Condensed Phase Materials Research Articles

Crystalline molecular machines: function, phase order, dimensionality, and composition

The design of molecular machines is stimulated by the possibility of developing new materials with complex physicochemical and mechanical properties that are responsive to external stimuli. Condensed-phase matter with anisotropic molecular order and controlled dynamics, also defined as amphidynamic crystals, offers a promising platform for the design of bulk materials capable of performing such functions. Recent studies have shown that it is possible to engineer molecular crystals and extended solids with Brownian rotation about specific axes that can be interfaced with external fields, which may ultimately be used to design novel optoelectronic materials. Structure/function relationships of amphidynamic materials have been characterized, establishing the blueprints to further engineer sophisticated function. However, the synthesis of amphidynamic molecular machines composed of multiple "parts" is essential to realize increasingly complex behavior. Recent progress in amphidynamic multicomponent systems suggests that sophisticated functions similar to those of simple biomolecular machines may eventually be within reach.

C–H Bond Activation by Oxygen-Centered Radicals over Atomic Clusters

Saturated hydrocarbons, or alkanes, are major constituents of natural gas and oil. Directly transforming alkanes into more complex organic compounds is a value-adding process, but the task is very difficult to achieve, especially at low temperature. Alkanes can react at high temperature, but these reactions (with oxygen, for example) are difficult to control and usually proceed to carbon dioxide and water, the thermodynamically stable byproducts. Consequently, a great deal of research effort has been focused on generating and studying chemical entities that are able to react with alkanes or efficiently activate C-H bonds at lower temperatures, preferably room temperature. To identify low-temperature methods of C-H bond activation, researchers have investigated free radicals, that is, species with open-shell electronic structures. Oxygen-centered radicals are typical of the open-shell species that naturally occur in atmospheric, chemical, and biological systems. In this Account, we survey atomic clusters that contain oxygen-centered radicals (O(-•)), with an emphasis on radical generation and reaction with alkanes near room temperature. Atomic clusters are an intermediate state of matter, situated between isolated atoms and condensed-phase materials. Atomic clusters containing the O(-•) moiety have generated promising results for low-temperature C-H bond activation. After a brief introduction to the experimental methods and the compositions of atomic clusters that contain O(-•) radicals, we focus on two important factors that can dramatically influence C-H bond activation. The first factor is spin. The O(-•)-containing clusters have unpaired spin density distributions over the oxygen atoms. We show that the nature of the unpaired spin density distribution, such as localization and delocalization within the clusters, heavily influences the reactivity of O(-•) radicals in C-H bond activation. The second factor is charge. The O(-•)-containing clusters can be negatively charged, positively charged, or neutral overall. We discuss how the charge state may influence C-H bond activation. Moreover, for a given charge state, such as the cationic state, it can be demonstrated that local charge distribution around the O(-•) centers can also significantly change the reactivity in C-H bond activation. Through judicious synthetic choices, spin and charge can be readily controllable physical quantities in atomic clusters. The adjustment of these two properties can impact C-H bond activation, thus constituting an important consideration in the rational design of catalysts for practical alkane transformations.

Shortening scattering duration by detuning purifies Raman spectra of complex systems

We highlight Resonant Raman scattering as a dynamical process with a finite duration time that results from not only the irreversible homogeneous broadening but also the reversible dephasing caused by the detuning from the resonance, which acts as a camera shutter with a regulated scattering duration. This provides a practical tool of controlling the scattering time. We show that the typical dephasing rates in gas and condensed matter phases are not sufficiently high to make the scattering fast. The detuning from the resonance leads to a strong purification of Raman spectrum from the contamination by higher overtones and soft modes.

“There is no such animal (אין חיה כזו)”—Lessons of a discovery

The discovery of quasicrystals by Dan Shechtman in the early 1980s was a conspicuous event in materials science not only because it led to the production of a plethora of new materials but also because it signified the demise of a dogma in the science of condensed phase materials concerning symmetry restrictions. Having the discovery recognized was not easy and it required stamina on Shechtman’s part. The story of the quasicrystal discovery offers a set of lessons that might be useful to remember in similar situations.

