Showing papers in "Faraday Discussions of The Chemical Society in 1985"
TL;DR: In this article, a wide-angle X-ray diffraction was used to measure the global chain orientation, and in the case of the first polymer the meridional scattering has been analysed in terms of diffraction from an isolated straight random chain.
Abstract: Uniaxially oriented samples of thermotropic random copolyesters of hydroxybenzoic and hydroxynaphthoic acids, and of hydroxybenzoic acid and ethylene terephthalate, have been characterised in detail using wide-angle X-ray diffraction. The technique was used to measure the global chain orientation, and in the case of the first polymer the meridional scattering has been analysed in terms of diffraction from an isolated straight random chain. Optical microstructures were observed between crossed polars, with circularly polarized light and in plane-polarised light with no analyser present. Interpretation of the contrast seen shown that the material is optically biaxial and leads to the conclusion that, in the liquid-crystalline phase, there is long-range correlation of the rotations of the molecules about their chain axes. The polymers are examples of ‘biaxial nematics’. Thermal analysis indicates that the solid phase has a higher level of conformational order than the melt and that the melting range is very broad. Annealing below the melting point leads to the development of localised regions of enhanced order, especially in specimens with a high degree of overall orientation. The combination of electron diffraction and electron microscopy provides evidence that the lateral growth of ordered entities does not necessarily require regular sequences within the otherwise random copolymer molecules. Small crystals based on identical but non-periodic sequences within the molecules are (NPL crystals) proposed as being consistent with the experimental evidence and the implications of a brief statistical analysis.
157 citations
TL;DR: The infrared spectra of CO and NO adsorbed at 77 K on sintered alkali-metal halide films (LiF, NaCl, KCl and NaI) and on Sintered polycrystalline oxides (MgO and NiO) are reported in this paper.
Abstract: The infrared spectra of CO and NO adsorbed at 77 K on sintered alkali-metal halide films (LiF, NaCl, KCl and NaI) and on sintered polycrystalline oxides (MgO and NiO) are reported. The most intense bands have been assigned to monomeric (CO) and dimeric (N2O2) species adsorbed on (100) faces. The CO molecules are adsorbed in a localized form on surface cations with orientation perpendicular to the (100) planes and form a regular array of parallel oscillators which interact via dynamic and static effects. The dynamic polarizability (αv) of CO adsorbed on alkali-metal halides and magnesium oxide is very small (αv≈ 0.02–0.03), while that of CO on NiO is one order of magnitude larger. The molecules of CO adsorbed on (100) faces also interact through the solid via inductive effects. NO is adsorbed on (100) planes of alkali-metal halides and MgO in the dimeric cis form; on NiO monomeric nitrosylic species predominate.
120 citations
TL;DR: In this article, high-resolution inelastic scattering of low-energy (8-17 meV) helium atoms from the (111) surfaces of the three noble metals, Cu, Ag and Au, has been used to determine the surface-phonon dispersion curves for 〈112〉 and 》110〉 azimuths out to the zone boundary.
Abstract: High-resolution inelastic scattering of low-energy (8–17 meV) helium atoms from the (111) surfaces of the three noble metals, Cu, Ag and Au, has been used to determine the surface-phonon dispersion curves for 〈112〉 and 〈110〉 azimuths out to the zone boundary. The new measurements for Cu and Au reveal two sharp dispersion curves corresponding to vibrational modes with a high spectral density at the surface, as observed previously for Ag. The lower-frequency curve is attributed to the Rayleigh mode and the higher-frequency curve to a longitudinal mode, which appears at the surface because of a significant 50–70% weakening in the lateral force constants in the uppermost surface layer. The results are compared with several recent lattice-dynamical calculations.
110 citations
TL;DR: In this article, optical observations of a number of main-chain thermotropic liquid-crystal polymers were carried out at elevated temperatures using an apparatus that is capable of providing a controlled translational oscillatory shearing motion to samples of typically 1-10 µm thickness.
Abstract: We report optical observations of a number of main-chain thermotropic liquid-crystal polymers. In situ measurements were carried out at elevated temperatures using an apparatus that is capable of providing a controlled translational oscillatory shearing motion to samples of typically 1–10 µm thickness. In static samples we observe either a general birefringence with local scattering from line defects which we believe to be disclinations, or in thicker samples a texture dominated by light scattering due to the presence of dence disclinations. Superimposed shearing appears to result in both the multiplication of disclinations and the progressive decrease in the distance between individual disclinations. At high shear rates pure birefringence in the direction of flow is observed. Finally we report on the optical relaxation behaviour of oriented samples together with an explanation for the skin core effect observed when thermotropic liquid-crystal polymers are extruded through dies.
