Incoherent subharmonic light scattering in isotropic media

Incoherent subharmonic light scattering in isotropic media is a new kind of nonlinear light scattering, which involves single input photon and multiple output photons of equal frequency. We investigate theoretically the dependence of the subharmonic scattering intensity on the hyperpolarizability of molecules and the incident intensity using nonlinear optics theory similar to that used for Hyper-Rayleigh scattering and degenerate optical parametric oscillators. It is derived that the subharmonic scattering intensities grow exponentially or superexponentially with the hyperpolarizability of molecules and the incident intensity.     

Inelastic scattering of low-energy electrons by metastable atoms

Cross sections for inelastic collisions of low-energy electrons with metastable atoms in the case of close coupling between different atomic states are obtained in the quasi-statistical approximation. Processes with metastable C, N, O and rare-gas atoms are considered. The calculated cross sections and rate constants are compared with experimental and numerical calculations.     

X-ray scattering by black foam films

The x-ray scattering by the three types of black foam films (common black, Newton black, and stratified black films) was experimentally studied. A special device in which flat black films with an area of ca. 2 cm² can be produced was developed and x-ray diffraction patterns were obtained by a vertical diffractometer. The three types of films differ significantly in their x-ray reflections, which proves that they have different structure. For common black films, the comparison of observed and calculated intensities lead to a model, which corresponds to the three-layer model. The Newton black films exhibit diffraction trace with only one highly asymetric peak and there is, as of yet, no unambigous interpretation. The patterns of the stratified black films have several pronounced sharp peaks corresponding to the areas of different films with a given thickness.     

Light and X-ray scattering by polystyrene glasses

Summary Polarized and depolarized components of the light scattered by atactic polystyrene samples prepared from monomer of varying purity were investigated at 25 °C. In addition, small angle X-ray measurements were made on a well purified sample. These investigations show that the angular dependence and the magnitude of the scattered intensity are strongly influenced by dust-like solid impurities. The scattering observed with polystyrene glasses prepared from the purest monomer is consistent with a model in which the glass is a structureless liquid-like material. No evidence for any ordered regions is seen in these clean glasses by either light or X-ray scattering.Further consideration shows, however, that light and X-ray scattering are not very sensitive to the presence of small isotropic regions. While depolarized light scattering may be more sensitive to order through orientational correlations, quantitative interpretation of the H _v intensities is difficult. Nevertheless, from the observation that the light scattering is unaffected by thermal history, one can be reasonably certain that no structure is present beyond that expected in a supercooled liquid.

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Zusammenfassung Polarisierte und depolarisierte Komponenten des gestreuten Lichtes von ataktischen Polystyrol-Proben hergestellt von Monomeren mit verschiedenen Reinheitsgraden, wurden mit Hilfe eines Lichtstreuphotometers bei 25 °C untersucht. Ferner wurden Röntgenkleinwinkelmessungen an einigen Proben von höchster Reinheit durchgeführt. Diese Untersuchungen ergaben, daß sowohl die Winkelabhängigkeit als auch die Höhe der Intensität durch Staub und feste Verunreinigungen stark beeinflußt wird. Die Streuung von Polystyrolen im Glaszustand, die von höchstreinen Monomeren hergestellt wurden, stimmte mit dem Modell eines strukturlosen, flüssigkeitsähnlichen Glases überein. Licht- und Röntgenkleinwinkelstreuung zeigten keinen Hinweis auf geordnete Gebiete.Theoretische Überlegungen zeigen, daß die Lichtund Röntgenstreuung durch kleine isotrope Gebiete wenig beeinflußt wird. Während die depolarisierte Lichtstreuung empfindlicher gegen Ordnung durch Orientierungskorrelationen sein kann, sind quantitative Aussagen über dieH _v Intensitäten schwierig. Da die Lichtstreuung durch Wärmebehandlung nicht beeinflußt wurde, kann man mit ziemlicher Sicherheit sagen, daß es nicht mehr Struktur gibt als man es von einer unterkühlten Flüssigkeit erwarten würde.

