A dendrimer is a hyperbranched macromolecule that is grown from a central core generation by generation. We demonstrate through Monte Carlo simulations of a dendrimer model in two dimensions that when the terminal mon...
A dendrimer is a hyperbranched macromolecule that is grown from a central core generation by generation. We demonstrate through Monte Carlo simulations of a dendrimer model in two dimensions that when the terminal monomers and the interior monomers interact through an effective Flory-Huggins parameter χ, an angularly dependent domain structure forms in the molecule for sufficiently large χ. Based on the simulation data, we further show that the formation of the domain pattern is a smooth crossover, not a phase transition, from a mixed structure.
The dynamical interaction effects on the Coulomb explosion and the stopping power are studied for MeV H2+ ions in grazing collisions with a solid surface. The surface dynamical potential induced by the molecular ions ...
The dynamical interaction effects on the Coulomb explosion and the stopping power are studied for MeV H2+ ions in grazing collisions with a solid surface. The surface dynamical potential induced by the molecular ions is obtained by using a local frequency-dependent dielectric function for a semi-infinite electron gas with damping. A set of equations describing the motion of the center of mass and the Coulomb explosion is obtained by including the repulsive force of the surface atoms, the bare Coulomb force, and the dynamical forces. The total energy loss for the molecular ion is calculated, taking into account a contribution from both the collective excitations of the conduction electrons and the single-electron excitations of each of the substrate atoms.
The dissociation of a diatomic molecule by a chirped infrared laser pulse is modeled by a Morse oscillator interacting with a classical electric field with a time-dependent frequency. Our previous classical analysis i...
The dissociation of a diatomic molecule by a chirped infrared laser pulse is modeled by a Morse oscillator interacting with a classical electric field with a time-dependent frequency. Our previous classical analysis in terms of bucket dynamics, in which systems within the single-node separatrices (buckets) in phase space are trapped and undergo convection to highly excited states, is found to be applicable to the more general cases of nonlinear chirping and using a realistic dipole moment function for the molecule. This route of excitation leads to a much lower dissociation threshold laser intensity when compared to the chaotic diffusion route for monochromatic excitation. Time-dependent quantum mechanical calculations of the dissociation probability based on the split-operator method are performed. It is found that the classical and quantum results agree well, and the classical resonances appear also in the quantum probabilities. Hence the classical method can be used to investigate various characteristics of the chirped pulse excitation and dissociation processes.
Using a Si(Li) x-ray spectrometer, we have measured the respective fractions of L1, L2, and L3 x rays in the L x-ray spectrum emitted in the 157Tb−157Gd electron-capture decay. Using, in addition, our previously measu...
Using a Si(Li) x-ray spectrometer, we have measured the respective fractions of L1, L2, and L3 x rays in the L x-ray spectrum emitted in the 157Tb−157Gd electron-capture decay. Using, in addition, our previously measured value for the ratio of L and K x rays, we deduce values for the fluorescence and Coster-Kronig yields of the L1 subshell. These are ω1=0.101±0.005, f12=0.166±0.020, and f13=0.287±0.014. The f12 and f13 values are significantly below the predictions of the independent-particle model (IPM). The deduced L1 level width of 3.7 eV is also significantly below the IPM width of 4.6 eV.
The detailed response of an Si(Li) detector to the manganese K x-rays from a 55 Fe source was studied by least-squares fitting various spectrum models using a generalized variant of the GUPIX code. The peak-to-backgro...
The gaseous products evolved from solid hydrogenated amorphous carbon (HAC) under UV irradiation have been sampled using time of flight (TOF) mass spectrometry. A notable feature is the appearance of an extended seque...