89 citations
TL;DR: In this paper, a phenomenological model to account for low-shear-rate region of shear thinning of viscosity is presented, accompanied by lack of net orientation and by a dependence of texture and rheology on shear history.
Abstract: Liquid-crystal polymers often exhibit a low-shear-rate region of shear thinning of viscosity, accompanied by lack of net orientation and by a dependence of texture and rheology on shear history. A phenomenological model to account for this region is presented. The model uses Marrucci's description of stable domains whose size is a function of stress. The interaction of these domains is estimated from the continuum theory of liquid crystals. The shear-rate-dependent viscosity and normal stress of the polydomain fluid are calculated from the interaction energy and size of the domain by analogy with non-equilibrium molecular-dynamics calculations for small-molecule fluids. The shear-rate dependences are similar to those observed experimentally. Other tests of the model, including optical observations and measurement of dynamic viscosity, are proposed.
71 citations
TL;DR: In this article, two rigid/flexible thermotropic polyesters based on 1,10-decane bisterephthaloyl chloride with hydroquinone or methyl hydroquinones were examined by transmission electron microscopy and electron diffraction.
Abstract: Two rigid/flexible thermotropic polyesters based on 1,10-decane bisterephthaloyl chloride with hydroquinone or methyl hydroquinone have been investigated by transmission electron microscopy. Oriented thin films were prepared by melt spreading on phosphoric acid. After thermal quenching to freeze-in the liquid-crystalline state, films were examined at room temperature using electron microscopy and electron diffraction. High local orientation within 10 µm diameter areas was observed by electron diffraction. Layer-line spacings showed that the molecules adopted a highly aligned conformation. Multiple equatorial reflections in the unsubstituted polymer and the formation of a single-crystal-like texture on annealing the frozen liquid-crystalline state in the methyl-substituted polymer suggest biaxial chain packing in the liquid-crystalline state. Bright-field images of the quenched films were essentially featureless, but equatorial dark field and polarized-light optical images revealed an alternating band structure. Annealing below the crystal-melting transition results in the growth of lamellae which decorate the overall pattern of molecular order in the precursor frozen liquid-crystalline state. The 200 A thick lamellae provide high contrast and resolution for observation of characteristic mesophase texture and defects. The molecular trajectory across the transverse bands is found to be sawtoothed rather than serpentine. Two types of disclination lines have been observed: non-singular S=±1 disclination loops lying in the plane of the films with their loop axes oriented along the overall chain-axis direction and S=±½ disclination lines lying normal to the film surface.The semicrystalline microstructure consists of thin lamellae separated by frozen liquid-crystal regions. The single-crystal-like texture of the oriented lamellae, together with the nature of the meridional scattering, suggests a biaxial smectic liquid-crystalline phase for these polymers. The liquid-crystalline state is not believed to consist of an intrinsic polydomain structure but rather a continuous liquid-crystalline medium containing various types of disclinations.
69 citations
TL;DR: In this article, the stability and the molecular order of thermotropic polymers comprising rigid groups connected by flexible spacers are found to be dominated primarily by the characteristics (configurational partition function) of highly extended conformers, which are favored for packing.
Abstract: The stability and the molecular order of nematic states of thermotropic polymers comprising rigid groups connected by flexible spacers are found to be dominated primarily by the characteristics (configurational partition function) of highly extended conformers, which are favoured for packing. Moreover, both the macroscopic consideration of the enthalpies and the entropies of isotropic–nematic transitions and the microscopic probe by deuterium n.m.r. of labelled chains lead to the conclusion that the highly extended conformers are selected preferentially in polymeric nematogens. A number of unique results exhibited by polymer liquid crystals, i.e. strong effects of even–odd character of polymethylene spacers, abnormalities associated with —C(O)O— links [compared with —O— and —OC(O)— links] between rigid (phenylene) groups and polymethylenes and drastic differences between polymethylene and polyoxyethylene spacers, can all be attributed quantitatively to this conformational ordering, which is the most prominent feature distinguishing polymer liquid crystals from their monomeric counterparts. The orientational order parameters of a nematic polymer measured from both D and H n.m.r. spectra of labelled chains are found to be quite high, ca. 0.8 throughout the nematic region. These findings on conformational order and orientational order of polymer liquid crystals are compared with theoretical predictions based on ideal lattice chains and worm-like chains.