Electron emission from condensed phase material induced by fast protons.

Monte Carlo track simulation has become an important tool in radiobiology. Monte Carlo transport codes commonly rely on elastic and inelastic electron scattering cross sections determined using theoretical methods supplemented with gas-phase data; experimental condensed phase data are often unavailable or infeasible. The largest uncertainties in the theoretical methods exist for low-energy electrons, which are important for simulating electron track ends. To test the reliability of these codes to deal with low-energy electron transport, yields of low-energy secondary electrons ejected from thin foils have been measured following passage of fast protons. Fast ions, where interaction cross sections are well known, provide the initial spectrum of low-energy electrons that subsequently undergo elastic and inelastic scattering in the material before exiting the foil surface and being detected. These data, measured as a function of the energy and angle of the emerging electrons, can provide tests of the physics of electron transport. Initial measurements from amorphous solid water frozen to a copper substrate indicated substantial disagreement with MC simulation, although questions remained because of target charging. More recent studies, using different freezing techniques, do not exhibit charging, but confirm the disagreement seen earlier between theory and experiment. One now has additional data on the absolute differential electron yields from copper, aluminum and gold, as well as for thin films of frozen hydrocarbons. Representative data are presented.

Formation of nascent soot and other condensed-phase materials in flames

Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Noncontact detection of homemade explosive constituents via photodissociation followed by laser-induced fluorescence

Noncontact detection of the homemade explosive constituents urea nitrate, nitromethane and ammonium nitrate is achieved using photodissociation followed by laser-induced fluorescence (PD-LIF). Our technique utilizes a single ultraviolet laser pulse (approximately 7 ns) to vaporize and photodissociate the condensed-phase materials, and then to detect the resulting vibrationally-excited NO fragments via laser-induced fluorescence. PD-LIF excitation and emission spectra indicate the creation of NO in vibrationally-excited states with significant rotational energy, useful for low-background detection of the parent compound. The results for homemade explosives are compared to one another and 2,6-dinitrotoluene, a component present in many military explosives.

Role of Molecular Structure on X-ray Diffraction in Uniaxial and Biaxial Phases of Thermotropic Liquid Crystals

X-ray diffraction is one of the most definitive methods to determine the structure of condensed matter phases, and it has been applied to unequivocally infer the structures of conventional calamitic and lyotropic liquid crystals. With the advent of bent-core and tetrapodic mesogens and the discovery of the biaxial nematic phase in them, the experimental results require more careful interpretation and analysis. Here, we present ab-initio calculations of X-ray diffraction patterns in the isotropic, uniaxial nematic, and biaxial nematic phases of bent-core mesogens. A simple Meier-Saupe-like molecular distribution function is employed to describe both aligned and unaligned mesophases. The distribution function is decomposed into two, polar and azimuthal, distribution functions to calculate the effect of the evolution of uniaxial and biaxial nematic orientational order. The calculations provide satisfactory semiquantitative interpretations of experimental results. The calculations presented here should provide a pathway to more refined and quantitative analysis of X-ray diffraction data from the biaxial nematic phase.

Valence-Bond Order (VBO): A New Approach to Modeling Reactive Potential Energy Surfaces for Complex Systems, Materials, and Nanoparticles.

The extension of molecular mechanics to reactive systems, metals, and covalently bonded clusters with variable coordination numbers requires new functional forms beyond those popular for organic chemistry and biomolecules. Here we present a new scheme for reactive molecular mechanics, which is denoted as the valence-bond order model, for approximating reactive potential energy surfaces in large molecules, clusters, nanoparticles, solids, and other condensed-phase materials, especially those containing metals. The model is motivated by a moment approximation to tight binding molecular orbital theory, and we test how well one can approximate potential energy surfaces with a very simple functional form involving only interatomic distances with no explicit dependence on bond angles or dihedral angles. For large systems the computational requirements scale linearly with system size, and no diagonalizations or iterations are required; thus the method is well suited to large-scale simulations. The method is illustrated here by developing a force field for particles and solids composed of aluminum and hydrogen. The parameters were optimized against both interaction energies and relative interaction energies. The method performs well for pure aluminum clusters, nanoparticles, and bulk lattices and reasonably well for pure hydrogen clusters; the mean unsigned error per atom for the aluminum-hydrogen clusters is 0.1 eV/atom.

