Filipe Ferreira da Silva

{We report on an initial investigation into the transport of electrons through a gas cell containing 1 mTorr of gaseous furfural. Results from our Monte Carlo simulation are implicitly checked against those from a corresponding electron transmission measurement. To enable this simulation a self-consistent cross section data base was constructed. This data base is benchmarked through new total cross section measurements which are also described here. In addition, again to facilitate the simulation, our preferred energy loss distribution function is presented and discussed.}

{We report absolute elastic differential cross sections for electron interactions with the C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluoro-2-butyne (2-C4F6), and hexafluorocyclobutene (c-C4F6). The incident electron energy range is 1.5-200 eV, and the scattered electron angular range for the differential measurements varies from 15 degrees to 150 degrees. In all cases the absolute scale of the differential cross section was set using the relative flow technique, with helium as the reference species. Atomic-like behaviour in these scattering systems is shown here for the first time, and is further investigated by comparing the elastic cross sections for the C4F6 isomers with other fluorinated molecules, such as SF6 and CnF6 (n = 2, 3, and 6). We note that for all the six-F containing molecules, the scattering process for electron energies above 30 eV is indistinguishable. Finally, we report results for calculations of elastic differential cross sections for electron scattering from each of these isomers, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements is found to be quite remarkable in all cases. (C) 2014 AIP Publishing LLC.}

{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.}

{We present negative ion formation from collisions of neutral potassium atoms with D-ribose (C5H10O5), the sugar unit in the DNA/RNA molecule. From the negative ion time-of-flight (TOF) mass spectra, OH- is the main fragment detected in the collision range 50-100 eV accounting on average for 50% of the total anion yield. Prominence is also given to the rich fragmentation pattern observed with special attention to O- (16m/z) formation. These results are in sharp contrast to dissociative electron attachment experiments. The TOF mass spectra assignments show that these channels are also observed, albeit with a much lower relative intensity. Branching ratios of the most abundant fragment anions as a function of the collision energy are obtained, allowing to establish a rationale on the collision dynamics. (C) 2013 AIP Publishing LLC.}

{Negative ion formation by low-energy electron impact to germane (GeH4) has been performed in an electron energy region from 6 to 11 eV with an energy resolution of similar to 500 meV. Anion efficiency curves of four anions have been measured. Product anions are observed mainly in the 6-11 eV energy region, yielding GeHx- (x= 0-3). Comparative studies with methane (CH4) and silane (SiH4) are also presented with the most intense signals observed at 14 amu (CH2-) 31 amu (SiH3-) and 75 amu (GeH3-) from CH4, SiH4 and GeH4, respectively. Fragmentation into these negative ions is attributed to resonant dissociative electron attachment processes. 2011 Elsevier B.V. All rights reserved.}

{High-resolution vacuum ultraviolet photoabsorption measurements in the wavelength range of 115-320 nm (10.8-3.9 eV) have been performed together with comprehensive relativistic time-dependent density functional calculations (TDDFT) on the low-lying excited sates of tungsten hexacarbonyl, W(CO)(6). The higher resolution obtained reveals previously unresolved spectral features of W(CO)(6). The spectrum shows two higher-energy bands (in the energy ranges of 7.22-8.12 eV and 8.15-9.05 eV), one of them with clear vibrational structure, and a few lower-energy shoulders in addition to a couple of lower-energy metal-to-ligand charge-transfer (MLCT) bands reported in the literature before. Absolute photoabsorption cross sections are reported and, where possible, compared to previously published results. On the basis of this combined experimental/theoretical study the absorption spectrum of the complex has been totally re-assigned between 3.9 and 10.8 eV under the light of spin-orbit coupling (SOC) effects. The present comprehensive knowledge of the nature of the electronically excited states may be of relevance to estimate neutral dissociation cross sections of W(CO)(6), a precursor molecule in focused electron beam induced deposition (FEBID) processes, from electron scattering measurements.}

{In this Letter we present a high resolution VUV photoabsorption spectrum of hexafluoro-1,3-butadiene (1,3-C4F6), over the wavelength range 113-247 nm (11.0-5.0 eV). The spectrum reveals several new features not previously reported in the literature. The assignment of the observed valence and Rydberg transitions and the associated vibronic series is presented based on our recent ab initio calculations on the vertical excitation energies of C4F6 isomers. The dominant excitation has been assigned to the upsilon(1)' (a) C=C stretching mode in the (5(1)A <- 1(1)A, 3p(a) <- pi(a)(20a)) and (7(1)A <- 1(1)A, 3p(b) <- pi(b)(19b)) transitions, with mean energies of 0.201 and 0.188 eV, respectively. The measured absolute photoabsorption cross section has been used to calculate the photolysis lifetime of 1,3-C4F6 in the upper stratosphere (20-50 km). (c) 2012 Elsevier B.V. All rights reserved.}