54 citations
TL;DR: In this article, the authors measured the critical volumes of fractions of cellulose-based polymers in dialkyl phthalate solvents over a large temperature range and compared the results with predictions of theories for the phase separation of freely jointed and worm-like chains.
Abstract: Cellulosic derivatives form an interesting range of polymer liquid crystals. They belong to a class of mesophases in which the polymer chain is stiff or semiflexible and is chemically homogeneous. The cellulose backbone, a single-stranded homopolymer of β-linked 1,4-anhydroglucose units, contains three hydroxy groups per anhydroglucose unit which provide convenient sites for substitution reactions, leading to a wide variety of cellulose ethers and esters. Many of these derivatives form cholesteric liquid-crystalline phases in suitable solvents, and some derivatives with relatively large (but non-mesogenic) side chains form both lytropic and thermotropic liquid crystals.Published observations on the phase separation of cellulose-based polymers indicate that the mesophases form at critical volume fractions of polymer ranging from 0.3 to 0.5 for high molar mass samples at room temperature. The critical volume fractions for a given polymer and solvent decrease with increasing molar mass to approach these asymptotic values. The critical volume fraction increases with temperature. The nature of the solvent is also important; highly polar or acidic solvents generally favour mesophase separation at lower critical concentrations than simple organic solvents. Furthermore, whereas heavily substituted derivatives with long flexible side-chains form mesophases easily in a wide range of solvents, the more familiar derivatives with smaller substituents seem to form mesophases with specific solvents, but not with others.We have measured the critical concentrations of fractions of (acetoxypropyl)cellulose in dialkyl phthalate solvents over a large temperature range and compared the results with predictions of theories for the phase separation of freely jointed and worm-like chains. The results indicate that chain geometry and stiffness are the major factors controlling liquid-crystalline phase formation and that anisotropic inter-chain interactions play a minor role. Thus the chemical structure of polymer and solvent govern the phase separation indirectly in these systems, primarily through effects on the chain conformation.
54 citations
TL;DR: In this article, a sub-monolayer film of fluid nitrogen adsorbed on a graphite surface has been studied using the molecular-dynamics technique, where the surface is modelled as a static external field using a Fourier expansion in the reciprocal lattice vectors of the graphite basal plane.
Abstract: A sub-monolayer film of fluid nitrogen adsorbed on a graphite surface has been studied using the molecular-dynamics technique. The molecule–molecule potential consists of a two-centre Lennard-Jones interaction with additional coulombic interactions between partial charges. The surface is modelled as a static external field using a Fourier expansion in the reciprocal lattice vectors of the graphite basal plane.Simulations are performed at a variety of coverages at a temperature close to 75 K. The isosteric enthalpy of the model is in good agreement with experiment. The underlying surface lattice modifies the structure of the adsorbed liquid and there is significant out-of-plane orientational ordering at all the coverages studied.
44 citations
TL;DR: In this article, a wide variety of polymers based on rigid, linear aromatic ester mesogenic units, with and without flexible or rigid non-mesogenic spacers, have been prepared and characterized for their ability to form a liquid-crystalline melt, the type of phase formed, their transition temperatures and the morphology of the mesophase.
Abstract: The molecular variables which control the structure–property relationships in thermotropic liquid-crystalline polyesters are under investigation in this laboratory. A wide variety of polymers based on rigid, linear aromatic ester mesogenic units, with and without flexible or rigid non-mesogenic spacers, have been prepared and characterized for their ability to form a liquid-crystalline melt, the type of phase formed, their transition temperatures and the morphology of the mesophase. Flexible spacers reduce both the melting and clearing temperatures, and the type and length of the spacer can determine whether a nematic, cholesteric or smectic phase is formed. Variations in the structure of the rigid mesogenic group, both in the specific type and arrangement of the aromatic ester groups and the presence of pendant substituent, also cause profound changes in the properties of the mesophase melt formed. Copolymers containing rigid non-mesogenic units in random sequence distributions with closely related mesogenic units have been characterized for the effects of composition on thermotropic properties, including the parameter of ‘degree of liquid crystallinity’. Of interest in all series of polymers studied is the critical limit of non-mesogenic unit content beyond which either liquid crystallinity no longer occurs or monotropic behaviour is observed.
