We present results on the valence level excitation, ionization and dissociation of adenine, using time-of-flight mass spectrometry and synchrotron radiation, in the vacuum ultraviolet (VUV) range of 12-21 eV. The measurements were performed using a gas-phase (Ne) harmonics filter in order to eliminate contributions from higher-order harmonics. Mass spectra were obtained using the photoelectron-photoion coincidence technique (PEPICO). The relative abundances for each ionic fragment and their mean kinetic energy release have been determined from the analysis of the corresponding peak shapes in the mass spectra. Comparison with the available photoelectron spectra and previous measurements allowed the assignment of the main features in the spectra. A discussion on the dissociative photoionization channels of this molecule has also been included. Due to our harmonics-free incident photon beam we were able to propose new appearance energy (AE) for the most important ionic channels in this energy range. The precursor ion, C(5)H(5)N(5)+, is the most abundant species (40% at 15 eV and 20% at 20 eV), which confirms the high stability of adenine upon absorption of VUV photons. We have observed other intense fragment ions such as: C(4)H(4)N(4)+, C(3)H(3)N(3) (+), C(2)H(2)N(2)+ and HCNH+. The production of the neutral HCN fragment represents up to 40% of the dissociative channels for this molecule as induced by VUV photons.

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Ionic fragmentation of the sublimated amino acid DL-proline has been studied using time-of-flight mass spectrometry and synchrotron radiation. Total ion yield and mass spectra were recorded in the 13 to 21.6 eV energy range. Partial ion yields have been calculated for the produced fragments and the results analyzed in a comparative way. Mass spectrum of proline previously obtained at 21.21 eV using photons from a discharge lamp (He I), was used as reference in the comparison to the synchrotron radiation based spectra. The loss of the COOH fragment represents the most probable dissociation pathway following the photoionization of DLproline in the valence region. These are the fi rst results of total and partial ion yields spectra for this molecule in its gas phase in the valence region using time-of-flight spectrometry.

In this work, we report a systematic search of metastable C6H n 2+ (n = 1–6) dications from electron impact time-of-flight measurements of several benzene derivatives in combination with global minimum search based on the genetic algorithm. Our theoretical calculations reveal that the C6H n 2+ (n < 6) global minimum structures are completely different from that of the benzene dication, featuring linear carbon chains and/or cyclopropenylium moieties. Experimentally, the doubly charged species were investigated for a wide range of electron impact energies, from 20 to 2000 eV, for benzene and several monosubstituted compounds containing either electron-withdrawing or -donating groups. Furthermore, the C6H n 2+ production, evaluated from the yields of the dications with respect to that of the parent ion (or parent dication), was compared to those obtained from charge exchange in the doubly charged 2E spectra and electron impact experiments available in the literature. The yields of the long-lived benzene dications were contrasted to those analogues formed in chlorobenzene. Moreover, the formation of C6H n 2+ species is strongly dependent on the nature of substituent groups, with electron-withdrawing ones favoring the dication formation.

Abstract.We present experimental studies on the photoionization and photodissociation processes (photodestruction) of gaseous amino acids and nucleobases in interstellar and interplanetary radiation analogs conditions. The measurements have been undertaken at the Brazilian Synchrotron Light Laboratory (LNLS), employing vacuum ultraviolet (VUV) and soft X-ray photons. The experimental set up basically consists of a time-of-flight mass spectrometer kept under high vacuum conditions. Mass spectra were obtained using a photoelectron photoion coincidence technique. We have shown that the amino acids are effectively more destroyed (up to 70-80%) by the stellar radiation than the nucleobases, mainly in the VUV. Since polycyclic aromatic hydrocarbons have the same survival capability and seem to be ubiquitous in the ISM, it is not unreasonable to predict that nucleobases could survive in the interstellar medium and/or in comets, even as a stable cation.

Plant volatiles are emitted by plants in response to several forms of stress, including interaction with energetic photons. In the present work, we discuss the interaction of extreme UV and soft X-ray photons with a plant volatile, vanillin. The single and double (multiple) ionization of the vanillin molecule have been studied for the first time using time-of-flight mass spectrometry and VUV and soft X-ray photons (synchrotron radiation, at 12.0 eV, 21.2 eV, 130 eV, 310 eV, 531 eV, and 550 eV). At 12.0 and 21.2 eV, only singly charged species are observed and the parent ion, C8H8O3 (+), is the dominant species. Energy differences for some selected fragments were calculated theoretically in this energy region. At 130 eV, direct double and triple ionization of the valence electrons may occur. The fragmentation increases and CHO(+) becomes one of the main cations in the mass spectrum. The molecular ion is still the dominant species, but other fragments, such as C6H5O(+), begin to present similar intensities. At 310 eV, C 1s electrons may be ionized and Auger processes give rise to dissociative doubly ionized cations. Ionization around the O 1s edge has been studied both at the 531 eV resonance and above the ionization edge. Resonant and normal Auger processes play a significant role in each case and a large fragmentation of the molecule is observed at both photon energies, with intense fragments such as CHO(+) and CH3 (+) being clearly observed. A near edge X-ray absorption fine structure spectrum of the vanillin molecule was obtained around the O 1s ionization threshold. In addition, the fragmentation of vanillin has also been studied using a fast beam of electrons (800 eV), for the sake of comparison.

We report on the single- and double-charge photofragment formation by synchrotron radiation, following C 1s core excitation and ionization and Cl 2p inner excitation and ionization of chlorobenzene, C6H5Cl. From...

The inorganic composition of the bark and leaf of a plant from the Amazon rainforest, Andira surinamensis, was determined using two non-destructive, multi-element techniques: X-ray fluorescence (XRF) and Rutherford backscattering spectrometry (RBS). XRF measurements were made using both a conventional X-ray source and synchrotron radiation. It was observed that although magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and potassium are present in higher concentrations in the leaf, calcium is about three times more concentrated in the bark. Manganese, iron, copper, zinc, strontium and barium were also detected, with barium showing a concentration above the minimum toxicity level for plants. Chemical speciation of sulfur, performed using the X-ray absorption near edge structure (XANES) technique, showed that sulfur is present in several oxidation states, with a much larger contribution from the inorganic sulfate in the leaves. The article evidences that the combined use of synchrotron radiation and non-destructive multielement techniques allows for an efficient and accurate determination of the inorganic composition and chemical speciation in plants.