C.A. Lucas

Valence electronic excitation of the SiF4 molecule: generalized oscillator strength for the 5t2 ? 6a1 transition and ab initio calculation

The electronic excitation of the silicon tetrafluoride (SiF4) molecule has been studied using the angle-resolved electron energy-loss technique, at 1.0 keV incident electron energy, in the 0-50 eV energy range with an angular range of 1.5°-20.0°. The absolute generalized oscillator strength (GOS) for the 5t2→6a1 electronic transition, located at 13.0 eV, has been determined. A minimum has been observed in the GOS for this transition at K2 = 1.4 au. We have also determined the absolute elastic and inelastic differential cross sections at 1 keV. In order to help in the interpretation of the experimental results, ab initio calculations have been performed for the vertical valence transitions and ionization energies for the SiF4 molecule. Configuration-interaction calculations, including single and double excitations (CI-SD) and the symmetry-adapted-cluster expansion (SAC) were used. The CI-SD approach was also employed to obtain the optical oscillator strength for the 5t2→6a1 transition.

Excitation of the methyl methacrylate molecule in the VUV range by angle-resolved EELS

Abstract Angle-resolved electron energy loss spectra for the methyl methacrylate molecule have been measured at an impact energy of 1 keV. The spectra cover an excitation energy range of 0 to 100 eV and an angular range of 2° to 10°. Within our knowledge, this is the first gas-phase excitation spectrum for MMA in this energy range. At small angles the spectra are dominated by an intense peak at 6.7 eV followed by a broad band centered at about 16 eV. As the scattering angle increases, the intensity maximum in the spectra shifts towards higher energies. The generalized oscillator strength (GOS) for the 6.7 eV peak has been determined in the 0.090 to 1.093 au K 2 range. The absolute elastic differential cross section has also been obtained through normalization of the relative data to the theoretical value, determined by using the independent atom model (IAM) approximation. CI-level theoretical calculations have been also performed in order to assign the low-lying energy states. The experimentally determined value for the optical oscillator strength (0.18) for the 6.7 eV peak showed a good agreement with the theoretical result.

FRAGMENTAÇÃO DE MOLÉCULAS PELA LUZ SÍNCROTRON E POR ELÉTRONS RÁPIDOS. I. O CASO DO HALOTANO, C2F3HClBr

The photofragmentation of a core-excited halogenated compound, Halotane (C2F3HClBr), generally used as anesthetic by inhalation, has been studied using high energy photons and electrons near C 1s ionization edge (~ 300 eV), using time-of-flight mass spectrometry in multicoincidence mode. We observe strong differences between the molecular fragmentation induced by photons and electron impact.

Absolute differential cross sections for elastic and inelastic electron scattering from benzene with 1 kev impact energy

The absolute generalized oscillator strength and absolute inelastic differential cross section have been determined, as a function of the momentum transfer, for the 1E1u + 1B1u ← 1Ag transition in the benzene molecule. A previously described electron energy-loss spectrometer, featuring a Wien-filter velocity analyser, has been used. Employing 1000 eV incident electron energy and 1.0 eV energy resolution, absolute oscillator strength distribution as a function of electron energy loss was obtained in the 2–8° scattering angle range. The absolute elastic differential cross section was also determined spanning an angular range of 2°–14°. The photoabsorption spectrum converted from the electron energy-loss spectrum was compared with the interstellar extinction curve where we confirm the proportionality between the cross section and extinction.

Generalized oscillator strength for the transition of anthracene, as determined by electron energy-loss spectroscopy

As part of a systematic quantitative study of the angular dependence of valence-shell electron-impact excitation of molecules, we have determined, for the first time, the absolute inelastic differential cross section and the generalized oscillator strength (GOS) for the transition (5.2 eV) in anthracene . A minimum in the GOS has been observed at a value of around 0.2 au. The GOS values were determined from the relative inelastic differential cross section, measured with an electron-impact spectrometer, which uses a Wien-filter velocity analyser. The incident electron energy was 1 keV, the energy resolution was 1.0 eV and the energy-loss spectra were obtained in the range of - . The absolute elastic differential cross section was determined in the - angular range.

Excitação eletrônica das moléculas de metacrilato de metila e estireno na região do ultravioleta de vácuo

Angle-resolved electron energy-loss spectra have been measured for the methyl methacrylate (MMA) and styrene molecules in the 0 - 50 eV energy range. The spectra have been obtained at 1 keV incident energy, with an energy resolution of 0.8 eV and covering an angular range of 2.0 to 7.0 degrees. Within our knowledge, this is the first gas-phase excitation spectrum for MMA and styrene in this energy range. The spectra of MMA at small scattering angles are dominated by an intense peak at 6.7 eV followed by a broad band centered at about 16 eV. In the case of styrene, six bands can be observed in the spectra. Based on the angular behaviour of the excitation spectra of these molecules, the low-lying peaks observed are considered to be associated predominantly with dipole-allowed processes. In both cases, new bands can be observed for excitation energies greater than 20 eV. This could be associated with dipole-forbidden transitions to shake-up and doubly-excited states.

Valence-shell angle-resolved electron energy loss spectra of SF6

Electron-energy-loss spectra of the SF6 molecule have been measured in the valence-shell region (0 - 100 eV) covering an angular range of 1.0° - 15.0°. The spectra have been measured at an impact energy of 1.0 keV. The angular dependence of the spectra showed the presence of three forbidden transitions in the region below 40 eV. Above 40 eV interesting features appear at higher scattering angles, which can be associated to nondipole many-body processes. A new feature was observed at 76 eV. The absolute elastic differential cross section has been obtained through normalization of the relative data to the experimental values found in the literature. The absolute inelastic differential cross sections for the 11.5 eV and 13.2 eV transitions have been determined. The generalized oscillator strength (GOS) for the 11.5 eV peak has been also determined in the 0.05 to 1.3 a.u. K2 range. The experimentally determined value for the optical oscillator strength (0.312) for the 11.5 eV peak obtained through extrapolation of the GOS curve showed good agreement with the data found in the literature.