Peter Haaland

Dissociative attachment in silane

Cross sections for production of SiH−n from silane by electron impact from 6–12 eV have been measured with Fourier transform mass spectrometry techniques. The peak cross sections decrease and thresholds increase for the more extensively dissociated silicon hydride anions: σa(SiH−3)=1.9×10−18 cm2; σa(SiH−2)=0.8×10−18 cm2; σa (SiH−)=0.2×10−18 cm2. The cross sections for production of SiH−4 and H− are less than 8×10−21 cm2 over the 6–12 eV energy range. Ab initio calculations of the anion thermochemistry show that if the neutral products include molecular hydrogen substantial excitation of the anion or neutral fragments is required. The rates of anion production calculated from dissociative attachment cross sections show that attachment of electrons to radicals or excited species are the most important source of negative ions in silane plasmas.

Ion chemistry in octafluorocyclobutane, c-C4F8

Abstract Cross-sections for electron impact ionization of octafluorocyclobutane (c-C4F8) have been measured from 10 to 200 eV by Fourier transform mass spectrometry. No parent ion is observed, and over half of the dissociative ionization yields C2F4+ and C3F5+. Eleven other fluorocarbon cations are produced with smaller cross-sections, giving a total ionization cross-section of (1.6±0.2)×10−15 cm2 between 80 and 200 eV. Only CF2+ and C2F3+ react further with the parent molecule to yield C3F5+ as the primary product. No evidence of cationic polymerization was found. F− and C4F8− are formed by electron attachment at energies below 10 eV, but neither reacts further with c-C4F8.

Dissociative ionization of silane

Abstract The dissociative ionization cross-sections of silane (SiH 4 ), have been measured by Fourier transform mass spectrometry. Neither the molecular ion SiH 4 + nor the hydrogen ions H 2 + , H + are formed by electron impact. Cross-sections for the other silicon hydride ions at 50 eV are: σ(SiH 3 + ) = (0.5 ± 0.08) × 10 −16 cm 2 , σ(SiH 2 + ) = (0.74 ± 0.11) × 10 −16 cm 2 , σ(SiH + ) = (0.21 ± 0.03) × 10 −16 cm 2 , and σ(Si + ) = (0.19 ± 0.03) × 10 −16 cm 2 .

Electronic structure and properties of strained polymers: 2. Rigid-rod PBI, PBO and PBZT

Abstract A theoretical study of single-chain moduli employing the molecular-orbital semi-empirical AM1 (Austin model 1) approach is presented for the poly( p -phenylene benzobisimidazole) (PBI), poly( p -phenylene benzobisoxazole) (PBO) and poly( p -phenylene benzobisthiazole) (PBZT) rigid-rod polymers. This computational study is important for comparisons of mechanical properties for the series of related polymers. An analysis of molecular deformation on tension and compression of these rigid-rod polymers offers an insight into tensile and compressive processes at the molecular level, allowing their relative ordering by theoretical tensile and/or compressive strength and moduli. Furthermore, an examination of the electronic structures of these molecular systems containing various hetero atoms provides insight into the charge distribution and bond order changes on the application of strain. This approach will be useful for the relative prediction of the properties of polymers that have not yet been synthesized.

Cholesteric liquid crystals: Image contrast in the TEM

Abstract Bright-field image contrast of thermotropic cholesteric liquid crystalline materials in the transmission electron microscope (TEM) is investigated. Possible sources of contrast for these systems are discussed in terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Initial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed under normal viewing conditions. The periodic steady state contrast typically observed for these materials is the result of beam damage. Furthermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due to a preferred fracture path in ...

Electronic structure and properties of strained polymers: 1. Methods for polyethylene

Abstract The modulus and strain-dependent frequency spectrum of polyethylene have been computed from a cluster of C 6 H 12 units using the semi-empirical AM1 Hamiltonian and unrestricted Hartree—Fock wavefunctions. The computed modulus substantially exceeds experimental values, a result which derives from neglect of electron correlation in the calculations. Good qualitative and quantitive agreement with reported strain-dependent frequencies is found, suggesting that the method describes cubic anharmonicity in the bonding while systematically overestimating components of the Hessian.

Flowing afterglow synthesis of polythiophene films

Penning ionization of thiophene and 2,5‐dichlorothiophene by metastable argon in a flowing afterglow leads to the deposition of dense, uniform polythiophene films. Homogeneous kinetics have been examined using quadrupole mass spectrometry. The films have been characterized by spectroscopic ellipsometry, optical absorption, Rutherford backscattering spectrometry, and scanning force microscopy. The polymer, which has a nonlinear optical susceptibility χ(3)=10−11 esu at 602 nm, has been deposited on Si, quartz, and metal substrates.

Microlithographic patterning of polythiophene films

Lithographic patterning of micrometer scale structures in films of polythiophene, an organic material with good linear and nonlinear χ(3) optical properties, has been accomplished by two methods: direct deposition of polythiophene in SiO2 trenches and an oxygen plasma etch of polythiophene through a spin‐on glass mask.

Electronic structure of FO and its negative ions

Abstract Electronic structure calculations of FO and its anion have been performed at the QCISD(T) level of ab initio theory using 6−31 + G* and extended Gaussian basis sets. The adiabatic electron affinity of FO is calculated to be 2.08 ± 0.2 eV. The 1 Σ + anion, which is bound by 3 eV, dissociates to F + O − rather than lower energy products F − + O. The disposition of the anion and neutral potentials suggests intriguing results from experiments which would examine the vibrational dependence of electron attachments to FO.

Anisotropic stress and molecular moduli of C60

Abstract An exploration of mechanical properties at the molecular level based on inversion of the Hessian matrix is described. The molecular response of buckminsterfulleren (C 60 ) to isotropic and anisotropic stresses has been examined using semiempirical molecular orbital methods. Although C 60 is very stiff when compressed isotropically, its modulus is decreased by over an order of magnitude when the stresses are anisotropic