Showing papers on "Molecular solid published in 1983"

Solid state effects in the electronic structure of TiCl4 studied by XPS

Abstract: High resolution x‐ray photoelectron spectra of TiCl4 have been obtained in the solid state. Core levels, their shape‐up satellites and valence levels are described and discussed in detail. From comparison with gas phase XPS data, UPS spectra, and theoretical models from the literature, emerge specific characteristics of this molecular solid. Special attention is paid to peculiarities of chlorine orbitals, which are discussed in the light of similar data for organic and inorganic chlorides.

Mechanism switching and trapping of triplet-triplet energy transfer in an orientationally disordered molecular solid

Abstract: : The separation between the ions or molecules in disordered solids varies at random. The optical transition energy could also vary over a wide range for the different molecules or ions in the solid resulting in a large inhomogeneous line width (delta v inh). This allows energy transfer and spectral diffusion studies to be carried out between the same chemical species but at different local environments in these solids using lasers for excitation. Furthermore, when carried out at temperatures at which kT delta v inh, energy transfer becomes unidirectional, i.e., to molecules or ions having transition energies equal or lower than the laser-excited-donors within the inhomogeneous profile. This allows studies on the dependence of the rate and mechanism of the energy transfer on the acceptor concentration (i.e., on donor-acceptor separation) to be carried out by simply changing the laser wavelength within the inhomogeneous profile. By analyzing the temporal behavior of the emission intensity of the pulsed-laser-excited set of molecules or ions (donors), the mechanism of the excitation transfer can be elucidated. These types of studies are carried out on the triplet-triplet energy transfer in a unique type of disordered solid, orientationally disordered molecular solids, e.g., 1-bromo,4-chloronaphthalen (BCN) neat solid.

Shock Induced Molecular Excitation in Solids.

Abstract: Initiation of condensed explosives is studied on a molecular level with a quantum mechanical calculation of transition rates for shock induced transitions between the low lying internal molecular normal mode states in a molecular solid. The transition lifetimes are compared with, and show some correlation with, nitromethane pressure-time critical shock initiation data reported by de Longueville, Fauquignon, and Moulard.

Organic Molecular Aggregates Electronic Excitation and Interaction Processes Proceedings of the International Symposium on Organic Materials at Schloss Elmau, Bavaria, June 5-10, 1983

Abstract: I Basic Concepts, Methods and Results.- Excited State Interaction and Energy Transfer Between Molecules in Organic Crystals - Basic Experimental Methods and Results.- Electronic Excitations in Molecular Solids.- II Interaction of Electronic Excitations with Electromagnetic Radiation.- Excitation Spectroscopy of Triplet State Monomers, Aggregates and Excitons in Anthracene Crystals.- Photoemission from Molecular Crystals, Bandstructure and Resonance Effects.- New Type of Local Resonances in Thin Rough Films.- III Electronic Excitations and Spin Dynamics.- Electron Spin Echo Spectroscopy of One-Dimensional Excitons.- Energy Transfer in Molecular Crystals and Its Influence on Spin Resonance.- Ensemble Averaged Spin Pair Dynamics of Doublet and Triplet Molecules.- IV Interaction of Electronic Excitations with Lattice Vibrations.- Localization and Delocalization of an Exciton in the Phonon Field.- Vibronic Excitons in the Intermediate Coupling Regime.- Photo-Induced Electron or Excitation Transfer Enhanced During Vibrational Relaxation and Generalized Forster's Formula.- Molecular Aggregates in Liquids Resolved by a Novel Raman Spectroscopy.- V Excimers, Charge Transfer Excitons and Exciton Fission.- Charge Transfer Spectra of Aromatic Hydrocarbon Crystals.- Exciton Band Structure and Excimer Formation.- Fission and Radiation!ess Transitions in Organic Molecular Crystals in Highly Excited States.- VI Electronic Excitations in Disordered Systems.- Transport and Thermodynamics of Physical Systems with Fractal Geometry.- Hopping Transport in Disordered Systems.- Energy Transfer and Relaxation Processes as Studied by Picosecond Fluorescence Spectroscopy.- Theoretical Methods for the Analysis of Exciton Capture and Annihilation.- Excitation Transport in Naphthalene Aggregates: Mixed Crystals, Amorphous Thin Films and Polymeric Glasses.- VII Electronic Excitation of Impurities in Glasses and Polymers.- Dynamical Linewidth Effects of Hole Burning of Free Base Phthalocyanine in Polymers: Spectral Diffusion and Exchange Narrowing.- Theory of Dephasing of Impurities in Glasses.- VIII Conductivity and Superconductivity in Organic Materials.- Spin Resonance and Conductivity of F1uoranthenyl Radical Cation Salts.- Conducting Polymers Derived from Pyrrole.- Organic Superconductors: Quasi One-Dimensional Conductors, Anomalous Superconductors.- IX Electronic Excitations in Photosynthetic Systems.- Optical Investigations of Photosynthetic Systems.- Index of Contributors.