{Fragmentation of transient negative ions of tyrosine formed, through electron transfer in collisions with neutral potassium atoms is presented in the collision energy range from 30 to 75 eV. At low collision energies the dominating side chain channel observed corresponds to the cleavage of the bond from the para-position of the phenyl ring to the beta-C of the remaining moiety, but cleavage of the C-alpha-C-beta, bond is also observed. Further fragments are formed through cleavage of the C-alpha bond to the carbonyl group, through decomposition of the carboxyl group or through significant decomposition of the backbone. The dehydrogenated molecular anion is also observed with appreciable intensity. These results are discussed in the context of earlier studies on dissociative electron attachment to tyrosine and other amino acids, as well as within the role of the side chain in electron induced decomposition of this aromatic amino acid. Stabilization of the temporary molecular anion in the transient collision complex is discussed and we argue that this may have significant influence on the branching ratios observed. (C) 2016 Published by Elsevier B.V.}

{The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range. (C) 2015 AIP Publishing LLC.}

{Low energy electron attachment to gas phase royal demolition explosive (RDX) (and RDX-A3) has been performed by means of a crossed electron-molecular beam experiment in an electron energy range from 0 to 14 eV with an energy resolution of similar to 70 meV. The most intense signals are observed at 102 and 46 amu and assigned to C(2)H(4)N(3)O(2)- and NO(2)(-), respectively. Anion efficiency curves of 16 anions have been measured. Product ions are observed mainly in the low energy region, near 0 eV arising from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The remarkable instability of RDX to electron attachment with virtually thermal electrons reflects the highly explosive nature of this compound. The present results are compared to other explosive aromatic nitrocompounds studied in our laboratory recently. (C) 2009 American Institute of Physics. {[}doi:10.1063/1.3230116]}

{We report gas phase studies on NCO- fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO- production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO- upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO- formation in DEA. Site selectivity in the NCO- yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO- ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation.}

{Results of a detailed study on electron interactions with nitromethane (CH(3)NO(2)) embedded in helium nanodroplets are reported. Anionic and cationic products formed are analysed by mass spectrometry. When the doped helium droplets are irradiated with low-energy electrons of about 2 eV kinetic energy, exclusively parent cluster anions (CH(3)NO(2))(n) are formed. At 8.5 eV, three anion cluster series are observed, i.e., (CH(3)NO(2))(n)(-), {[}(CH(3)NO(2))(n)-H](-), and (CH(3)NO(2))(n)NO(2)(-), the latter being the most abundant. The results obtained for anions are compared with previous electron attachment studies with bare nitromethane and nitromethane condensed on a surface. The cation chemistry (induced by electron ionization of the helium matrix at 70 eV and subsequent charge transfer from He(+) to the dopant cluster) is dominated by production of methylated and protonated nitromethane clusters, (CH(3)NO(2))nCH(3)(+) and (CH(3)NO(2))nH(+). (C) 2011 American Institute of Physics. {[}doi:10.1063/1.3656680]}

{Gas phase electron attachment studies have been performed for musk ketone by means of a crossed electron-molecular beams experiment in an energy range from 0 to 15 eV with a resolution of similar to 70 meV. Additional measurements, utilizing a two-sector-field instrument, have been used to distinguish between possible isobaric products. Anion efficiency curves for 19 anions have been measured including a long-lived (metastable) non-dissociated parent anion which is formed at energies near 0eV. Many of the dissociative electron attachment products observed at low energy arise from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The present results are compared with previous aromatic nitrocompounds studied in our laboratory recently. Particularly the close similarity of musk ketone and the explosive trinitrotoluene is of special interest. (C) 2008 Elsevier B.V. All rights reserved.}

{We report on the first measurements of the electron impact electronic excitation cross sections for C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6), and hexafluoro-2-butyne (2-C4F6), measured at 100 eV, 3 degrees scattering angle, while sweeping the energy loss over the range 2.0-15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C4F6 isomers and classified according to the magnitude of the quantum defects (delta).}

{Here we report novel comprehensive investigations on the electronic state spectroscopies of isolated 2,4- and 2,6-difluorotoluene in the gas phase by high-resolution vacuum ultraviolet (VUV) photoabsorption measurements in the 4.4-10.8 eV energy range, with absolute cross-section values derived. We also present the first set of ab initio calculations (vertical energies and oscillator strengths), which we have used in the assignment of valence transitions of the difluorotoluene molecules, together with calculated ionization energies to obtain the Rydberg transitions for both molecules. The measured absolute photoabsorption cross sections have been used to estimate the photolysis lifetimes of 2,4- and 2,6-difluorotoluene in the Earth's atmosphere.}

{Total electron scattering cross sections for pyrazine in the energy range 10-500 eV have been measured with a new magnetically confined electron transmission-beam apparatus. Theoretical differential and integral elastic, as well as integral inelastic, cross sections have been calculated by means of a screening-corrected form of the independent-atom representation (IAM-SCAR) from 10 to 1000 eV incident electron energies. The present experimental and theoretical total cross sections show a good level of agreement, to within 10%, in the overlapping energy range. Consistency of these results with previous calculations (i.e., the R-matrix and Schwinger Multichannel methods) and elastic scattering measurements at lower energies, below 10 eV, is also discussed. (C) 2013 AIP Publishing LLC.}