     

Angular distributions for vibrationally inelastic nonresonant scattering of electrons by molecules

Polar graphs for differential cross section (dcs) called spatial dcs maps are presented as graphical representation of the angular distribution of vibrationally inelastic electron scattering by polyatomic molecules. The objective of this paper is to show that an intuitive understanding of the principal features of these graphs can be obtained from a simple analysis of the normal modes of vibration of the target molecule and plane-wave functions representing the scattering electron. The procedure is illustrated on the H2 and CH4 molecules.     

Investigation of humate-cetyltrimethylammonium complexes by small-angle X-ray scattering

The structural examination of the complexes formed between humic acid and cationic surfactants has environmental implications. A humic acid (HA) dissolved in 0.1 M NaOH (5 g/L) was reacted with a cationic surfactant (hexadecyltrimethylammonium bromide or CTAB) at initial solution concentrations of 1, 5, 10, 20, 30, 40 and 50 mM. The HA precipitated at CTAB concentrations of 20, 30, and 50 mM but the complexes were soluble at 40 mM and below 20 mM. The charge neutralization between humic acid anions and CTAB micelles and the subsequent charge reversal due to hydrophobic interactions explain the behavior of the HA-CTAB complexes. The HA solution (5 g/L), reaction products (supernatants and precipitates), and pure cationic surfactant solutions were studied by the small-angle X-ray scattering (SAXS) technique in order to determine the structure of HA-CTAB complexes. The scattering intensity (I(q)) of various HA-CTAB systems were recorded over a range of scattering vectors (q=0.053-4.0 nm(-1)). HA forms networks in an alkaline solution with a characterization length of 7.8 nm or greater. The HA-CTAB precipitates and the 50-mM CTAB solution gave d(100) and d(110) reflections of a hexagonal structure. The hexagonal array of cylindrical CTAB micelles has a lattice parameter of 5.01 nm in pure solution, and the parameter decreases in the order: 4.96, 4.91, and 4.85 nm for the precipitates of HA-CTAB (50, 30, and 20 mM, respectively), indicating that the structure of CTAB micelles was disturbed by the addition of HA. The molecular properties and behavior of HA in solution were discussed.     

Measurement of Compton scattering on bound electrons by the coincidence method

A review of results of the new method for measuring the Compton scattering on bound electrons in germanium, introduced by the presented authors, is given. It is based on the application of two detectors that operate in the coincidence mode. One detector is used as the scatterer and the other as the detector of scattered radiation. Two conditions, simultaneity of pulses from the two detectors and constant energy sum, result in very clean spectra in broad energy regions. Normalization of the Compton spectra to the Ge K X-ray escape peaks, which are measured simultaneously with the Compton spectrum, gives reliable double-differential Compton-scattering cross sections on an absolute scale. Several versions of the impulse approximation are compared to the cross sections obtained by the present method for incident photon energies in the range from 60 to 105 keV. The non-relativistic impulse approximation gives the best agreement to the experimental data. We point out the suitability of the new method for an investigation of the incoherent scattering function at small photon momentum transfer.     

Nuclear excitation in <Superscript>176</Superscript>Lu by positron annihilation on K-shell electrons

In the process of nuclear excitation in positron-electron annihilation (NEPEA) experimental results have been found to be larger than theoretical predictions. In our previous works we have analyzed the NEPEA experimental data for 1078 keV in ¹¹⁵In and for 1330 keV in ¹¹¹Cd, respectively, by applying our model of indistinguishable quantum oscillators. In the present work we extend this model to estimate the cross section of nuclear excitation in positron-electron annihilation for ¹⁷⁶Lu from the experimentally measured effective cross section of σ ^eff = (2.6±0.9)·10⁻²⁹ cm². The estimated NEPEA cross section of σ=2.7·10⁻²⁶ cm² is in very good agreement with that of the theoretical prediction σ ^th=2.2·10⁻²⁶ cm².     

Quasi-elastic scattering of electrons in image-potential states

Image-potential states provide a model system to study electron scattering at surfaces. With time-, energy- and angle-resolved two-photon photoemission quasi-elastic intraband and resonant interband scattering processes can be identified and resolved. The scattering sources are related to phonons and to imperfections of the surface such as defects and steps.     