Surface Chemical Information from Electron- and Photon-Stimulated Desorption

Abstract: Electron- and Photon-Stimulated Desorption can directly or indirectly provide atomic and structural information on the chemical state of surfaces and adsorbates on surfaces. Information is contained in the variation of desorption yield with chemical state, ion energy distributions, ion angular distributions, and in desorption thresholds and spectral dependence. Current models for desorption discuss both ionic and covalent systems and provide considerable insight into the environment of desorbing species. Widely studied examples include oxygen on metals, CO on metals, hydrogen on metals and semiconductors, and cryogenic molecular solids as models for desorption. All of these systems show variations in ion yield with chemical state of the desorbing species which in many cases fit existing models and in many cases suggest new mechanisms of desorption. Ion energy and angular distributions provide signatures of the type of state yielding desorption and the local structure of the desorption site. Desorption spectra can provide electronic and structural details of the bonding site from the analysis of near edge and extended fine structure. Near edge structure can also be used as a signature by comparison to systems of known chemical environment. The above examples of the combined use of the different variations of the desorption experiment are discussed.

Dynamics of Reactions in molecular Solids

Abstract: The dynamics of several thermal and photochemical reactions in the solid-state are investigated by laser Raman spectroscopy, electronic spectroscopy and a rigid-body motion analysis of the thermal parameters of x-ray diffraction studies. Raman phonon spectroscopy along with electronic emission spectroscopy is used to identify the reaction mechanism. The possibility of occurence of any lattice and/or molecular intermediate is examined, by Raman spectroscopy of both phonons and intramolecular vibrations. The new concept of phonon-assisted reaction is tested by examining the role of an overdamped oscillation in a thermal reaction and that of a strong electron-phonon coupling in a photochemical reaction.

Activation of electronic conduction in molecular solids by adsorbed water

Abstract: Adsorbed water molecules are found to activate electronic conduction in several molecular solids. Experimental data on Malachite Green and Rosaniline are presented to illustrate the phenomenon.

Magnetic Resonance in Ion-Radical Organic Solids

Abstract: The remarkable physical properties of ion-radical molecular solids have opened up a new area of solid-state chemistry and physics. The synthetic problem is to stabilize open-shell and mixed-valent molecular arrays while suppressing the recombination of adjacent radicals. Recent chemical studies emphasize mixed-valent systems based on π-molecular cation and anion radicals, on transition-metal complexes, on macrocyclic ligands, and on doped polymers. The subsequent characterization of conducting, semiconducting, or paramagnetic molecular solids draws on a bewildering array of conventional and novel techniques, including electron paramagnetic resonance (epr) and nuclear magnetic resonance (nmr) results discussed in this review. The problem is to determine, in the solid state, the delocalization, interactions, or relaxation of unpaired electronic moments. Solid-state models for ion-radical solids are still fragmentary but have been successfully applied in specific cases. Magnetic resonance methods have been notably useful in delineating the proper starting point for physical models.