44 citations
TL;DR: The underlying theme of the lecture is the need to recognize the synergism and affinity that characterizes research on monomeric and polymeric liquid crystals.
Abstract: The relationship between low-molar-mass or monomeric liquid crystals and polymeric liquid crystals is explored by examining the historical development of experimental and theoretical aspects of the latter materials. Polymeric liquid crystals are initially contrasted with conventional, non-mesogenic, flexible polymer chains (random coils), and then semiflexible and rigid-rod polymers are examined. Both thermotropic and lyotropic phases are described, with emphasis placed on the former; in particular, the relationship between dimers and polymers belonging to the semi-flexible category: [graphic omitted](mesogenic core)—(flexible spacer)[graphic omitted]n. The review concludes by drawing attention to the curious behaviour of polypeptide liquid crystals (cholesteric compensation). The underlying theme of the lecture is the need to recognize the synergism and affinity that characterizes research on monomeric and polymeric liquid crystals.
TL;DR: The stochastic-trajectory approach to simulating the dynamics of gas-surface interactions is described in this article, including techniques for handling infrequent events such as desorption.
Abstract: The ‘stochastic-trajectory’ approach to simulating the dynamics of gas–surface interactions is described, including techniques for handling ‘infrequent events’ such as desorption. The method is applied to interaction of an isolated Xe atom with a clean Pt(111) surface. Calculated time-of-flight distributions of Xe atoms scattered from the surface, as a function of scattering angle, are in good agreement with experiment. The time required for a ‘trapped’ atom to settle down into its binding site and the relative efficiencies of normal and tangential energy accommodation are also examined.
TL;DR: A brief background to electro-optic effects in sidechain polymer liquid crystals has been given in this article, where it has been shown that ultra-high contrast and resolution laser writing may be achieved with both aligned and scattering textures.
Abstract: A brief background to electro-optic effects in side-chain polymer liquid crystals has been given. Electro-optic effects in dyed and undyed smectic polysiloxanes are described and it has been shown that these high-contrast optical effects may be written on the ca. 100 ms timescale and are stored in the smectic phase above Tg. It has also been shown that ultra-high contrast and resolution laser writing may be achieved with both aligned and scattering textures. Finally, it has been demonstrated that the siloxane polymers may be added to monomeric liquid crystals to control their viscoelastic properties. This has led to a desirable improvement in these properties and the polymer characteristics likely to lead to further improvements have been discussed.
TL;DR: In this article, the current status of the macroscopic linear mechanical properties of nematic liquid crystals formed from solutions of rigid or semirigid rod-like polymers is summarized.
Abstract: We summarize the current status of our studies of the macroscopic linear mechanical properties of nematic liquid crystals formed from solutions of rigid or semirigid rod-like polymers. The polymer system we have studied is a racemic mixture of poly(benzyl glutamate) dissolved in a mixture of dioxane and methylene chloride. We have also studied nematics formed from colloidal suspensions of tobacco mosaic virus which may be viewed as an ideal model system representing a rigid-rod polymer solution. We review briefly the current theoretical understanding of the elastic and viscous parameters characterizing a nematic. Then we discuss our experiments, both Frederiks-transition studies and quasielastic Rayleigh scattering. The Frederiks transition in its well known simple forms is not easily utilized in these systems, mainly because of the intervention of a number of phenomena not normally encountered in low-molecular-weight liquid crystals. However, these phenomena have been analysed theoretically and can be used to extract information on elastic and viscous properties. Quasielastic Rayleigh scattering on well oriented single crystals has proved to be an excellent technique for measuring ratios of elastic and viscous parameters. We describe the scattering geometries we have used and our results.
TL;DR: In this paper, the dynamics of energy exchange between gas atoms and crystal surfaces, with consequent trapping and desorption, is discussed in terms of a simple model for the gas-surface potential involving motion in a conservative attractive potential and fast energy-exchanging collisions between gas and surface atoms.