{In the present study, dissociative electron attachment (DEA) measurements with gas phase HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, C4H8N8O8, have been performed by means of a crossed electron-molecular beam experiment. The most intense signals are observed at 46 and 176 u and assigned to NO2 (-) and C3H6N5O4 (-), respectively. Anion efficiency curves for 15 negatively charged fragments have been measured in the electron energy region from about 0-20 eV with an energy resolution of 0.7 eV. Product anions are observed mainly in the low energy region, near 0 eV, arising from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The remarkable instability of HMX towards electron attachment with virtually zero kinetic energy reflects the highly explosive nature of this compound. Substantially different intensity ratios of resonances for common fragment anions allow distinguishing the nitroamines HMX and royal demolition explosive molecule (RDX) in negative ion mass spectrometry based on free electron capture.}

{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. (C) 2014 Elsevier B.V. All rights reserved.}

{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. (C) 2014 Elsevier B.V. All rights reserved.}

{RationaleSecondary electrons with an energy distribution below 100 eV are formed when high-energy particles interact with matter. In the focused electron beam induced deposition, high-energy beams are used to decompose organometallic compounds on surfaces. We investigated the electron ionisation of WCl6 and dissociative electron attachment to WCl6 in the gas phase in order to better understand the decomposition mechanism driven by secondary electrons. MethodsA double-focusing mass spectrometer coupled with a Nier-type ion source was used to perform the present studies. The electron ionisation studies were performed with an electron energy of 70 eV and dissociative electron attachment studies in the energy range of similar to 0-14 eV. ResultsTungsten hexachloride rapidly oxidises, leading to the formation of a mixture of pure WCl6 and WCl4O together with WCl2O2 species. The fragmentation of the three chlorinated compounds is effective, although electron ionisation to WCl6 leads to W+ in contrast with WCl2O2 and WCl4O leading to WO2+ and WO+, respectively, as lighter fragments. With regard to electron attachment, decomposition of the precursor molecules is observed; however, W- was not detected within the detection limit of the instrument. ConclusionsElectron ionisation and dissociative electron attachment (DEA) to WCl6, WCl4O and WCl2O2 lead to strong fragmentation. In electron ionisation, the fragmentation by loss of chlorine atoms was observed for both WCl6 and the oxidised species. Additionally, the loss of all chlorine ligands is observable for WCl6 as well as the oxidised species. The DEA results have shown dissociation by the scission of chlorine atoms as well as by the scission of an oxygen atom. The formation of chlorine and oxygen anions was observed, indicating the formation of a neutral counterpart containing the metal atom, free to be attacked by the next electron. Copyright (c) 2016 John Wiley & Sons, Ltd.}

{Electron transfer in potassium collisions with alanine (C3H7NO2) and valine (C3H11NO2) 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 a 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 rationalised in terms of autodetachment inhibition, whereas at higher collision 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. (C) 2014 Elsevier B.V. All rights reserved.}

{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.}

{We report elastic differential cross sections (DCSs) for electron interactions with acetone, C3H6O. The incident electron energy range was 7.0-50 eV, and the scattered electron angular range for the differential measurements varied from 10 degrees to 120 degrees. The calculated cross sections were obtained with two different methodologies, the Schwinger multichannel method with pseudopotentials (SMCPP), and the independent-atom method with screening-corrected additivity rule (IAM-SCAR). The present elastic DCSs have been found to agree well with the results of IAM-SCAR calculations above 20 eV, and also with the SMC calculations below 30 eV. Although some discrepancies were found for lower energies, the agreement between the SMCPP data and the DCSs obtained with the IAM-SCAR method improves, as expected, as the impact energy increases. Comparison with previous DCSs shows good agreement albeit the present data is extended down to lower electron impact energies. We find a low-lying pi{*} shape resonance located at 2.6 eV, in agreement with recent results on electron collisions with acetone {[}M. G. P. Homem et al., Phys. Rev. A 92, 032711 (2015)]. The presence of a sigma{*} resonance is also discussed.}

{The first detailed study of electron attachment to amino acid clusters is reported. The amino acids chosen for investigation were glycine, alanine, and serine. Clusters of these amino acids were formed inside helium nanodroplets, which provide a convenient low temperature (0.37 K) environment for growing noncovalent clusters. When subjected to low energy (2 eV) electron impact the chemistry for glycine and alanine clusters was found to be similar. In both cases, parent cluster anions were the major products, which contrasts with the corresponding monomers in the gas phase, where the dehydrogenated products ({[}AA(n)-H](-), where AA=amino acid monomer) dominate. Serine clusters are different, with the major product being the parent anion minus an OH group, an outcome presumably conferred by the facile loss of an OH group from the beta carbon of serine. In addition to the bare parent anions and various fragment anions, helium atoms are also observed attached to both the parent anion clusters and the dehydrogenated parent anion clusters. Finally, we present the first anion yield spectra of amino acid clusters from doped helium nanodroplets as a function of incident electron energy. (C) 2010 American Institute of Physics. {[}doi: 10.1063/1.3429743]}

