Low-energy electron and X-ray degradation of biomedical plasma-fluoropolymer

Plasma-deposited thin films of fluoropolymer on metallic substrates were degraded by low-energy (1-100 eV) electrons and X-ray irradiation to simulate irradiation conditions of implanted coated stents in the human body during diagnostic procedures using high energy radiation. The desorption of anions and cations from the surface of the films induced by 1-100 eV electrons was recorded by mass spectrometry. The electron energy dependence of the emission of F⁻ exhibited a resonant peak at 12.9 ± 0.4 eV, showing the formation of a transient excited anion and a monotonic rise at higher energies, associated to dipolar dissociation. In the positive ion mode, the fragments F⁺, CF⁺, CF₂⁺, CF₃⁺, C₃F₃⁺, C₂F₄⁺ and C₂F₅⁺ were observed. Emission thresholds were measured and laid above 25 eV. The shape of the cation emission curves versus electron energy showed no resonant process. X-ray degradation was studied by X-ray photoelectron spectroscopy for different exposure times. Loss of fluorine in -CF₂ groups was observed and damage mechanisms were proposed.     

Total cross sections for electron scattering by polyatomic molecules at 10–1000 eV: C2H2, C2H4, C2H6, C3H6, C3H8 and C4H8

A model complex optical potential (composed of static, exchange, polarization and absorption terms) is employed to calculate the total (elastic and inelastic) electron-atom scattering cross sections from the corresponding atomic wave function at the Hartree-Fock level. The total cross sections (TCS) for electron scattering by their corresponding molecules (C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈ and C₄H₈) are firstly obtained by the use of the additivity rule over an incident energy range of 10–1000 eV. The qualitative molecular results are compared with experimental data and other calculations wherever available, good agreement is obtained in intermediate-and high-energy region.     

Competition between reactivity and relaxation in free electron attachment to molecules

The reactivity of the C₆F₅X (X=F, Cl, Br, I) molecules following low energy (0–15 eV) electron attachment is studied in the gas phase under single collision conditions, free molecular clusters and condensed molecules by means of crossed beams and surface experiments. All four molecules exhibit a very prominent resonance for low energy electron attachment (<1 eV, attachment cross section >10⁻¹⁴ cm²). Under collision free conditions thermal electron capture generates long lived molecular parent anions C₆F₅X^−*. Along the line Cl, Br, I dissociation into X⁻+C₆F₅ and X+C₆F₅-increasingly competes until for X=1 only chemical fragmentation is observed on the mass spectrometric time scale. In free molecular clusters chemical fragmentation is quantitatively quenched at low energies in favour of associative attachment yielding undissociated, relaxed ions (C₆F₅X)⁻ _n,n≥1. A further dissociative resonance at 6.5 eV in C₆F₅Cl is considerably enhanched in clusters. If these molecules are finally condensed on a solid surface, one observes a prominent Cl⁻ desorption resonance at 6.5 eV. While the quantitative quenching of the chemical reactivity at low energies is due to the additional possibilities of energy dissipation under aggregation, the enhanched reactivity at 6.5 eV is interpreted by the conversion of a core excited open channel resonance in single molecules into a closed channel (Feshbach) resonance when it is coupled to environmental molecules.     

Measurement of electron densities by a microwave cavity method in 13.56-MHz RF plasmas of Ar,CF4, C2F6, and CHF3

Electron densit ies have been determined /or RF plasmas that were generated within a microwave resonant cavity by measuring the difference of the resonance frequencies with and without plasma. Since that method only yields a value of the electron density weighted ouer the microwave electric field distribution, to obtain real values an assumption on the spatial distribution of the electron density had to he made. Spatial profiles were taken of the emission of a 4s–5p Ar line at 419.8 not (with a small Ar admixture). The electron densities have been determined as a function of pressure and RF power in Ar, CF₄, C₂ F₆ and CHF, plasmas. The results indicate that the electron density for the last three gases decreases as a function of pressure above 50 m Torr. Typical values for the electron density for the investigated parameter range are 1–6 · 10³ cm^–3. Furthermore, the electron density is the lowest in gases with a high attachment cross .section.     

Ion pair formation in alkali-SF6 collisions: Dependence on collisional and vibrational energy

The dependence of ion pair formation in collisions of fast alkali atoms (K, Na and Li) with SF₆ on the initial relative kinetic energy and the internal energy of the target molecule has been studied by the crossed molecular beam method. Using a mass spectrometer we have measured total cross sections for negative ion formation as a function of translational and internal energy. Collision energies ranged from threshold up to 35 eV and SF₆ source temperatures were varied from 300 K to 850 K.By means of an inverse Laplace transform of the measured cross sections, we have determined total specific cross sections for each negative ion depending on the SF₆ vibrational energy and at fixed relative kinetic energy.The relative importance of both collisional and internal energy in promoting the electron transfer process is discussed for the various reaction channels in terms of a collision model. An essential feature of this model is the stretching of the S-F molecular ion bond during the collision. The product show complete relaxation in the threshold region, i.e., vibrational and collisional energy are equivalent: This holds for the SF⁻₆ formation only near threshold and for the SF⁻₅ and F⁻ formation up to about 2 eV above threshold. In the post-threshold region the effect of the internal energy on the cross section dominates over that of the translational energy.From these measurements the adiabatic electron affinity of SF₆ is inferred to be 0.32 ± 0.15 eV, T = 0 K. Some other thermodynamic data are deduced: EA(SF₅) > 2.9 ± 0.1 eV (T = 300 K) and D₀(SF⁻₅-F) = 1.0 ± 0.1 eV.     