Photon-Stimulated Ion Desorption from Condensed Molecules: N2, CO, C2H2, CH3OH, N2O, D2O, and NH3

Abstract: Results of photon-stimulated ion desorption measurements from the surfaces of solid molecular films are presented. In the photon-energy range < 35 eV, data on N2, CO, C2H2, and CH3OH illustrate the importance of including electron-correlation effects in describing the desorption process, while the low-energy threshold for NO+ desorbed from N20 clearly indicates a oneelectron process. The N+ yield from adsorbed N2 resembles the O+ yield from CO due to hole localization on the N atom nearest the surface. K-shell excitation spectra of NH3 and D2O show that excitations of Rydberg orbitals are severely perturbed by hydrogen bonding in the bulk and, to a lesser extent, on the surface. Similar spectra of N2 show that reneutralization effects observed in N2 adsorbed on Ni are due to interaction with the metal and are not intrinsic to the N2 molecule.

Low-temperature energy trapping and emission line profile of disordered solids

Abstract: : In a disordered solid, where random excitation energy and donor-acceptor separation is expected, low temperature energy transfer between a high energy excited molecule to a lower energy acceptor might not be complete. As a result, the emission profile of the solid at low temperature is determined by the energy distribution of the emission of the trapping sites. Predictions based on these ideas are used and a fit is made to the 4.2K observed phosphorescent profile of 1-bromo, 4-chloronaphthalene, an orientationally disordered molecular solid. The theoretical fit to the observed emission profile is discussed in terms of the possible energy transfer mechanism(s) in this solid. (Author)

Point Defects and Diffusion in Molecular Solids

Abstract: Molecular solids comprise atoms or uncharged molecules bound in the lattice by van der Waals forces. This is the largest group of all known materials as it includes the organic solids, which are virtually infinite in number due to the unique facility of carbon for homopolar and heteropolar covalent bonding. It also exhibits the widest variation in the nature of the basic stuctural unit, ranging from the single atoms of the rare gas solids to the macromolecules of polymers and biological materials. This in turn leads to a corresponding range in the complexity of the potential describing the intermolecular forces in the solid. Van der Waals forces are relatively weak and even in hydrogen-bonded materials, which are usually included as a sub-section of molecular solids, the intermolecular forces are small compared to the intramolecular forces. This accounts for the characteristic properties of the simpler molecular solids – low melting point, low lattice energy, softness and friability.

Charge states in polymers: application to triboelectricity

Abstract: A mathematical model of localized molecular-ion, and molecular-exciton states in a dielectric medium is described. This model is shown to provide a quantitative description of the lineshapes and temperature dependence of photoemission and UV absorption from molecular glasses and pendant-group polymers. Its extension to describe electron transfer in these systems is indicated. Attention is focussed on polystyrene, poly(2-vinyl pyridine), and molecular glasses of 2-ethyl benzene, isopropylbenzene and 2-ethyl pyridine as model systems. Application of the model to describe contact charge exchange in copolymers of polystyrene, poly(methyl methacrylate) and poly(2-vinyl pyridine) correctly predicts the steady-state charge exchange among these materials.

Electron-Stimulated Desorption from Condense Branched Alkanes

Abstract: The electron-stimulated dissociation or desorption (ESD) of hydrocarbons is of relevance to the study of radiation-induced surface damage in organic solids and polymers, surface analysis, and radiation-induced surface chemistry. A number of such studies have been carried out on gas phase hydrocarbon systems [1]. Caution should be observed, however, in extrapolating gas phase results to the solid, since dissociation in the gas phase often occurs after a conversion of electronic excitation energy to vibrational energy which is inherently localized due to the isolation and finite extent of the molecule [2]. In an organic solid or polymer, however, the vibrational energy may be dissipated prior to dissociation, and the excitation energy must be localized in some other manner if dissociation is to occur. Condensed organic molecular solids offer the experimenter several other conveniences, including a wellcharacterized chemical structure, ease of desorption and the ability to systematically alter chemical structure as readily as in a gas phase study.

Mass transport in solids. Vol. 97

Abstract: This book (based on a NATO sponsored Advanced Study Institute, France, 1981) attempts to present a general survey of atomic transport in solids, highlighting those areas where research has been especially active in recent years. Topics include the kinetics of atomic transport in crystals; ionic conductivity; defect calculations for ionic and semi-ionic materials; computer experiments on point defects and diffusion; correlation effects in diffusion; neutron scattering studies of diffusion in solids; diffusion in a temperature gradient; point defects and diffusion in molecular solids; diffusion in semiconductors; diffusion in stoichiometric close-packed oxides; highly defective oxides; non-stoichiometry and disorder in oxides; interfacial effects in mass transport in ionic solids; the surface properties of ionic materials; corrosion; mass transport in heterogeneous catalysts; and electrochemical applications of superionic conductors. Abstracts submitted by participants of the NATO ASI are included in the appendix in order to show the diversity of contemporary research in this field.