Abstract: The dynamics of energy exchange between gas atoms and crystal surfaces, with consequent trapping and desorption, is discussed in terms of a simple model for the gas-surface potential involving motion in a conservative attractive potential and fast energy-exchanging collisions between gas and surface atoms. Integral recurrence relations for the velocity distributions of atoms leaving the surface after N hops are derived and used to study the approach to the asymptotic steady state for large N. For a Boltzmann incoming distribution this steady-state distribution is non-Boltzmann, as are the velocity distributions for small N, but in such a way that the total velocity distribution for all atoms leaving the surface is Boltzmann, as required by detailed balance. Residence-time distributions are also calculated, both for Boltzmann and for mono-energetic incident beams. For high incident-beam energies these show a pronounced peak at short residence times (prompt inelastic scattering) together with a decay at long-residence times which is ultimately exponential. If the gas–surface collisions are ‘perfectly equilibrating’ the residence-time distribution is purely exponential in form. For high incident-beam energies the probability of trapping is zero at very low and at very high surface temperatures, with a maximum in between. For low incident-beam energies the probability of trapping decreases monotonically from one at low temperatures to zero at high temperatures. Arguments are presented indicating that many of these qualitative features are independent of the simplifications of our model. A general relation between mean residence time and equilibrium adsorption is stated. The implications of these results for the kinetics of physisorption and chemisorption are discussed.
TL;DR: In this paper, a broad overview of the static and dynamic surface properties of rare-gas overlayers was provided, including elastic diffraction, selective adsorption and inelastic single-phonon scattering by angle-resolved time-of-flight.
Abstract: We review our experiments involving He scattering from the surfaces of ordered rage-gas overlayers physisorbed on Ag(111). The rare gases studied were Ar, Kr and Xe. The measurements were performed on thin-film structures which were grown on a layer-by-layer basis, from monolayer to thick film, in order to study how the surface properties changed with increasing distance from the substrate. Three types of experiments are discussed; elastic diffraction, selective adsorption and inelastic single-phonon scattering by angle-resolved time-of-flight. These experiments, taken together, provide a broad overview of the static and dynamic surface properties of rare-gas overlayers. They also provide information about the forces which govern the adatom–adatom and adatom–substrate interactions.
TL;DR: An intermediate nematic mesophase state has been found to occur in carbonaceous pitches during their thermal transformation to coke and carbon as discussed by the authors, and the present state and future possibilities of the technology based on these phenomena is discussed.
Abstract: An intermediate nematic mesophase state has been found to occur in carbonaceous pitches during their thermal transformation to coke and carbon. The molecules responsible for mesophase formation are, by and large, disc-like and mostly aromatic, and their distribution of molecular weights is quite critical. The liquid crystal exhibits uniaxial-negative optical behaviour and can be easily oriented by mechanical and magnetic forces.Mesophase pitches are usually two-phase systems in the liquid state and the mesophase becomes a nematic glass when cooled to the solid state. The two phases can be separated and their constitutions and physical behaviour determined. In certain aspects, the coexisting isotropic and anisotropic phases are remarkably similar.Because of the unique structural and rheological characteristics of the carbonaceous mesophase and its ordering and hardening behaviour, it has been possible to prepare highly oriented, high-performance products such as carbon fibres from mesophase pitch.The present state and future possibilities of the technology based on these phenomena is discussed.
TL;DR: In this paper, X-ray analyses of the structures of a group of aromatic copolyesters that form liquid-crystalline melts are described, and it is shown that their positions and intensities are predicted by a model consisting of an assembly of parallel chains of completely random monomer sequences.
Abstract: The X-ray analyses of the structures of a group of aromatic copolyesters that form liquid-crystalline melts is described. X-ray patterns of melt-spun fibres of wholly aromatic copolymers prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation and there is also evidence for three-dimensional order. The meridional maxima are aperiodic, and it is shown that their positions and intensities are predicted by a model consisting of an assembly of parallel chains of completely random monomer sequences. Analyses of the scattering by aperiodic chains are summarized, first for point monomers and subsequently for an atomic model. From the breadth of some of the meridional maxima it is possible to estimate the persistence or correlation length for the stiff-chain conformation, which is found to be between 9 and 13 monomer units, depending on the HBA/HNA ratio. Determination of the three-dimensional structure is initiated via calculation of the cylindrically averaged transform for the random chain, followed by consideration of the interference effects caused by chain packing.