Electron scattering and dissociative attachment by SF6 and its electrical-discharge by-products

Discrete electron-molecule processes relevant to SF₆ etching plasmas are examined. Absolute, total scattering cross sections for 0.2–12-eV electrons on SF₆, SO₂, SOF₂, SO₂F₂, SOF₄, and SF₄, as well as cross sections for negative-ion formation by attachment of electrons, have been measured. These are used to calculate dissociative-attachment rate coefficients as a function ofE/N for SF₆ by-products in SF₆.     

Partial cross sections and autoionization resonances in the valence-shell photoemission from solid acetylene

Photoelectron energy distribution curves from solid acetylene were measured for excitation energies up to 30 eV using synchrotron radiation. The partial cross sections are discussed in relation to theory and other experiments. The 1π_u valence-band cross section shows a pronounced autoionization resonance with a shoulder and a peak 2.5 and 5.0 eV above the vacuum level. This originates from the resonant decay of the 2σ _u → 1π_g valence excitation (discrete shape resonance).     

Excitation of alkali atoms in collisions with molecules. Population of the states K(62S) and K(52-P)

Radiation at 4050 and 6930 A has been observed in collisions of fast potassium atoms with N₂, O₂, CO, NO, CO₂, C₂H₄ and C₆H₆. Emission at both wavelengths is weak compared to that at 7680 Å. The threshold energy for emission at 6930 Å has been found to be equal to the excitation energy (3.4 eV) of the state K(6²S) for N₂ and NO, but higher (5.5 eV) for CO.     

Shape resonances and partial photoemission cross sections of solid SF6 and CCl4

Photoelectron energy distribution curves from solid films of SF₆ and CCl₄ have been measured in the photon energy range 10 ? hr ? 40 eV using synchrotron radiation. The binding energies, peak widths and relative partial cross sections have been determined. In the photoelectron spectra a 1:1 correspondence to the gas phase is observed for the occupied molecular orbitals, and a straightforward assignment of the occupied valence bands emerges. Furthermore, the cross sections of the individual orbitals show for both samples great similarities to the gas phase. For SF₆ detailed structures are visible in the cross sections which are only partly interpreted as shape resonances. A new assignment for the 6t_1u shape resonance is proposed and the resonance energies are related to X-ray absorption and electron scattering data. Furthermore a comparison of the total photoemission cross section to the optical reflection spectrum of solid SF₆ is presented. For CCl₄ less structures are ob served in the partial cross sections. They are all interpreted as shape resonances. An energetic scheme of the virtual orbitals is proposed for CCl₄.     

Total ionization and electron attachment cross sections of CCl2F2 by electron impact

The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl₂F₂ (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10^?19 m². The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10^?20 m², and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV.     

Electron impact dissociation of cyanobenzene radical cations by ion cyclotron resonance spectroscopy

Trapped ion cyclotron resonance spectroscopy has been used for the first time to study the electron impact dissociation of ions. Fragmentation of C₆H₅CH⁺ to produce C₆H⁺₄ and HCN is observed to occur at low electron energies (3–9 eV). The extent of dissociation is observed to be linear in emission current, rising from a threshold at 3.0 ± 0.5 eV to a maximum cross section estimated to be 6A² at 7.5 ± 0.5 eV. The implications of these results are discussed.     

Mass spectra of the thioethers C6F5SC6H4X

The mass spectra of a number of thioethers of the type p-XC₆H₄SC₆F₅ (X = H, Cl, NO₂, SC₆F₅, CO₂C₂H₅, C₆H₅) have been examined. These spectra have been compared with those of (C₆H₅)₂S and (C₆F₅)₂S. Fragmentation patterns have been deduced from metastable peaks. The spectrum of p-ClC₆H₄SC₆F₅ has been studied in more detail.     