{Kinetic-energy release distributions have been obtained from the width and shapes of the time-of-flight (TOF) negative ion mass peaks formed in collisions of fast potassium atoms with D-Ribose (DR) and tetrahydrofuran (THF) molecules. Recent dissociative ion-pair formation experiments yielding anion formation have shown that the dominant fragment from D-Ribose is OH {[}D. Almeida, F. Ferreira da Silva, G. Garcia, P. Limao-Vieira, J. Chem. Phys. 139, 114304 (2013)] whereas in the case of THF is O{[} D. Almeida, F. Ferreira da Silva, S. Eden, G. Garcia, P. Limao-Vieira, J. Phys. Chem. A 118, 690 (2014)]. The results for DR and THF show an energy distribution profile reminiscent of statistical degradation via vibrational excitation and partly due to direct transformation of the excess energy in translational energy.}

{This Letter reports on the first measurements of the lowest lying triplet states as studied by electron energy loss spectroscopy for C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6) and hexafluoro-2-butyne (2-C4F6). This study has been performed at an incident electron energy of 30 eV, 30 degrees, whilst sweeping the energy loss over the range 2.0-15.0 eV. The electronic state spectroscopy has been investigated and the assignments supported by multi-reference quantum chemical calculations. The transition to the steepest electronic excited potential energy curve, is suggested to be dominant for 2-C4F6, due to the large broadening in its energy profile. (C) 2013 Elsevier B.V. All rights reserved.}

{The interaction of low-energy electrons with biomolecules plays an important role in the radiation-induced alteration of biological tissue at the molecular level. At electron energies below 15eV, dissociative electron attachment is one of the most important processes in terms of the chemical transformation of molecules. So far, a common approach to study processes at the molecular level has been to carry out investigations with single biomolecular building blocks like pyrimidine as model molecules. Electron attachment to single pyrimidine, as well as to pure clusters and hydrated clusters, was investigated in this study. In striking contrast to the situation with isolated molecules and hydrated clusters, where no anionic monomer is detectable, we were able to observe the molecular anion for the pure clusters. Furthermore, there is evidence that solvation effectively prevents the ring fragmentation of pyrimidine after electron capture.}

{We present the experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectra of phenol covering for the first time the full 4.3-10.8 eV energy-range, with absolute cross sections determined. Theoretical calculations on the vertical excitation energies and oscillator strengths were performed using time-dependent density functional theory and the equation-of-motion coupled cluster method restricted to single and double excitations level. These have been used in the assignment of valence and Rydberg transitions of the phenol molecule. The VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the 6.401 eV transition, which is here assigned to the 3s sigma/sigma{*}(OH) <- 3 pi(3a `') transition. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of phenol in the earth's atmosphere (0-50 km). Published by AIP Publishing.}

{Absolute differential cross sections (DCSs) for electron interaction with BF3 molecules have been measured in the impact energy range of 1.5-200 eV and recorded over a scattering angle range of 15 degrees-150 degrees. These angular distributions have been normalized by reference to the elastic DCSs of the He atom and integrated by employing a modified phase shift analysis procedure to generate integral cross sections (ICSs) and momentum transfer cross sections (MTCSs). The calculations of DCSs and ICSs have been carried out using an independent atom model under the screening corrected additivity rule (IAM-SCAR). The present elastic DCSs have been found to agree well with the results of IAM-SCAR calculation above 20 eV, and also with a recent Schwinger multichannel calculation below 30 eV. Furthermore, in the comparison with the XF3 (X = B, C, N, and CH) molecules, the elastic DCSs reveal a similar angular distribution which are approximately equal in magnitude from 30 to 200 eV. This feature suggests that the elastic scattering is dominated virtually by the 3-outer fluorine atoms surrounding the XF3 molecules. The vibrational DCSs have also been obtained in the energy range of 1.5-15 eV and vibrational analysis based on the angular correlation theory has been carried out to explain the nature of the shape resonances. Limited experiments on vibrational inelastic scattering confirmed the existence of a shape resonance with a peak at 3.8 eV, which is also observed in the vibrational ICS. Finally, the estimated elastic ICSs, MTCSs, as well as total cross sections are compared with the previous cross section data available. (C) 2015 AIP Publishing LLC.}

{Here we report electron impact ionization studies with the amino acid valine in different environments, i.e., (i) isolated in the gas phase, (ii) embedded in superfluid helium droplets and (iii) co-embedded with water in superfluid helium droplets. Mass spectra are presented for all three environments for which changes in the fragmentation pattern of valine upon ionization are investigated. Comparison is made with previous electron impact ionization and photoionization studies with valine in the gas phase which confirms the fragile nature of this amino acid. Embedding valine in cold superfluid helium droplets leads to the formation of highly abundant protonated valine clusters. Co-embedding water with valine in helium droplets reduces fragmentation of valine.}

{Electron transfer in alkali-molecule collisions to gas phase thymine and uracil yielding H- formation is selectively controlled in the energy range between 5.3 and 66.1 eV. By tuning the collision energy, electron transfer from the alkali to partly deuterated thymine, methylated thymine at the N-1 and methylated uracil at the N-3 positions, H- loss proceeds not only through the breaking of the (C-H) against (N-H) bonds but also through N-1 against N-3 sites. Such selectivity, as far as bond and site are concerned, is here reported for the first time by electron transfer induced dissociation experiments in alkali-molecule collisions. DOI: 10.1103/PhysRevLett.110.023201}