Charge separation at evaporation and vapor growth of ice and water

A mathematical model of the interface charging at evaporation and growth of ice and water phases from vapor is proposed. This model takes into account the competition between the two mechanisms of charge separation, one of which is based on protons and the other involves orientational defects. The first mechanism leads to the accumulation of a positive charge by ice and water during evaporation, while the second one provides negative charge accumulation. The protonic mechanism dominates at low velocities of the evaporation front with respect to the condensed phase material (lower than 10−11–10−9 m/s). At high rates, the mechanism based on orientational defects is dominant. When vapor is condensed, and, correspondingly, the ice and water phases grow, the charge polarity is opposite to the polarity in the case of evaporation. The proposed model adequately describes the experimentally observed interface electric current and the signs of phase charges during evaporation and condensation.

Detection of condensed-phase explosives via laser-induced vaporization, photodissociation, and resonant excitation

We investigate the remote detection of explosives via a technique that vaporizes and photodissociates the condensed-phase material and detects the resulting vibrationally excited NO fragments via laser-induced fluorescence. The technique utilizes a single 7 ns pulse of a tunable laser near 236.2 nm to perform these multiple processes. The resulting blue-shifted fluorescence (226 nm) is detected using a photomultiplier and narrowband filter that strongly block the scatter of the pump laser off the solid media while passing the shorter wavelength photons. Various nitro-bearing compounds, including 2,6-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) were detected with a signal-to-noise of 25 dB. The effects of laser fluence, wavelength, and sample morphology were examined.

Internally Consistent Ion Volumes and Their Application in Volume-Based Thermodynamics

"Volume-based thermodynamics" (VBT) relates the thermodynamics of condensed-phase materials to their formula unit (or molecular) volumes, V(m). In order to secure the most accurate representation of these data, the volumes used are to be derived (in order of preference) from crystal structure data or from density or, in the absence of experimental data, estimated by ion-volume summation.

High-capacity hydrogen storage in lithium and sodium amidoboranes

The safe and efficient storage of hydrogen is widely recognized as one of the key technological challenges in the transition towards a hydrogen-based energy economy. Whereas hydrogen for transportation applications is currently stored using cryogenics or high pressure, there is substantial research and development activity in the use of novel condensed-phase hydride materials. However, the multiple-target criteria accepted as necessary for the successful implementation of such stores have not yet been met by any single material. Ammonia borane, NH3BH3, is one of a number of condensed-phase compounds that have received significant attention because of its reported release of approximately 12 wt% hydrogen at moderate temperatures (approximately 150 degrees C). However, the hydrogen purity suffers from the release of trace quantities of borazine. Here, we report that the related alkali-metal amidoboranes, LiNH2BH3 and NaNH2BH3, release approximately 10.9 wt% and approximately 7.5 wt% hydrogen, respectively, at significantly lower temperatures (approximately 90 degrees C) with no borazine emission. The low-temperature release of a large amount of hydrogen is significant and provides the potential to fulfil many of the principal criteria required for an on-board hydrogen store.

