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The electronic state spectroscopy of carbonyl sulphide, COS, has been investigated using high resolution vacuum ultraviolet photoabsorption spectroscopy and electron energy loss spectroscopy in the energy range of 4.0-10.8 eV. The... more
The electronic state spectroscopy of carbonyl sulphide, COS, has been investigated using high resolution vacuum ultraviolet photoabsorption spectroscopy and electron energy loss spectroscopy in the energy range of 4.0-10.8 eV. The spectrum reveals several new features not previously reported in the literature. Vibronic structure has been observed, notably in the low energy absorption dipole forbidden band assigned to the (4π←3π) ((1)Δ←(1)Σ(+)) transition, with a new weak transition assigned to ((1)Σ(-)←(1)Σ(+)) reported here for the first time. The absolute optical oscillator strengths are determined for ground state to (1)Σ(+) and (1)Π transitions. Based on our recent measurements of differential cross sections for the optically allowed ((1)Σ(+) and (1)Π) transitions of COS by electron impact, the optical oscillator strength f0 value and integral cross sections (ICSs) are derived by applying a generalized oscillator strength analysis. Subsequently, ICSs predicted by the scaling are...
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Time-of-flight (TOF) negative ion mass spectra have been obtained in collisions of 20-100 eV neutral potassium atoms with tetrahydrofuran (C4H8O), an analogue for the sugar unit in DNA/RNA. Major enhancements in O(-) and C2H3O(-)... more
Time-of-flight (TOF) negative ion mass spectra have been obtained in collisions of 20-100 eV neutral potassium atoms with tetrahydrofuran (C4H8O), an analogue for the sugar unit in DNA/RNA. Major enhancements in O(-) and C2H3O(-) production were observed compared with earlier dissociative electron attachment (DEA) experiments. In further contrast with DEA, no evidence was observed for dehydrogenated parent anions, and three new fragment anions were detected: CH(-), C2(-), and C2H(-). These contrasting results for potassium impact and DEA highlight significant differences in the reaction pathways initiated by the two electron delivery processes.
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The electronic state spectroscopy of pyrimidine C(4)H(4)N(2) has been investigated using both high resolution VUV photoabsorption in the energy range 3.7 to 10.8 eV (335 to 115 nm) and lower resolution electron energy loss in the range 2... more
The electronic state spectroscopy of pyrimidine C(4)H(4)N(2) has been investigated using both high resolution VUV photoabsorption in the energy range 3.7 to 10.8 eV (335 to 115 nm) and lower resolution electron energy loss in the range 2 to 15 eV. The low energy absorption band, assigned to the (pi*) <-- 7b(2)(n(N)) (1(1)B(1)<-- 1(1)A(1)) transition, at 3.85(4) eV and the vibrational progressions superimposed upon it have been observed for the first time, due to the availability of a high-resolution photon beam (0.075 nm), corresponding to 3 meV at the midpoint of the energy range studied. Vibronic coupling has been shown to play an important role dictating the nature of the observed excited states, especially for the lowest (1)B(1) state. The 2(1)B(1) state is proposed to have its origin at 7.026 eV according to the vibrational excitation reported in this energy region (7.8-8.4 eV). New experimental evidence of 4(1)A(1) state with a maximum cross section at 8.800 eV is supported by previous ab initio quantum chemical calculations. Rydberg series have been assigned converging to the three lowest ionisation energy limits, 9.32 eV ((2)B(2)), 10.41 eV ((2)B(1)) and 11.1 eV ((2)A(1) + (2)A(2)) with new members reported for the first time and classified according to the magnitude of the quantum defects (delta). Additionally, the absolute differential cross section for inelastic electron scattering has been measured for the most intense band from 6.9 to 7.8 eV assigned to (1)pipi* (3(1)A(1) + 2(1)B(2)).