{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-, C-2(-), and C2H-. These contrasting results for potassium impact and DEA highlight significant differences in the reaction pathways initiated by the two electron delivery processes.}

{We present the results obtained in dissociative electron attachment to Semtex in the gas phase by making use of a crossed electron-molecular beam experiment. Energy dependence of partial cross sections has been measured in an electron energy range from 0 to 12 eV with an energy resolution of similar to 80 meV. Semtex 1A and H are mixtures of two other explosives, i.e., royal demolition explosive - RDX (hexogen) and pentaerythritol tetranitrate - PETN (pentrite), appearing in different fractions. In fresh samples product anions from both explosives can be detected together with anions originating from volatile markers. After some time we only observe product ions formed upon dissociative electron attachment to PETN, which are not observed for a mixture of pure RDX and PETN. For both Semtex samples, an additional anionic feature at 210 m/z can be assigned to the binder butyl rubber. (C) 2011 Elsevier B.V. All rights reserved.}

{Electron transfer in alkali-molecule collisions with gas phase acetic acid and its deuterated analogues resulting in OH- formation requires considerable internal rearrangement in the temporary negative ion. At a collision energy well above the threshold of negative ion formation, electron transfer from potassium to CH3COOH/CH3COOD and CD3COOH results not only in H transfer from CH3 to COOH/COOD, but also in H release from COOH and subsequent rearrangement to eliminate OH. These processes are also investigated by theoretical post-Hartree-Fock and DFT calculations. The combination of both studies reveals that the most favourable intermediate mechanism occurs via diol formation. Such intramolecular H transfer is reported here for the first time in the context of electron transfer induced dissociation experiments in alkali-molecule collisions. A comprehensive fragmentation study is presented and dissociation mechanisms are suggested.}

{Electron attachment to formamide clusters in helium nanodroplets is reported for the first time. In contrast to the gas phase, parent anions are seen following low energy electron attachment to both the monomer and the small clusters. This is attributed to formation of dipole (or quadrupole) bound anions. In addition to the bare anions, the mass spectra also show the monomer and clusters with attached helium atoms. The affinity for attaching helium atoms strongly varies with cluster size; for example, the dimer anion is more than 10 times more likely to bind one or more helium atoms than the monomer. Possible binding sites for the helium atoms are discussed.}

{Electron impact ionization of helium nano-droplets containing several 10(4) He atoms and doped with CCl4 or SF6 molecules is studied with high-mass resolution. The mass spectra show significant clustering of CCl4 molecules, less so for SF6 under our experimental conditions. Positive ion efficiency curves as a function of electron energy indicate complete immersion of the molecules inside the helium droplets in both cases. For CCl4 we observe the molecular parent cation CCl4+ that preferentially is formed via Penning ionization upon collisions with He{*}. In contrast, no parent cation SF6+ is seen for He droplets doped with SF6. The fragmentation patterns for both molecules embedded in He are compared with gas phase studies. Ionization via electron transfer to He+ forms highly excited ions that cannot be stabilized by the surrounding He droplet. Besides the atomic fragments F+ and Cl+ several molecular fragment cations are observed with He atoms attached. (C) 2008 Elsevier B.V. All rights reserved.}

{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 (4 pi <- 3 pi) ((1)Delta <- (1)Sigma(+)) transition, with a new weak transition assigned to ((1)Sigma(-) <- (1)Sigma(+)) reported here for the first time. The absolute optical oscillator strengths are determined for ground state to (1)Sigma(+) and (1)Pi. transitions. Based on our recent measurements of differential cross sections for the optically allowed (spectroscopy and (1)Pi) transitions of COS by electron impact, the optical oscillator strength f(0) value and integral cross sections (ICSs) are derived by applying a generalized oscillator strength analysis. Subsequently, ICSs predicted by the scaling are confirmed down to 60 eV in the intermediate energy region. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of carbonyl sulphide in the upper stratosphere (20-50 km). (C) 2015 AIP Publishing LLC.}

{Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H(n)(+). The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H(n)(+). The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered H(n)(+) up to n congruent to 120 by their mass excess that distinguishes them from He(n)(+), mixed He(m)H(n)(+), and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n = 6, pronounced drops in the abundance of even-numbered cluster ions are seen at n = 30 and 114, which suggest icosahedral shell closures at H(6)(+)(H(2))(12) and H(6)(+)(H(2))(54). Possible isomers of H(6)(+) are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory (C) 2008 American Institute of Physics. {[}DOI: 10.1063/1.3035833]}

{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. Spinorbit 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 sigma{*}(C-Br) <- nBr and sigma{*}(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).}

{In this work, we report theoretical and experimental cross sections for elastic scattering of electrons by chlorobenzene (ClB). The theoretical integral and differential cross sections (DCSs) were obtained with the Schwinger multichannel method implemented with pseudopotentials (SMCPP) and the independent atom method with screening corrected additivity rule (IAM-SCAR). The calculations with the SMCPP method were done in the static-exchange (SE) approximation, for energies above 12 eV, and in the static-exchange plus polarization approximation, for energies up to 12 eV. The calculations with the IAM-SCAR method covered energies up to 500 eV. The experimental differential cross sections were obtained in the high resolution electron energy loss spectrometer VG-SEELS 400, in Lisbon, for electron energies from 8.0 eV to 50 eV and angular range from 7 degrees to 110 degrees. From the present theoretical integral cross section (ICS) we discuss the low-energy shape-resonances present in chlorobenzene and compare our computed resonance spectra with available electron transmission spectroscopy data present in the literature. Since there is no other work in the literature reporting differential cross sections for this molecule, we compare our theoretical and experimental DCSs with experimental data available for the parent molecule benzene. Published by AIP Publishing.}