Diffusion in Nanoporous Phases: Size Dependence and Levitation Effect

Self-diffusivity, D, of diffusants in widely differing mediums such as liquids (e.g., solution), porous solids (e.g., guests in zeolites), or ions in polar solvents exhibit strong size dependence. We discuss the nature of the size dependence observed in these systems. Altogether, different theoretical approaches have been proposed to understand the nature of size dependence of D not only across these widely differing systems but even in just one medium or class of systems such as, for example, ions in polar solvents. But molecular dynamics investigations in the past decade have shown that the size dependence of self-diffusion in guest-porous solids could have origins in the mutual cancellation of forces that occurs when the size of the diffusant is comparable to the size of the void. The effect leading to the maximum in D is known as the levitation effect (LE). Such a cancellation is a consequence of symmetry. This effect exists in all porous solids irrespective of the geometrical and topological details of the pore network provided by the solid. Recent studies show that the levitation effect and size-dependent diffusivity maximum exists for uncharged solutes in solvents. One of the consequences of this is the breakdown in the Stokes-Einstein relationship over a certain range of solute-solvent size ratio. Experimental measurements of ionic conductivity over the past hundred years have found the existence of a size-dependent diffusivity maximum leading to violation of the Walden's rule for ions in polar solvents. Molecular dynamics simulations and experimental data suggest that even this maximum has its origin in LE. Simulation studies of impurity atom diffusion in close-packed solids as well as ions in superionic and other solids suggest the existence of a size-dependent diffusivity maximum in these materials as well. The levitation effect is a universal effect leading to a maximum in diffusivity of a diffusant in a variety of condensed matter phases. The only condition for its existence appears to be the presence of van der Waals or electrostatic interactions.

Light and matter: electromagnetism, optics, spectroscopy and lasers

Preface. 1 Electromagnetic radiation. 1.1 Brief history of the interaction of light and matter. 1.2 Light in vacuum. 1.3 Matter-source of light. 2 Phenomenology of light propagation in matter. 2.1 Absorption of light. 2.2 Nonlinear absorption. 2.3 Index of refraction. 2.4 Optical phenomena in nonisotropic media. 2.5 Electric field effects. 2.6 Acousto-optic effects. 2.7 Magnetic field effects. 3 The interaction of light and matter. 3.1 Lorentz force law. 3.2 Motion of a charged particle in static electric and magnetic fields. 3.3 Motion of a bound electron in an electromagnetic field. 3.4 Radiation due to acceleration of charges. 3.5 Multipole radiation. 3.6 Scattering of a light wavepacket. 3.7 Cooling and trapping of atoms. 4 Magnetic phenomena, constitutive relations and plasmas. 4.1 Magnetic moments. 4.2 Magnetization. 4.3 Magnetic resonance. 4.4 Polarization and magnetization as source terms. 4.5 Atomistic derivation of macroscopic electromagnetism and the constitutive relations. 4.6 Microscopic polarizability and macroscopic polarization. 4.7 Dielectric relaxation. 4.8 Plasmas 275 5 Quantum description of absorption, emission and light scattering. 5.1 Charged particle in an electromagnetic field. 5.2 Absorption and emission. 5.3 Rayleigh and Raman scattering. 5.4 Thomson scattering. 6 Spectroscopy. 6.1 Atoms. 6.2 Molecules. 6.3 Diatomic molecules. 6.4 Polyatomic molecules. 6.5 Condensed-phase materials. 7 Lasers. 7.1 Laser dynamics. 7.2 Threshold. 7.3 Steady state. 7.4 Pulsed laser operation. 7.5 Cavity modes. 7.6 Amplified spontaneous emission. 7.7 Laser linewidth. 7.8 Laser coherence. 7.9 Specific laser systems. 8 Nonlinear optics. 8.1 Expansion of the polarization in the electric field. 8.2 Phase-matching. 8.3 Second harmonic generation. 8.4 Three-wave mixing. 8.5 Third harmonic generation. 8.6 Self-focusing and self-phase modulation. 8.7 Four-wave mixing. 8.8 Stimulated Raman processes. 8.9 Stimulated Brillouin processes. 8.10 Nonlinear matter-wave optics. 9 Quantum-optical processes. 9.1 Interaction of a two-level system with an electromagnetic field. 9.2 Liouville-von Neumann equation for the density matrix. 9.3 Three-level system. 9.4 Coherent states and squeezed states. 9.5 The Jaynes-Cummings model. 9.6 Interaction between modes of a quantum field. 10 Light propagation in optical fibers and introduction to optical communication systems. 10.1 Fiber characteristics. 10.2 Transverse modes of an optical fiber. 10.3 Nonlinear processes in fibers. 10.4 Fiber-optic communication systems. Appendices. Appendix A: vector analysis. Appendix B: Electromagnetism and Maxwell's equations. Appendix C: Quantum mechanics and the Schr..odinger equation. Appendix D: perturbation theory. Appendix E: Fundamental constants. References. Bibliography. Index.