TL;DR: In this paper, a mixed overlap between adsorbates and defects is used to investigate adsorbate diffusion in thermal He-beam scattering, showing that the cross-section overlap is substantial even at low coverages and the degree of overlap depends on the nature of the adorbate distribution along the surface.
Abstract: The recent application of the scattering of thermal He beams to the study of disordered adsorbates is featured by both an extreme sensitivity (coverages down to θ≈ 0.001 are accessible) and non-destructiveness. The sensitivity is a consequence of the very large cross-section (ca. 100 A) for diffuse scattering exhibited by adsorbates. The cross-section overlap is substantial even at low coverages and the degree of overlap depends on the nature of the adsorbate distribution along the surface, i.e. on the mutual interaction of the adsorbates. Thus information on the latter can be obtained from the former; this is the overlap approach. A recent development has shown that this is also true if ‘monovacancy’ is substituted for ‘adsorbate’.Examples of the use of the overlap approach are given. Information, inaccessible so far, on the mutual interaction of adsorbates and of vacancies, on the details of the adsorption and of the vacancy-creation processes and on two-dimensional phase transitions are obtained. Similarities and differences between adsorbate and vacancy behaviour and their influence on thermal He scattering are discussed. Finally, a mixed overlap between adsorbates and defects is used to investigate adsorbate diffusion.
TL;DR: In this paper, the homologous series of thermotropic nematic main-chain polyesters has been investigated and a layered structure develops progressively with increasing chain length, the sharpness and intensity of the SAX diffraction peak increasing with molecular weight.
Abstract: The homologous series of thermotropic nematic main-chain polyesters [graphic omitted] has been investigated. In the case of even values of n, a smectic phase appears at n= 18. An odd–even oscillation of nematic order parameters associated with the mesogen is observed near the clearing temperature, Tc. At the same time, in the vicinity of Tc, the relative order of mesogen and spacer is approximately the same for odd and even values of n. In unoriented specimens of polymer with spacer length n= 10, a layered structure develops progressively with increasing chain length, the sharpness and intensity of the SAX diffraction peak increasing with molecular weight. However, the degree of order remains that of a cybotactic nematic, based on measurements of the complex viscosity and miscibility behaviour. No SAX diffraction peak is observed with spacer length n= 7.
TL;DR: In this paper, two β relaxation processes were found: the βL relaxation is believed to be caused by diffusional segmental motion of the flexible spacers located in isolated amorphous isotropic regions of the material and the βU process is associated with the ‘ether’ sequences located in the glassy SC domains.
Abstract: The molecular dynamics of the polyester (—OC—[Ph]3—CO—O—[CH2–CH2—O—]4)n, which has a liquid-crystalline smectic C (SC) phase, have been investigated by the e.s.r. spin-probe, dielectric relaxation and carbon-13 solid-state n.m.r. techniques. The low-temperature, high-frequency γ relaxation preserves the characteristics of the local in-chain motions of polyethers. The apparent activation energy corresponds to a very simple flexible structure. The most likely mechanism of this γ relaxation might be the local tg+t ↔ tg–t transition in the central ‘ether’ units. Two β relaxation processes were found: the βL relaxation is believed to be caused by diffusional segmental motion of the flexible spacers located in isolated amorphous isotropic regions of the material and the βU process is associated with the ‘ether’ sequences located in the glassy SC domains. The increasing rate of rotation of the phenyl rings about their C1—C4 axis is related to the crystal ↔ SC transition. Note that this motional process of the phenyl rings does not alter the mean orientation of the mesogenic groups.
TL;DR: In this article, the repulsive surface interaction potential of H2 and D2 molecules at thermal energies from the Ag(110) surface has been studied by means of nozzle-beam techniques.
Abstract: The scattering of H2 and D2 molecules at thermal energies from the Ag(110) surface has been studied by means of nozzle-beam techniques. Diffraction peaks of comparable intensities are present along both the [001] and the [1text-decoration:overline10] azimuthal directions, indicating a two-dimensional repulsive corrugation. The diffraction intensities were resolved over a wide range of incident angles and energies. The experimental H2 data are analysed theoretically to obtain information on the repulsive molecule–surface interaction potential. The calculations are performed (a) with the hard-corrugated-wall potential, (b) with a soft-corrugated-wall model and (c) performing close-coupling calculations for a flexible phenomenological potential. The best-fit interaction potential shows that the two-dimensional repulsive corrugation has a weak dependence on energy. A comparison is made between the H2–Ag(110) and He–Ag(110) potentials and the H2 and He diffraction from the Ag(111) and Cu(110) surfaces.
