Molecular orientation of asphaltenes and PAH model compounds in Langmuir-Blodgett films using sum frequency generation spectroscopy

Asphaltenes are an important class of compounds in crude oil whose surface activity is important for establishing reservoir rock wettability which impacts reservoir drainage. While many phenomenological interfacial studies with crude oils and asphaltenes have been reported, there is very little known about the molecular level interactions between asphaltenes and mineral surfaces. In this study, we analyze Langmuir-Blodgett films of asphaltenes and related model compounds with sum frequency generation (SFG) vibrational spectroscopy. In SFG, the polarization of the input (vis, IR) and output (SFG) beams can be varied, which allows the orientation of different functional groups at the interface to be determined. SFG clearly indicates that asphaltene polycyclic aromatic hydrocarbons (PAHs) are highly oriented in the plane of the interface and that the peripheral alkanes are transverse to the interface. In contrast, model compounds with oxygen functionality have PAHs oriented transverse to the interface. Computational quantum chemistry is used to support corresponding band assignments, enabling robust determination of functional group orientations.     

Nanostructure fabrication by reactive-ion etching of laser-focused chromium on silicon

6 plasma. We show that the result can be an array of parallel wires as narrow as 68 nm or an array of parallel Si trenches as narrow as 85 nm. The laser-focused deposition process is inherently parallel, so a large area is patterned simultaneously with an accurate periodicity of 212.78 nm. This method represents a novel way to make large, coherent arrays of sub-100 nm-size structures. Received: 20 March 1997     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Quasi-free scattering of pions on 3, 4He

Differential cross sections for ^{3, 4}He(π, π') are presented for angles of 60° and 120° at 200 MeV and 120° at 295 MeV. The momentum spectra for the scattered pions are dominated by a peak attributed to quasi-free scattering from individual nucleons. There are significant differences between the scattering on ³He and ⁴He as well as between the scattering of π⁺ and π^? on ³He. The data are compared to a simple model incorporating the free π+N scattering amplitudes. Modifications for the structure and dynamics of the target nucleus are discussed.     

Stereochemically versatile synthesis of the C1-C12 fragment of tedanolide C

A flexible synthesis of the C1-C12 fragment of Tedanolide C has been accomplished in eight steps from 2-methyl-2,4-pentadienal. Asymmetric hydroformylation of a 1,3-diene allows for the late-stage generation of either C10 epimer with complete catalyst control. Diastereoselective addition of an isobutyryl β-ketoester dianion to an α,β-disubstituted chiral aldehyde sets the C5 stereochemistry while installing the geminal dimethyl unit. Differential protection of a syn-1,3-diol is performed as a highly efficient single-pot operation.     

Hydrogenic systems,frequency standards and fundamental constants

Hydrogenic (two-body) systems are the only atomic systems for which uncertainties in calculations of the energy levels approach the current state of the art in frequency measurement. This article discusses progress in the theory and measurement of transition frequencies in hydrogenic systems. These studies have relevance to the determination of fundamental constants and the testing of physical theories, especially quantum electrodynamics. A set of high accuracy calculable frequency standards could also be realized by using hydrogenic systems.     

Iron pentacarbonyl: are the axial or the equatorial iron-carbon bonds longer in the gaseous molecule?

The structure of iron pentacarbonyl, Fe(CO)(5), was reinvestigated by gas-phase electron diffraction using an experimental rotational constant available from the literature as a constraint on the structural parameters. The study utilized a B3LYP/6-311+G(d) ab initio quadratic force field, scaled to fit observed infrared wavenumbers, from which were calculated corrections for the effects of vibrational averaging on distances and certain other quantities useful for the structural analysis. The results confirm that the equatorial Fe-C bonds are longer than the axial ones, an important difference with the structure in the crystal where the equatorial Fe-C bonds are the shorter. Some distance (r(g)/A) and vibrational amplitude (l(alpha)/A) parameter values with estimated 2sigma uncertainties based on assumption of D(3h) symmetry are [r(Fe-C)] = 1.829(2), r(Fe-C)(eq) - r(Fe-C)(ax) = 0.032(20), [r(C=O)] = 1.146(2), r(C=O)(eq) - r(C=O)(ax) = 0.006(27), r(Fe-C)(ax) = 1.810(16), r(Fe-C)(eq) = 1.842(11), r(C=O)(ax) = 1.142(23), r(C=O)(eq) = 1.149(16), l(Fe-C)(ax) = l(Fe-C)(eq) = 0.047(5), and l(C=O)(ax) = l(C=O)(eq) = 0.036(3).     

Squeezing flow of elastic liquids

The theoretical analysis is made of the relation between applied force and plate separation for squeezing flows of viscoelastic liquids between closely-spaced parallel disks. The lubrication approximation and the quasi-steady-state assumption are employed in the development. Elastic effects are incorporated through inclusion of normal stresses. Solutions are presented for liquids with power-law viscometric functions, and a numerical procedure is used for fluids having viscometric functions of arbitrary form. For fast and slow squeezing, calculated values of t_{$\text{1}\text{2}$}, the time required to squeeze out half the fluid, are found to agree with the constant force data of Leider [1,2].     

The atom-cavity system as a generator of quadrature squeezed states

This paper describes experiments with a coupled atom-cavity system generating quadrature squeezed states of light. A wide range of parameters was explored and a regime was found where a beam of laser light with significant power (0.17 mW) and good noise suppression (measured 18±3%, inferred 50±10%) was observed. An analysis of the exact phase of the noise suppression shows it to be a minimum uncertainty state with reduced noise in a combined amplitude/phase quadrature. The observations are in good qualitative agreement with a full quantum theory of squeezing in optical bistability.     

T=1, spin-dipole strength in40Ca

Spin-dipole strength in⁴⁰Ca has been studied by inelastic scattering of 500-MeV protons and the dipole response in⁴⁰Ca is compared with the spin-dipole data from the⁴⁰Ca(p, n) reaction and nonspin-dipole data from photonuclear studies.