Development and applications of the interfacial tension between water and organic or biological surfaces

The interfacial tension ( γ SW) between a condensed-phase material (S) and water (W) is one of the most important terms occurring (directly or indirectly) in the major surface thermodynamic combining rules, such as the different variants of the Dupré equation, as well as the Young and the Young–Dupré equations. Since the late 1950s, γ SL (where L stands for liquid in general) could be correctly expressed, as long as one only took van der Waals attractions and electrical double layer repulsions into account, i.e., as long as both S and L were apolar. However for interfacial interactions taking place in water among apolar as well as polar solutes, particles or surfaces, γ SW was not properly worked out until the late 1980s, due in particular to uncertainties about the treatment of the polar properties of liquid water and other condensed-phase materials. In this review the historical development of the understanding of these polar properties is outlined and the polar equation for γ SW, as well as the equations derived there from for the free energies of interaction between apolar or polar entities, immersed in water (Δ G SWS) are discussed. Also discussed is the role of the various terms of Δ G SWS, in hydrophobic attraction (the “hydrophobic effect”), hydrophilic repulsion (“hydration forces”) and in the quantitative expression of hydrophobicity and hydrophilicity. The DLVO theory of attractive and repulsive free energies between particles immersed in liquids, as a function of distance between suspended particles, was extended to allow its use in the expression of the polar interactions occurring in water. Finally, the free energy term, Δ G SWS and the related γ SW, have been directly linked to the aqueous solubility of organic and biological solutes, which allows the determination of interfacial tensions between such solutes and water from their solubilities.

The Role of Phosphorus in Chemical Evolution

AbstractFor Abstract see ChemInform Abstract in Full Text.

Experimental Benchmarking of Pu Electronic Structure

AbstractThe standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]

The role of phosphorus in chemical evolution

In this tutorial review we consider the role of phosphorus and its compounds within the context of chemical evolution in galaxies. Following an interdisciplinary approach we first discuss the position of P among the main biogenic elements by considering its relevance in most essential biochemical functions as well as its peculiar chemistry under different physicochemical conditions. Then we review the phosphorus distribution in different cosmic sites, such as terrestrial planets, interplanetary dust particles, cometary dust, planetary atmospheres and the interstellar medium (ISM). In this way we realize that this element is both scarce and ubiquitous in the universe. These features can be related to the complex nucleosynthesis of P nuclide in the cores of massive stars under explosive conditions favouring a wide distribution of this element through the ISM, where it would be ready to react with other available atoms. A general tendency towards more oxidized phosphorus compounds is clearly appreciated as chemical evolution proceeds from circumstellar and ISM materials to protoplanetary and planetary condensed matter phases. To conclude we discuss some possible routes allowing for the incorporation of phosphorus compounds of prebiotic interest during the earlier stages of solar system formation.

Shock Induced Reaction Observed via Ultrafast Infrared Absorption in Poly(vinyl nitrate) Films

Ultrafast laser shock generation methods and broad-band infrared reflection absorption spectroscopy have provided evidence that shock-induced chemistry in a condensed-phase energetic material occurs on a time scale of tens of picoseconds and involves the nitro group as a primary reactant. Femtosecond broad-band infrared reflection absorption spectroscopy was used to monitor films of the energetic polymer poly(vinyl nitrate) during shock loading and rarefaction. At ∼18 GPa, poly(vinyl nitrate) films exhibited loss of absorption in the nitro group stretch modes that did not recover upon rarefaction, providing an indication of initial chemical reaction. At pressures ≤9 GPa, the observed spectral changes are ascribed to thin film optical interference effects, without chemical reaction. The loss of infrared absorption required an induction time of tens of picoseconds after shock passage, supporting reaction mechanisms that require vibrational energy transfer rather than prompt reaction.