{The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the pi{*}(17a' <- sigma(15a') and 1 pi{*}(10a{''}) <- 1 pi(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km).}

{We report a novel experimental setup for studying collision induced products resulting from the interaction of anionic beams with a neutral gas-phase molecular target. The precursor projectile was admitted into vacuum through a commercial pulsed valve, with the anionic beam produced in a hollow cathode discharge-induced plasma, and guided to the interaction region by a set of deflecting plates where it was made to interact with the target beam. Depending on the collision energy regime, negative and positive species can be formed in the collision region and ions were time-of-flight (TOF) mass-analysed. Here, we present data on O-2 precursor projectile, where we show clear evidence of O- and O-2(-) formation from the hollow cathode source as well as preliminary results on the interaction of these anions with nitromethane, CH3NO2. The negative ions formed in such collisions were analysed using time-of-flight mass spectrometry. The five most dominant product anions were assigned to H-, O-, NO-, CNO- and CH3NO2- .}

{We report experimental and theoretical studies on ring cleavage enhancement in collisions of potassium atoms with uracil/thymine to further increase the understanding of the complex mechanisms yielding such fragmentation pathways. In these electron transfer processes time-of-flight (TOP) negative ion mass spectra were obtained in the collision energy range 13.5-23.0 eV. We note that CNO- is the major ring breaking anion formed and its threshold formation is discussed within the collision energy range studied. Such a decomposition process is supported by the first theoretical calculations to clarify how DNA/RNA pyrimidine base fragmentation is enhanced in electron transfer processes yielding ion-pair formation.}

{We report integral cross sections for electron interactions with tungsten and beryllium atoms in the incident electron energy range from 0.1 up to 5000 eV. The calculated cross sections are obtained for electron-atom scattering processes represented by a complex potential. For tungsten, ionization cross sections are discussed in the electron energy region from threshold up to 5000 eV against the available data from the Deutsch-Mark formalism and a semi-empirical complex scattering potential. Although a reasonable agreement for the ionization cross sections has been found in the overlapping energy region, inconsistencies on the integral inelastic cross sections from the previous semi-empirical approach based on a complex scattering potential ionization contribution, are now amended and comprehensively explained. For beryllium atoms an excellent agreement with previous `state-of-the-art' scattering theory calculations has been found for the integral elastic cross sections. However, the partial contribution of the excitation and ionization channels to the inelastic part shows serious discrepancies which deserve further investigations. Calculated elastic differential cross sections for tungsten are also reported from 0.1 to 5000 eV for scattering angles from 0 degrees to 180 degrees. The present set of cross sectional data may be of relevance for the plasma fusion community.}

{Gas phase dissociative electron attachment (DEA) measurements to pentaerythritol tetranitrate (PETN) are performed in a crossed electron-molecular beam experiment at high-energy resolution and high sensitivity. DEA is operative at very low energies close to similar to 0 eV showing unique features corresponding to a variety of fragment anions being formed. There is no evidence of the parent anion formation. The fragmentation yields are also observed for higher electron energies and are operative via several resonant features in the range of 0-12 eV. In contrast to nitroaromatic compounds, PETN decays more rapidly upon electron attachment and preferentially low-mass anions are formed. The dominant fragment ion formed through DEA is assigned to the nitrogen trioxide NO(3)(-) and represents about 80% of the total anion yield. Further intense ion signals are due to NO(2)(-) (11%) and O(-) (2.5%). The significant instability of PETN after attachment of an electron with virtually no kinetic energy confers a highly explosive nature to this compound.}

{Here, we report elastic differential cross sections (DCSs) for electron scattering from water in the incident energy range of 2-100 eV. Furthermore, we present a complete study on the electronic excitation of the (a) over tilde B-3(1) and (A) over tilde B-1(1) states at electron impact energies of 15, 20, and 30 eV and in the scattering angle range of 10. -130.. Integral cross sections (ICSs) are determined from the DCSs. Measuring elastic DCSs in various experimental conditions confirmed the reproducibility of the data. The present results agree with the data previously obtained from a conventional collimating tube gas source. Ambiguities associated with the unfolding procedure of the electron energy loss (EEL) spectra for the electronic excitations have been reduced by comparison against the EEL spectrum at high electron impact energy and for small scattering angle. The reliability of the extracted DCSs is improved significantly for optically forbidden contributions from the overlap of the (a) over tilde B-3(1) and (A) over tilde B-1(1) electronic states. The BEf-scaling model is also confirmed to produce the integral cross section for the optical allowed transition of the (A) over tilde B-1(1) state in the intermediate electron energy region above 15 eV.}