TL;DR: In this article, a geometrical model which includes local competitions between coiling and orientational correlations and which uses the methods developed for the representation of frustration in curved spaces is given.
Abstract: Some recent observations of defects in main-chain nematic and chiral polymers are reviewed in order to obtain a better understanding of the nature of molecular configurations. Important parameters are (a) the chain length, the density of free ends and the length of flexible spacers and (b) the orientational correlations between chains. The first topic, (a), is discussed with respect to observations of defects in uniaxial nematics; the second, (b), is discussed in the light of observations of biaxial nematics and of cholesteric textures of rigid polymers of biological interest. The elements of a geometrical (structural) model which includes local competitions between coiling and orientational correlations and which uses the methods developed for the representation of frustration in curved spaces are given.
TL;DR: In this paper, a low-energy electron-diffraction study of incommensurate nitrogen physisorbed on graphite was performed and a triangular two-out structure with tilt predicted by Harris and Berlinsky was observed.
Abstract: New results of a low-energy electron-diffraction study of incommensurate nitrogen physisorbed on graphite are reported. A triangular incommensurate ‘two-out’ structure with tilt of the type predicted by theory of Harris and Berlinsky is observed. The molecular orientation is estimated on the basis of steric considerations and a kinematic approximation for the diffraction intensities. The estimated orientation and structure are found to be in reasonable agreement with the predictions of theory when the nitrogen–nitrogen and nitrogen–graphite interaction parameters currently available are used.
TL;DR: Very low-energy scattering experiments for He atoms colliding with an LiF(001) surface were described in this paper for beam energies down to 4 meV, where good agreement between experiment and theory was found, but there are small discrepancies which should allow the potential to be refined.
Abstract: Very low-energy scattering experiments are described for He atoms colliding with an LiF(001) surface. The apparatus is capable of performing experiments for beam energies down to 4 meV. The He–LiF results are compared with close-coupling calculations on the interaction potential of Celli et al.(J. Chem. Phys., 1985, 83, 2504). Very good agreement between experiment and theory is found, but there are small discrepancies which should allow the potential to be refined.
TL;DR: In this paper, the value of the critical temperature Tc for physically adsorbed atoms and methane molecules was derived using perturbation theory, using the equation of state of a 2D Lennard-Jones fluid as a reference.
Abstract: Thermodynamic properties of films depend on both the interactions within the adsorbate and those between the adsorbate and the substrate. We investigate these in relation to the problem of the two-dimensional (2D) liquid–vapour critical point. Our focus is the value of the critical temperature Tc for physically adsorbed atoms and methane molecules. The calculations employ perturbation theory, using the equation of state of a 2D Lennard-Jones (LJ) fluid as a reference. Explicitly included effects are deviations of the two-body potential from LJ shape, substrate modification of the interaction (leading to different 2D and 3D potentials) and three-body interactions within the adsorbate. Each of these contributes a measurable shift of Tc from the value computed with LJ theory and the 3D potential. The final predictions are in semiquantitative agreement with experimental data for gases on graphite.
TL;DR: Weare et al. as mentioned in this paper showed that the long-range triple-dipole correction alone is often used to represent the total non-additive correction, e.g. in calculations of the binding energy of rare-gas solids.