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ABSTRACT Electron scattering from Tetrahydrofuran (C4H8O) was investigated over a wide range of energies. Following a mixed experimental and theoretical approach, total scattering, elastic scattering and ionization cross sections as well... more
ABSTRACT Electron scattering from Tetrahydrofuran (C4H8O) was investigated over a wide range of energies. Following a mixed experimental and theoretical approach, total scattering, elastic scattering and ionization cross sections as well as electron energy loss distributions were obtained.
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ABSTRACT We report the formation of demethylation in 3-methyl-uracil (3meU) and 1-methyl-thymine (1meT), i.e. (3meU-CH3)- and (1meT-CH3)-, through potassium-molecule collisions at different potassium kinetic energies. Study of the... more
ABSTRACT We report the formation of demethylation in 3-methyl-uracil (3meU) and 1-methyl-thymine (1meT), i.e. (3meU-CH3)- and (1meT-CH3)-, through potassium-molecule collisions at different potassium kinetic energies. Study of the threshold of formation of this fragment can provide a value of threshold energy that can be compared with DEA studies.
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ABSTRACT Electron scattering from pyrimidine (C4H4N2) was investigated over a wide range of energies. Following different experimental and theoretical approaches, total, elastic and ionization cross sections as well as electron energy... more
ABSTRACT Electron scattering from pyrimidine (C4H4N2) was investigated over a wide range of energies. Following different experimental and theoretical approaches, total, elastic and ionization cross sections as well as electron energy loss distributions were obtained.
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ABSTRACT Clustering effects in the differential and integral electron scattering cross sections have been investigated by calculating their magnitudes for different water cluster configurations: specifically for the dimer, trimer and... more
ABSTRACT Clustering effects in the differential and integral electron scattering cross sections have been investigated by calculating their magnitudes for different water cluster configurations: specifically for the dimer, trimer and tetramer. By assuming that condensation effects can be described in terms of modifications to the single molecule electron scattering cross sections, induced by the surrounding molecules, effective data for liquid water is also provided. Comparison with previous calculations in liquid water has been carried out through the inelastic mean free path, i.e. the mean length between successive inelastic collisions. Although a satisfactory agreement for high energies, above 100 eV, has been found, for lower energies some discrepancies remain and so will be the subject of further investigation. As an example of modelling electron interactions in soft matter, the present set of data has been employed to simulate single electron tracks in liquid water. This was achieved using our Low Energy Particle Track Simulation (LEPTS) code.
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ABSTRACT In this study we report negative ion fragmentation patterns obtained from time-of-flight (TOF) mass spectrometry in potassium-uridine collisions at 30, 70 and 100 eV. The dominant fragments from uridine are also the most abundant... more
ABSTRACT In this study we report negative ion fragmentation patterns obtained from time-of-flight (TOF) mass spectrometry in potassium-uridine collisions at 30, 70 and 100 eV. The dominant fragments from uridine are also the most abundant from the individual units in potassium collisions with uracil, tetrahydrofuran (THF) and D-ribose. We observe no dehydrogenated parent anion formation. Special emphasis is given to the dissociation mechanisms lending support to the breaking of the N-glycosidic bond as an initial step in the fragmentation of the transient negative ion (TNI). Within their role as sugar unit surrogates, THF and D-ribose (DR) provide useful information for the study of uridine, and presumably other nucleosides from the point of view of DNA/RNA subunits integrity in the biological environment.
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ABSTRACT Interactions between 1 and 4 keV anions (H-, O-, and OH-) and gas-phase molecules (nitromethane, water, ethanol, and methanol) have been studied using quadrupole mass spectrometry of the product anions and cations. The low... more
ABSTRACT Interactions between 1 and 4 keV anions (H-, O-, and OH-) and gas-phase molecules (nitromethane, water, ethanol, and methanol) have been studied using quadrupole mass spectrometry of the product anions and cations. The low collision velocities (0.07-0.40 v(Bohr)) provide favourable conditions for electron transfer from the anion projectile to the neutral target molecule yielding negative ion formation, while strong competition with cation formation is also observed. Relative production of fragment cations increases with H- impact energy and with projectile mass when energy is constant. Considered together, these results suggest a momentum dependence on collisional energy deposition. As far as negative ion production is concerned, comparisons with previous free electron attachment studies are drawn as a starting point for the interpretation of the anion fragmentation channels. For nitromethane and water, the present anion fragmentation patterns are substantially different to the free electron attachment data. Conversely the fragmentation channels of ethanol and methanol anions only show clear dependence on the electron attachment/transfer process in terms of the relative anion yields. (C) 2011 Elsevier B.V. All rights reserved.