{The processes of dissociative excitation (DE) and dissociative ionisation with excitation (DIE) of iron pentacarbonyl, Fe(CO)(5), have been studied using a crossed electron-molecule beam experimental apparatus (Electron Induced Fluorescence Apparatus, EIFA). Using EIFA we were able to record the emission spectrum of the molecule in the UV-VIS range, as well as the photon efficiency curves initiated by electron impact. The emission spectrum of Fe(CO)(5) initiated by impact of 50 eV electrons was recorded in the spectral range between 200 nm and 470 nm. It shows a high density of emission lines and bands (mainly iron lines and carbonyl bands). Additionally, we have measured photon efficiency curves (PECs) as a function of the electron impact energy for several lines and bands. On the basis of the PECs we have discussed the reaction mechanism and the energetics of the reactions associated with the DE and DIE processes.}

{Dissociative electron attachment (DEA) measurements to nitromethane, CH(3)NO(2), in the gas phase have been revisited by making use of a high mass-resolution sector field instrument. Anion efficiency curves for 16 negatively charged fragments have been measured in the electron energy region from about 0 to 16 eV with an energy resolution of similar to 1 eV. Eight new anions have been detected, CH(2)NO(2)(-), CHNO(2)(-), CH(2)NO(-), H(2)NO(-), CH(3)(-), CH(2)(-), CH(-) and H(-). The five most dominant product anions are NO(2)(-), O(-), OH(-), CN(-) and CNO(-), all of them featuring two high-energy resonances at about 5 eV and 10 eV. Formation of CH2NO2- at low electron energies has been explained in terms of DEA to highly vibrationally excited molecules. The standard enthalpy of formation of CH(2)NO(2)(-) and H(-), CH(2)(-) and NO(-) radicals have been estimated as Delta(f)H(g)degrees(CH(2)NO(2)(-)) = -1.71 eV, Delta(f)H(g)degrees(H(-)) = 2.09 eV, Delta(f)H(g)degrees(CH(2)(-)) = 3.5 eV and Delta(f)H(g)degrees(NO(-)) = -1.93 eV, respectively. (c) 2007 Elsevier B.V. All rights reserved.}

{Electron impact ionization of argon clusters embedded in helium droplets is investigated. Superior mass resolution makes it possible to distinguish between nominally isobaric cluster ions. An abundance maximum for ArHe(12)(+) is unambiguously confirmed; the spectra also prove the formation of Ar(2)He(n)(+) complexes that had been claimed to fragment into pure Ar(2)(+). Distributions of larger argon cluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argon clusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argon cluster ions. Intriguing abundance anomalies are observed in distributions of argon cluster ions that contain water, nitrogen or oxygen impurities. The strong abundance of Ar(55)H(2)O(+), Ar(54)O(2)(+) and Ar(54)N(2)(+) contrasts with the virtual absence of slightly larger cluster ions containing the corresponding impurities. The features are probably related to enhanced cluster ion stability upon closure of the second icosahedral shell but the difference in magic numbers (54 versus 55) and the well-known reactivity of charged argon-nitrogen complexes suggest}

{H- and CNO- site and bond selectivity formation is shown in the context of atom-molecule collisions.}

{We present negative ion formation from collisions of 100 eV neutral potassium atoms with acetic acid (CH3COOH) and its deuterated analogue molecules (CH3COOD, CD3COOH). From the negative ion time-of-flight (TOF) mass spectra, OH-is the main fragment detected accounting on average for more than 25% of the total anion yield. The complex internal rearrangement processes triggered by electron transfer to acetic acid have been evaluated with the help of theoretical calculations at the DFT levels explaining the fragmentation channel yielding OH-.}

{In the present work we investigate the interaction of low energetic electrons with biologically relevant molecules. Our aim is to observe differences in the fragmentation pattern in the case of isolated molecules, bare clusters and water-solvated clusters. In this context we present the first results regarding electron attachment to nano-solvated biomolecule clusters.}

{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.}

{Dissociative electron attachment (DEA) to gaseous formamide, HCONH(2), has been investigated in the energy range between 0 eV and 18 eV using a crossed electron/molecule beam technique. The negative ion fragments have been comprehensively monitored and assigned to molecular structures by comparison with the results for two differently deuterated derivatives, namely 1D-formamide, DCONH(2), and N, N, D-formamide, HCOND(2). The following products were observed: HCONH(-), CONH(2)(-), HCON(-), OCN(-), HCNH(-), CN(-), NH(2)(-)/O(-), NH(-), and H(-). NH(2)(-) was also separated from O(-) by using high-resolution negative ion mass spectrometry. Four resonant dissociation channels can be resolved, the strongest ones being located between 2.0 and 2.7 eV and between 6.0 and 7.0 eV. CN(-) as the most abundant fragment and HCONH(-) are the dominant products of the first of these two resonances. The most important products of the latter resonance are NH(2)(-), CN(-), H(-), CONH(2)(-), and OCN(-). It is thus found that the loss of neutral H is a site-selective process, dissociation from the N site taking place between 2.0 and 2.7 eV while dissociation from the C site occurs between 6.0 and 7.0 eV. The suitability of these reactions and thus of formamide as an agent for electron-induced surface functionalisation is discussed.}