Abstract: Atom scattering from physisorbed overlayers is first briefly reviewed. While good agreement has been obtained between measurements and calculations both for diffraction scans and selective adsorption [H. Jonsson, J. H. Weare, T. H. Ellis, G. Scoles and U. Valbusa, Phys. Rev. B, 1984, 30, 4203; T. H. Ellis, G. Scoles, U. Valbusa, H. Jonsson and J. H. Weare, Surf. Sci., 1985, 155, 499; H. Jonsson, J. H. Weare, T. H. Ellis and G. Scoles, in preparation; J. M. Hutson and C. Schwartz, J. Chem. Phys., 1983, 79, 5179], detailed information on the scattering potential has not been inferred from the experimental data because of three highly correlated unknowns in the theoretical potential: (1) uncertainty in the probe–adatom pair potential, (2) uncertainty in the probe–substrate potential and (3) non-additive many-body corrections. New close-coupling calculations are presented here for the scattering of He from a commensurate overlayer of Kr on graphite and compared with recent selective adsorption data of Larese et al.(J. Z. Larese, W. Y. Leung, D. R. Frankl, N. Holter, S. Chung and M. W. Cole, preprint). This is a favourable system in that the He–Kr pair potential is well known and a semiempirical helium–graphite potential consistent with extensive experimental data on clean graphite has recently been constructed (H. Jonsson and J. H. Weare, in preparation). Good agreement is obtained when the theoretical potential only contains two-body contributions, but it is significantly improved by shifting the lowest bound-state energy by –0.12 meV. The long-range triple-dipole correction alone is frequently used to represent the total non-additive correction, e.g. in calculations of the binding energy of rare-gas solids. When applied to the present problem, the triple-dipole correction shifts the lowest bound state in the wrong direction by +0.16 meV. A correction due to thermal vibration further shift this level by +0.07 meV. There is, therefore, a 0.35 meV difference between the lowest bound-state energy of this theoretical potential and the experimental value. We estimate the uncertainty in the two-body terms [(1) and (2)] to be ±0.1 meV. Typically the uncertainty due to experimental parameters is estimated to be ±0.1 meV. The data therefore suggest that the total non-additive correction is not well represented by the long-range triple-dipole correction alone and that other many-body corrections of opposite sign need to be included. Three-body corrections to the short-range repulsive interaction have not been quantitatively estimated but are known to be attractive [W. J. Meath and R. A. Aziz, Mol. Phys., 1984, 52, 225).
TL;DR: In this paper, the lattice dynamics and molecular dynamics were used to investigate phonons propagating in monolayers, bilayers and trilayers of rare gases physisorbed on the Ag(111) surface.
Abstract: Lattice dynamics (LD) and molecular dynamics (MD) calculations have been used to investigate phonons propagating in monolayers, bilayers and trilayers of rare gases physisorbed on the Ag(111) surface. Adatom–surface potentials for the monolayers are represented by a Morse function whose well depth and curvature are parametrized to reproduce the latent heat of condensation and the frequency of the dispersionless phonon branch with atomic displacements perpendicular to the surface. In the bilayers and trilayers the second and third overlayers are assumed to interact with the surface via the known van der Waals terms. Realistic potentials are used to represent the interactions between the adatoms. If the interlayer separations are optimized with due allowance for thermal dilation effects the LD results are in agreement both with the MD calculations and with the available experimental data for the bilayers and trilayers. The sensitivity of the calculations to variations in the parameters of the model potential has been investigated.
TL;DR: The diffusive mobility of a rod-like molecule, ethane, adsorbed on the (0001) surface of graphite has been studied by incoherent quasielastic neutron scattering as a function of temperature as mentioned in this paper.
Abstract: The diffusive mobility of a rod-like molecule, ethane, adsorbed on the (0001) surface of graphite has been studied by incoherent quasielastic neutron scattering as a function of temperature. The measurements have been limited to the submonolayer domain (θ < 0.7 monolayer) and to the 10–122 K temperature range. At 10 K ethane molecules do not display any diffusive motion; at 53.5 K the herringbone solid structure exhibits reorientational motion of the methyl groups about the threefold carbon–carbon axis. Above the two-dimensional melting point (65 K) ethane molecules perform an isotropic rotational motion plus a diffusive translation which can be described, at 87 K, as a jump diffusion between equivalent surface sites (lattice liquid). At 122 K the adsorbed molecules display brownian motion (isotropic liquid).
TL;DR: In this paper, a theory of atomic scattering from a single adsorbate is given and numerical results for the total cross-sections of the He-Xe/Ag system are presented.
Abstract: The van der Waals interaction between an atom and an adsorbate consists of the usual two-body direct interaction as well as three-body interactions from the image force and an interference effect. The three-body interactions are caused by the presence of the surface and render the resulting potential anisotropic even when the adsorbate is spherically symmetrical. Expressions for the leading coefficients of the van der Waals forces and the corresponding reference-plane positions are given in terms of the polarizabilities of the atom, the adsorbate and the substrate. A theory of atomic scattering from a single adsorbate is given and numerical results for the total cross-sections of the He–Xe/Ag system are presented.