Electrical conductivity measurements of stannic iodide

Abstract: Conductivity measurements on insulating molecular materials can be of practical use, but are subject to considerable uncertainty because of variations in carrier density caused by extrinsic factors such as internal purity and perfection, ambient gases and photogeneration of carriers. During the past decade, single crystals of stannic iodide (SnI4) have been the subject of optical and electrical investigations. These studies have advanced the quantitative understanding of photodissociation [ 1 ], photolysis [2] and luminescence [3] of SnI4 materials. The optical absorption edge measures the gap between the conduction and valence bands, in the absence of excitation states and strong impurity absorptions. Whall and Juzova [4] have used the techniques Of transient photoconductivity to investigate the carrier transport in SnI4. Applied research has recently put forward SnI4 as a humidity proof transparent antistatic film coating [5]. SnI4 is an attractive molecular solid having a simple tetrahedral molecule which condenses into crystals of a simple cubic structure and with perfect octahedral habit. The tin and iodine atoms are covalenfly bonded while the intermolecular forces are entirely of van der Waals' type. As far as the authors are aware, electrical conductivity measurements of SnI4 have not been reported in the literature. In this paper, we have reported the behaviour of electrical conduction in the temperature range 30 to l l 0 ° C to understand the mechanism of electrical conduction in SnI4. Single crystals of SnI4 (Fig. i) used in the present investigation were grown by employing the gel-technique [6]. Orange to reddish octahedral SnI4 crystals up to 5 mm in size were obtained. The grown crystals were examined by scanning electron microprobe analysis, X-ray diffraction, density measurements and thermogravimetric analysis, these techniques confirmed that they were SnI4. By chemical analysis the percentage of tin and iodine in SnI4 were estimated as 15% and 83%, respectively. For pellet samples, the laboratory grown crystals were finely ground and the resulting powders, of average particle size 150/am, were compressed in a die under a pressure of 200 kg cm -2, giving a packing fraction of 0.764. The crystal or the pellet, as the case may be, was mounted between the flat platinum electrodes in an ordinary conductivity cell. It was then enclosed in a resistance heated furnace, and the temperature of the sample was monitored using

Properties of Isolated Hydrogen Molecules

Abstract: The characteristic property of the majority of molecular crystals is that the molecules in these solids retain their identity and that their intrinsic properties are modified only slightly by the intermolecular interaction. This is true in particular of the simplest molecular solids, composed of H2, D2, or HD molecules, at least at not too high pressures. The properties of these solids can be understood by treating the effect of the intermolecular interaction as a small perturbation on the properties of the free molecules. The analysis of the thermal, spectroscopic, and other properties of the solid hydrogens in terms of the properties of the free molecules and their interactions forms the main topic of this book. The free molecules are described to a high degree of accuracy by the Schrodinger equation for a system composed of two nuclei and two electrons moving under the influence of their mutual Coulomb interactions. In this first chapter we review the quantum theory of such systems, and we collect the available data on the free molecules to be used in the later chapters on the interacting molecules.

Simulation of shock-induced energy flux in molecular solids

Abstract: Computer molecular dynamics has been used to study the time evolution of the energy of diatomic molecules embedded in a monatomic host lattice when the system is shock loaded. Center-of-mass, rotational, and internal energies were each monitored. For H£ and CH groups in an iron host, the results demonstrate rapid and violent internal excitation of a totally athermal nature. The origins of this are discussed as are the reasons for the absence of a similar effect for a CH group in a carbon lattice. From these results for diatomic systems, it is argued that large molecules, similarly treated, may easily be excited to the point of rupture. If they are so situated (e.g., at or near a surface) that during, or shortly after, excitation they escape from the lattice, they will rupture rather than de-excite and thus generate molecular fragments (e.g., free radicals) that could, in the case of an explosive system, serve to initiate detonation.