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ABSTRACT Electron transfer in potassium collisions with alanine (C3H7NO2) and valine (C5H11NO2) is investigated at 15 and 100 eV. The fragmentation patterns obtained in the unimolecular decomposition through time-of-flight (TOF) mass... more
ABSTRACT Electron transfer in potassium collisions with alanine (C3H7NO2) and valine (C5H11NO2) is investigated at 15 and 100 eV. The fragmentation patterns obtained in the unimolecular decomposition through time-of-flight (TOF) mass spectrometry are compared for both amino acids as function of the collision energy. In the case of alanine, the most prominent feature in the collision regime is the relative decrease of the dehydrogenated parent anion signal with respect to the hydrogen anion as the collision energy increases. For low collision energies this can be rationalized in terms of autodetachment inhibition, whereas at higher collisions energies the negative molecular ion can be formed with an excess of internal energy which might even result in fragmentation. Regarding valine, such behaviour was not observed which has been interpreted as a result of side chain effect contributing to an increase of the internal degrees of freedom in comparison to alanine.
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Using the internationally recognized ultraviolet beam line (UV1) on the ASTRID synchrotron in Aarhus, Denmark since 2001 we have investigated the electronic state spectroscopy of a series of molecular species spanning over aeronomic and... more
Using the internationally recognized ultraviolet beam line (UV1) on the ASTRID synchrotron in Aarhus, Denmark since 2001 we have investigated the electronic state spectroscopy of a series of molecular species spanning over aeronomic and biological ...
ABSTRACT Electron transfer in potassium collisions with the simplest amino acid glycine has been investigated in the energy range from 20 to 100 eV. In the unimolecular decomposition, fragmentation patterns obtained through TOF mass... more
ABSTRACT Electron transfer in potassium collisions with the simplest amino acid glycine has been investigated in the energy range from 20 to 100 eV. In the unimolecular decomposition, fragmentation patterns obtained through TOF mass spectrometry show peculiar differences in comparison with dissociative electron attachment measurements. The most prominent feature in the collision regime is the relative decrease of the dehydrogenated parent anion signal with respect to the hydrogen anion as the collision energy increases. For low collision energies this can be rationalized in terms of autodetachment inhibition, whereas at higher collisions energies the negative molecular ion can be formed with an excess of internal energy which might even result in fragmentation. Metastable decay is here suggested to play a particular role leading to the formation of three anions assigned in the light of recent dissociative electron attachment studies. The formation of isobaric fragments with masses 15, 16 and 26 a. m. u. is also discussed on the basis of the energy available in the electron transfer process.
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ABSTRACT This colloquium describes an approach to incorporate into radiation damage models the effect of low and intermediate energy (0-100 eV) electrons and positrons, slowing down in biologically relevant materials (water and... more
ABSTRACT This colloquium describes an approach to incorporate into radiation damage models the effect of low and intermediate energy (0-100 eV) electrons and positrons, slowing down in biologically relevant materials (water and representative biomolecules). The core of the modelling procedure is a C++ computing programme named “Low Energy Particle Track Simulation (LEPTS)”, which is compatible with available general purpose Monte Carlo packages. Input parameters are carefully selected from theoretical and experimental cross section data and energy loss distribution functions. Data sources used for this purpose are reviewed showing examples of electron and positron cross section and energy loss data for interactions with different media of increasing complexity: atoms, molecules, clusters and condense matter. Finally, we show how such a model can be used to develop an effective dosimetric tool at the molecular level (i.e. nanodosimetry). Recent experimental developments to study the fragmentation induced in biologically material by charge transfer from neutrals and negative ions are also included.