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The gaseous products evolved from solid hydrogenated amorphous carbon (HAC) under UV irradiation have been sampled using time of flight (TOF) mass spectrometry. A notable feature is the appearance of an extended sequence of aromatic carbon clusters containing in excess of 30 carbon atoms and including fullerenes such as C-50, C-60, and C-70. Under certain conditions, clusters containing up to several hundred C atoms are detected. The radiation-induced decomposition of HAC is found to occur in a series of stages in which small (<20 atom) molecules including alkanes and unsaturated carbon chain (C=C)(n)H-m radicals with m = 1-4 are the first to be released. There is also evidence in these mass spectra for the ejection of small dehydrogenated polycyclic aromatic hydrocarbon (PAH) molecules. After these lighter molecules have been released, we observe a strong mass peak near 500 amu corresponding to clusters with similar to 40 C atoms. These clusters, which are protographitic aromatic structures, have been detected in IR spectra of HAC and show absorption and emission features at 3.3, 3.4, and 6.2 mu m and other wavelengths characteristic of PAH molecules. These data show that the decomposition of HAC in interstellar shocks may be a source of large PAH and fullerene molecules.
Thermal emission spectra from hydrogenated amorphous carbon (HAC) over the 2.5-15 mu m range have been obtained at temperatures between 573 and 773 K. These spectra are similar, but not identical, to absorption spectr...
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Thermal emission spectra from hydrogenated amorphous carbon (HAC) over the 2.5-15 mu m range have been obtained at temperatures between 573 and 773 K. These spectra are similar, but not identical, to absorption spectra of HAC samples subjected to the same thermal cycle. A distinct 3.29 mu m aromatic CH emission is found in samples heated to temperatures in excess of 723 K. This emission is also observed at lower temperatures in samples that have been thermally cycled to higher temperature. Laboratory emission spectra from HAC are shown to provide a good simulation of 3.2-3.6 mu m emission from dust in the extended atmospheres of post-asymptotic giant branch objects. The ratio of 3.4 and 3.3 mu m emission in such objects may be a useful indicator of evolutionary status.
The conformational properties of randomly branched polymers grafted at one end on a planar surface in the good solvent regime are investigated by using a Flory-type scaling theory and by solving a self-consistent-fiel...
The conformational properties of randomly branched polymers grafted at one end on a planar surface in the good solvent regime are investigated by using a Flory-type scaling theory and by solving a self-consistent-field model numerically. The average monomer height obtained from the self-consistent-field model is shown to be in agreement with the scaling behavior predicted from the scaling theory. The density profile is found to have a near-parabolic form with some discrepancies near the surface and the brush end.
We have used Brillouin light scattering and ellipsometry to measure the glass transition temperature Tg of thin polystyrene (PS) films as a function of the film thickness h for two different molecular weights Mw. Thre...
We have used Brillouin light scattering and ellipsometry to measure the glass transition temperature Tg of thin polystyrene (PS) films as a function of the film thickness h for two different molecular weights Mw. Three different film geometries were studied: freely standing films, films supported on a SiOx surface with the other film surface free (uncapped supported), and films supported on a SiOx surface and covered with a SiOx layer (capped supported). For freely standing films Tg is reduced dramatically from the bulk value by an amount that depends on both h and Mw. For h≲REE (the average end-to-end distance of the unperturbed polymer molecules), Tg decreases linearly with decreasing h with reductions as large as 60 K for both Mw values. We observe a large Mw dependence of the Tg reductions for freely standing films which provides the first strong evidence of the importance of chain confinement effects on the glass transition temperature of thin polymer films. For both the uncapped and capped supported films, Tg is reduced only slightly (<10K) from the bulk value, with only small differences in Tg (<4K) observed between uncapped and capped supported films of the same thickness. The results of our experiments demonstrate that the polymer-substrate interaction is the dominant effect in determining the glass transition temperature of PS films supported on SiOx.
Monte Carlo simulations were carried out to study the dynamics of randomly branched polymers in good solvents. Two types of time scales were observed: fast relaxation times corresponding to the internal contraction mo...
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Monte Carlo simulations were carried out to study the dynamics of randomly branched polymers in good solvents. Two types of time scales were observed: fast relaxation times corresponding to the internal contraction motions and slower relaxation times corresponding to the overall rotational motions of the polymers. The former is associated with autocorrelation functions that exhibit nonexponential decay behavior, a signature of the dynamics of random systems. The latter is associated with the usual exponential decay behavior, typical of linear or regularly branched polymers.
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