We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar CH 3 -X type organic molecules like methanol (CH 3 OH), methylamine (CH 3 NH 2 ) and acetonitrile (CH 3 CN) as alternative route for the production of H + 3 in interstellar and star-forming environments. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons with energies between 200 and 310 eV and time-of-flight (TOF) mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross-sections for H + 3 production by soft X-rays were determined. We have found that, among the channels leading to molecular dissociation, the H + 3 yield could reach values up to 0.7 per cent for single photoionization process and up to 4 per cent for process involving double photoionization. The H + 3 photoproduction cross-section due to the dissociation of the studied organic molecules by photons over the C1s edge (200-310 eV) were about 0.2-1.4 × 10 −18 cm 2 . Adopting the typical X-ray luminosity L X 10 31 erg s −1 which best fit the observational data for AFGL 2591 we derive an estimative for the H + 3 photoproduction rate due to methyl compound dissociation process. The highest value for the H + 3 column density from methanol dissociation by soft X-rays, assuming a steady state scenario, was about 10 11 cm 2 , which gives the fraction of the photoproduced H + 3 of about 0.05 per cent, as in the case of dense molecular cloud AFGL 2591. Despite the extreme small value, this represent a new and alternative source of H + 3 into dense molecular clouds and it is not been considered as yet in interstellar chemistry models.

The first ab initio calculations (vertical energies and oscillator strengths) are reported for the neutral electronic transitions of isoprene (2-methyl-1,3-butadiene), CH(2)CHC(CH(3))CH(2). The VUV photoabsorption spectroscopy of the molecule is presented in the energy range 4.6 to 10.8 eV (270-125 nm) with the highest resolution yet reported above 6.05 eV, revealing new spectral features. Valence and Rydberg transitions have been assigned in accordance with the theoretical results and the associated vibronic series have been analysed. The absolute photoabsorption cross sections at energies below 6.89 eV have been used to calculate the photolysis lifetime of isoprene in the upper stratosphere (20-50 km). Electron energy loss spectroscopy (EELS) measurements have enabled further photoabsorption cross sections to be derived in the range 9-28 eV. The first ab initio calculations have been carried out to determine excitation energies to the lowest energy ionic states of isoprene. The calculations are compared with the He(i) photoelectron spectrum (8 to 17 eV) and new vibrational structure is observed in the first photoelectron band.

We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115-310 nm (10.8-4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin-orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C-Br) ← nBr and σ*(C-Cl) ← n(Cl) transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0-40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20-50 km).

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