Vibronic effects in elastic scattering of electrons

A method for obtaining general equations for the scattered intensities from vibronic systems is given. The approximate formulas obtained are used to calculate the effects on the electron diffraction pattern for molecules with doubly degenerate electronic E terms interacting with e-type vibrations (Ee-type of problem). The results of the approximate calculations are compared to more precise results, based on numerical solution of the vibronic problem.     

X-ray scattering by water molecules studied by using synchrotron radiation

The energy spectra of free water molecules were measured at scattering angles 2θ ranging from 10.5° to 75.7°, using an angle-dispersive-type diffractometer and synchrotron radiation as an X-ray source. A silicon (111) monochrometer was used to obtain incident X-rays with the wavelengths of (1.543/n) Å (n = 1,3,4,5). Observed inelastic scattering peaks are clearly separated from eleastic ones at s values [s = (4π/λ) sin Å] larger than 8 Å^?1. The increase of the separation with an increasing s value was consistent with the classical theory of the Compton shift. The total (elastic plus inelastic) intensities were obtained over a range of s = 0.74–5.0 Å^?1. Experimental difference intensities Δσ_ee and Δσ_ne were obtained separately by combining the X-ray and high-energy electron scattering data. The experimental results are in reasonable agreement with the theoretical intensities calculated from SCF and CI molecular wave functions with a basis set of double-zeta plus polarization functions. © 1994 John Wiley & Sons, Inc.     

Incoherent inelastic neutron scattering studies of the torsional modes of hexachloroethane

From incoherent inelastic neutron scattering studies of solid C₂Cl₆ the in- and out-of-phase torsions about the C-C axis are assigned at 56 and 95 cm^?1, respectively. Using a model for the potential barriers in the solid the torsional frequency in the gas has been calculated to be 76.7 cm^?1 and the internal barrier to be 67.8 kJ mol^?1.     

Electronic properties of island thin films caused by surface scattering of electrons

We review theoretical and experimental works which substantiate that island metal films, contrary to bulk metals and similar to semiconductors and gas plasmas, are systems in which hot (nonequilibrium) electrons can be generated under stationary conditions. This is accomplished by passing a current or by laser irradiation of the islands films whose specific properties are caused by two major factors: (i) the electron-phonon interaction is strongly reduced in the islands with dimensions smaller that the electron mean free path in the bulk; (ii) considerable power can be fed into the islands without their destruction. The generation of the hot electrons results in electron and light emission from the island films.     

Elastic scattering of low and intermediate-energy electrons by Kr and Xe atoms

The many-body correlation forces, which act between the impinging electron and the bound electrons of the two heaviest rare gas atoms, are treated here using a newly developed correlation-polarisation potential that originates from the calculation of correlation energies in electronic bound-states of atoms and molecules. The new formulation of such forces, already tested by us for the lighter atomic targets, is particularly effective for the present systems and can be implemented very easily even for heavy atomic targets. The calculations reported in this paper show clearly that very good accord is obtained with more sophisticated theoretical treatments and with several experimental data on integral cross sections, momentum transfer cross sections and angular distributions.Von Humboldt — Prize Awardee 1992     

A simple model for the scattering of electrons by a rotating dipole

The coupled equations describing the interaction of one electron with a dipole and hard sphere are shown to be exactly soluble, even when the energy levels of the dipole are taken into account. This model is used to discuss the critical moment for binding the electron in the dipole field. The condition for the existence of Feshbach resonances is similarly discussed. When the model is applied to calculate scattering phase shifts, shape resonances are found.     

Polysiloxane network formation observed by time-resolved small-angle X-ray scattering

Time-resolved small-angle X-ray scattering was observed on systems undergoing gelation. A polysiloxane network was used as a model system, where the polysiloxane network was formed by hydrosylation of VT-M with F4-C. The Flory-Stockmayer model for gelation was found to be satisfactory to describe the polysiloxane network formation. No inhomogeneous distribution of densely crosslinked region exists, and in this respect the gel formed by the polysiloxane network is homogeneous. The invariance of the correlation length ξ evaluated from the Lorenzian term suggests that the local network architecture would not change by gelation.