dose 50 kGy).