RESUMO
We present an experimental and theoretical investigation of the photon interaction with formic acid in the vacuum-ultraviolet energy range. The absolute absorption cross sections and ionization efficiencies were measured in the 11.2-21.4 and 13.5-21.4 eV ranges, respectively, using a double-ion chamber technique. Photoionization and neutral-decay cross sections were derived from these results. From the present ionization cross sections and previously reported ionic dissociative branching ratios, the partial cross sections for dissociating processes were obtained. Theoretically, the photoionization cross sections and the asymmetry parameters of the photoelectron angular distributions for ionization out of the six outermost valence orbitals (10a', 2aâ³, 9a', 1aâ³, 8a', and 7a') were obtained in the energy range from near-threshold to 35 eV. For that, the Padé approximant technique along with the single-center partial-wave expansion method was applied to solve the Lippmann-Schwinger equation in the static-exchange-polarization level of approximation. This is the first theoretical investigation concerning the determination of the asymmetry parameters and photoionization cross sections of formic acid in the vacuum-ultraviolet energy range. Comparison is made between our results and the previous ones.
RESUMO
We have performed an experimental investigation into the interaction of vacuum-ultraviolet synchrotron radiation with pyridine molecules in the gas phase. Specifically, a double-ion chamber spectrometer was used to measure the absolute photoabsorption cross sections and the photoionization quantum yields from the ionization threshold to 21.5 eV. Moreover, photoionization and neutral-decay cross sections in absolute scale were derived from these data. In addition, the fragmentation pattern was investigated as a function of the photon energy by using a time-of-flight mass spectrometer and the photoelectron-photoion coincidence technique. Thus, the absolute partial ionization cross sections for each ionic fragment were obtained. Comparisons are made with experimental data available in the literature.
RESUMO
The photofragmentation dynamics of 1,1,1,2-tetrafluoroethane (R134a) with photon energies from 12 eV up to 320 eV, surrounding the C 1s edge is discussed. The ionic moieties were measured in coincidence with the ejected electrons (PEPICO mode), and detected as a function of the photon energy. Around the C K core edge, the fragmentation profiles are examined regarding the site specific excitation of the CH2FCF3 molecule. In the present case, site-selectivity is favored by the distinct chemical environments surrounding both C atoms. NEXAFS spectrum at the C 1s edge simulation has been obtained at the TDDFT level and excited state geometry optimization calculations have been performed at the inner-shell multiconfigurational self-consistent field level. Our observations indicate that the C(H2F) 1s excitation to a highly repulsive potential expels a fluorine atom leaving the heavier radical fragment C2F3H2* which relaxes to the fundamental state of the ion C2F3H2+. On the other hand, the excitation from the C(F3) 1s carbon to a repulsive state in the C-C bond, leads to a C-C bond cleavage, explaining the observed site specific fragmentation.