Dissociative charge exchange of C3F6

Charge exchange of neutral C₃F₆ by a variety of atomic and molecular ions in the 1 to 25 eV range of collision energies is used to characterize the energies associated with formation of [C₃F₆]⁺˙. The internal energy of the nascent [C₃F₆]⁺˙ ion, assessed by observing the degree to which it fragments, increases with the recombination energy of the charge-exchange reagent. The existence of excited states of the reagent ions is identified from the fragmentation behaviour of [C₃F₆]⁺˙ in the cases of [CS₂]⁺˙, NO⁺, O₂⁺˙, [NH₃]⁺˙ and possibly [CH₄]⁺˙. In addition, the data confirm that the [C₃F₆]⁺˙ parent ion fragments from both the ground state and a long-lived isolated electronic state. The latter is populated by near-resonant charge transfer. Translational excitation contributes relatively little to internal excitation of the charge-exchanged product ion and even less in the case of the isolated state.     

Neue Synthesen und Kristallstrukturen von Bis(fluorphenyl)quecksilber,Hg(Rf)2 (Rf = C6F5, 2, 3, 4, 6‐F4C6H, 2, 3, 5, 6‐F4C6H, 2, 4, 6‐F3C6H2, 2, 6‐F2C6H3)

New Syntheses and Crystal Structures of Bis(fluorophenyl) Mercury, Hg(R_f)₂ (R_f = C₆F₅, 2, 3, 4, 6‐F₄C₆H, 2, 3, 5, 6‐F₄C₆H, 2, 4, 6‐F₃C₆H₂, 2, 6‐F₂C₆H₃) Bis(fluorophenyl) mercury compounds, Hg(R_f)₂ (R_f = C₆F₅, C₆HF₄, C₆H₂F₃, C₆H₃F₂), are prepared in good yields by the reactions of HgF₂ with Me₃SiR_f. The crystal structures of Hg(2, 3, 4, 6‐F₄C₆H)₂ (monoclinic, P2₁/n), Hg(2, 3, 5, 6‐F₄C₆H)₂ (monoclinic, C2/m), Hg(2, 4, 6‐F₃C₆H₂)₂ (monoclinic, P2₁/c) and Hg(2, 6‐F₂C₆H₃)₂ (triclinic, P1) are described.     

A Theoretical and Experimental Study of the Interaction of C6F6 with Electron Donors

The NMR effects produced on the nitrogen absolute shieldings in a series of electron donors when they interact with hexafluorobenzene, C₆F₆, have been theoretically studied. The complexes have been optimized at the B3LYP/6-311++G** level and the NMR shieldings have been calculated using the GIAO method. The results obtained have allowed devising an experiment (C₆F₆···NCCH₃ complex) that is compatible with the theoretical calculations.     

(p, ϱ, T) for {(1−x)C6H6 + xC6D6} and {(1−x)C6H6 + xC6F6} in the range 298 to 373 K and 0.1 to 400 MPa

The behaviour of (p, ?, T) for C₆H₆, C₆F₆, and five mixtures, and of C₆D₆ and (0.5C₆H₆ + 0.5C₆D₆) has been determined at 298.2, 323.2, 348.2, and 373.2 K, and from 0.1 MPa, or saturation pressure, to the point of onset of solidification or to 400 MPa. The experimental results are tabulated and the isothermal densities are represented by a polynomial equation for the secant bulk modulus in terms of the pressure. The temperature and pressure dependence of the molar excess volume is described.     

Geometric shielding corrections for calculation of electron scattering by CO2, C2H2, CHCl3, CH2Cl2, CH3Cl,CHF3, CH2F2 and CH3F at 30–5000 eV

Taking into account the changes of the geometric shielding effect in a molecule as the incident electron energy varies, an empirical fraction, which depends on the energy of the incident electrons, the target's molecular dimension and the atomic and electronic numbers in the molecule, is presented. Using this empirical fraction, a new formulation of the additivity rule is proposed. Using the new additivity rule, the total cross sections of electron scattering by CO₂, C₂H₂, CHCl₃, CH₂Cl₂, CH₃Cl, CHF₃, CH₂F₂ and CH₃F are calculated at the Hartree–Fork level at 30–5000 eV. The quantitative total cross sections are compared with those obtained by experiments and other theories, and good agreement is obtained over a wide energy range, especially above 100 eV.     

Collisional ionization between fast alkali atoms and hexaflourobenzene

The formation of negative ions through collisions between alkali atoms and C₆F₆ is studied. Unlike all other systems studied to data the charge-transfer cross section producing C₆F^?(₆ is very small. A possible explanation for this anomaly is presented based upon competition with the flourine abstraction reaction.     

Total cross section of the SF6 molecule for elastic electron scattering

A multiple scattering method is used for determining the total cross section of elastic electron scattering for the SF₆ molecule in the energy range 10–60 eV.     

Differential cross section for CN− formation from dissociative electron attachment to the cyanogen molecule C2N2

The differential cross section for CN^? formaition from dissoiiativc attachment on C₂N₂ has been obtained in a crossed-beam experiment. Below 10 eV incident electron energy thc CN^? cross section shows two broad overlapping peaks with maxima at 5.4 and 7.3 eV corresponding to the formation of CN^? in its ground electronic state ¹σ⁺ plus the CN radical in the first excited state²π and the ground stale ²Σ+ respcctively