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Fundamentals of Analysis of Solids by Laser-Produced Plasmas
Published in Leon J. Radziemski, David A. Cremers, Laser-Induced Plasmas and Applications, 2020
The process of evaporation and heating continues until the electron density of the partially ionized gas becomes high enough for the heating of the gas by the inverse bremsstrahlung process to begin. The weakly ionized plasma becomes gradually coupled to the middle segment of the laser pulse, while remaining partially transparent to the laser beam so that direct heating of the target surface continues. The inverse bremsstrahlung process heats primarily the electrons in the presence of ions, resulting in free-to-free-state transitions. Such heated electrons increase the plasma temperature and, consequently, the electron density. At high laser powers, the electron density of the plasma can attain a value sufficiently high so that the core of the plasma becomes opaque. In this case, the frequency of plasma oscillation, as given by (nee2/nm)112, is comparable to or greater than the laser frequency and the laser beam is no longer able to penetrate the plasma and reach the condensed-phase target surface. Here, ne is the electron number density, e the electronic charge, and m the electron mass. Heating of the target surface can continue only indirectly by thermal conduction from the plasma.
An Introduction to Crystal Structures
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
In covalent bonding, the electrons are shared between two atoms, resulting in a build-up of electron density between the atoms. Covalent bonds are strong and directional. In a covalent bond, electronegative elements such as oxygen and nitrogen attract an unequal share of the bonding electrons, such that one end of the bond acquires a partial negative charge (δ−) and the other end acquires a partial positive charge (δ+). The separation of the negative and positive charge creates an electric dipole, and the molecule can align itself in an electric field. Such molecules are said to be polar and possess a permanent dipole moment. The partial electric charges on polar molecules can attract one another in a dipole–dipole interaction. The dipole–dipole interaction is about 100 times weaker than ionic interactions and falls off quickly with distance, as a function of 1/r3.
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Published in Chad A. Mirkin, Spherical Nucleic Acids, 2020
Robert J. Macfarlane, Matthew N. O’Brien, Sarah Hurst Petrosko, Chad A. Mirkin
Atoms rely on valency—the oriented overlap of atomic orbitals-to control molecular and crystallographic shape and symmetry. Among the tenets of valency is the relationship between electron density and bond strength: the greater the amount of shared electron density between two atoms, the stronger the bond. If this concept is extended to PAEs with anisotropic nanoparticle cores, one would expect particle orientations that align the largest faces of the particles in a parallel placement to be favored, as this would result in a greater number of DNA connections and create stronger bonds between particles. For example, two-dimensional triangular prism structures will form stronger “bonds” with their large triangular faces aligned parallel to one another, as compared to orientations that align their (relatively smaller) rectangular sides (Fig. 5.4). This effect results in triangular prisms assembled into 1D lamellar stacks. This relationship has been further demonstrated with octahedra, rod, and rhombic dodecahedra nanoparticle templates, where each structure assembles along the crystal facet that forms the greatest number of nucleic acid bonds [86]. The eighth design rule is therefore: PAEs based upon anisotropic particles with flat faces can be used to realize valency and will assemble into a lattice that maximizes the amount of parallel, face-to-face interactions between particles. These assemblies are also accessible by alternative bonding methods, such as with the pH-mediated association of carboxylic acid-terminated ligands attached to gold triangular prisms, demonstrating that anisotropic nanoparticle assembly is ligand general, where the ultimate structure is heavily influenced by the shape of the nanoparticle [120].
Oxidative desulfurization of fuels using gaseous oxidants: a review
Published in Journal of Sulfur Chemistry, 2022
Mohsen Adhami, Salman Movahedirad, Mohammad Amin Sobati
The types and amounts of sulfur-containing compounds vary with each hydrocarbon cut. Chemically, SCCs in fuel can appear in the form of either organic or inorganic molecules. Inorganic examples are hydrogen sulfides and pyrites [27]. Organic sulfur-containing molecules are the main sulfur sources in hydrocarbons. Thiols, sulfides, disulfides, thiolanes, thiophanes, benzothiophene (BT), dibenzothiophene (DBT), 4,6-dimethyl dibenzothiophene (DMDBT), benzonaphthothiophene, are examples of Organic Sulfur Compounds (OSCs) in fuel [24,28]. Each sulfur-containing substance behaves differently when subjected to the oxidation process. SCCs vary in molecular size, oxidation reactivity, and boiling point, as seen in Table 1 As the SCCs in hydrocarbon fuels turns from the aliphatic forms such as thiols, sulfides, and disulfides (with different hydrocarbon chain length) to the thiophenic forms, sulfur separation in HDS becomes more challenging due to molecular resonance stabilization [29]. There are also differences in the reactivity potentials of SCCs in thiophenic form due to the differences in electron densities [30]. In this regard, compounds with a higher electron density have a higher oxidation reaction capacity. Thus, the oxidation reactivity of thiophenic compounds is in the following order: 4,6-DMDBT > DBT > BT [31].