{The effect of incident electrons on acetic acid clusters is explored for the first time. The acetic acid clusters are formed inside liquid helium nanodroplets and both cationic and anionic products ejected into the gas phase are detected by mass spectrometry. The cation chemistry (induced by electron ionization at 100 eV) is dominated by production of protonated acetic acid (Ac) clusters, Ac(n)H(+), although some fragmentation is also observed. In the case of anion production (at 2.8 eV electron energy) there is a clear distinction between the monomer and the clusters. For the monomer the dominant product is the dehydrogenated species, {[}Ac-H](-), whereas for the clusters both the parent anion, Ac(n)(-), and the dehydrogenated species, {[}Ac(n)-H](-), have similar abundances. A particularly intriguing contrast between the monomer and cluster anions is that helium atoms are seen attached to the latter whereas no evidence of helium atom attachment is found for the monomer. This surprising observation is attributed to the formation of acyclic (head-to-tail) acetic acid clusters in helium nanodroplets, which have more favourable electronic properties for binding helium atoms. The acyclic clusters represent a local minimum on the potential energy surface and in the case of the dimer this is distinct from the cyclic isomer (the global minimum) identified in gas phase experiments.}

{Gas phase dissociative electron attachment (DEA) measurements with methyl-dialanine, C7H14N2O3, are performed in a crossed electron-molecular beam experiment at high energy resolution (similar to 120 meV). Anion efficiency yields as a function of the incident electron energy are obtained for the most abundant fragments up to electron energies of similar to 15 eV. There is no evidence of molecular anion formation whereas the dehydrogenated closed shell anion (M-H)(-) is one of the most dominant reaction products. Quantum chemical calculations are performed to investigate the electron attachment process and to elucidate site selective bond cleavage in the (M-H)(-) DEA-channel. Previous DEA studies on dialanine have shown that (M-H)(-) formation proceeds through abstraction of a hydrogen atom from the carboxyl and amide groups, contributing to two distinct resonances at 0.81 and 1.17 eV, respectively {[}D. Gschliesser, V. Vizcaino, M. Probst, P. Scheier and S. Denifl, Chem.-Eur. J., 2012, 18, 4613-4619]. Here we show that by methylation of the carboxyl group, all (calculated) thresholds for H-loss from the different sites in the dialanine molecule are shifted up to a maximum of 1.4 eV. The lowest lying resonance observed experimentally for (M-H)(-) remains operative from the amide group at the electron energy of 2.4 eV due to the methylation. We further study methylation-induced effects on the unimolecular dissociation leading to a variety of negatively charged DEA products.}

{Electron attachment to the explosive trinitrotoluene (TNT) embedded in Helium droplets (TNT@He) generates the non-decomposed complexes (TNT)(n)(-), but no fragment ions in the entire energy range 0-12 eV. This strongly contrasts the behavior of single TNT molecules in the gas phase at ambient temperatures, where electron capture leads to a variety of different fragmentation products via different dissociative electron attachment (DEA) reactions. Single TNT molecules decompose by attachment of an electron at virtually no extra energy reflecting the explosive nature of the compound. The complete freezing of dissociation intermediates in TNT embedded in the droplet is explained by the particular mechanisms of DEA in nitrobenzenes, which is characterized by complex rearrangement processes in the transient negative ion (TNI) prior to decomposition. These mechanisms provide the condition for effective energy withdrawal from the TNI into the dissipative environment thereby completely suppressing its decomposition.}

{Here we present negative ion formation driven by electron transfer in atom (potassium) molecule collision and dissociative electron attachment in sulphur containing compounds, probing the influence of sulphur atoms in the decomposition mechanism of two biological relevant molecules: thiaproline and taurine.}

{We present negative ion formation driven by electron transfer in atom (K) molecule (acetic acid) collisions. Acetic acid has been found in the interstellar medium, is also considered a biological related compound and as such studying low energy electron interactions will bring new insights as far as induced chemistry is concerned.}

{Electron transfer to uracil (U), 5-chlorouracil (5-ClU) and 5-fluorouracil (5-FU) yielding anion formation has been investigated in 30-100 eV potassium-molecule collisions. The rich fragmentation patterns of all three molecules suggest that electron transfer in collisions with electronegative neutrals may cause efficient damage to RNA. The main ring fragment anion in all the mass spectra was NCO(-) while the production of X(-) (X = F, Cl) was a strong decomposition of the halouracil temporary negative ions. Cl(-) was the most intense fragment anion in the 5-chlorouracil measurements, whereas NCO(-) production dominated in the U and 5-FU data. Arguments based on energetics and vibrational dynamics have been proposed to explain these differences. Electronic coupling between dipole- and valence-bound states may play a particularly important role in the fragmentation pathways of the 5-ClU parent anion. The stabilizing influence of the potassium cation following electron transfer (ionic scattering) on the observed fragmentation patterns is discussed, notably in the context of comparisons with free electron attachment processes.}

{Ion-pair formation has been studied in hyperthermal (30-100 eV) neutral potassium collisions with gas phase thymine (C(5)H(6)N(2)O(2)) and uracil (C(4)H(4)N(2)O(2)). Negative ions formed by electron transfer from the alkali atom to the target molecule were analysed by time-of-flight (TOF) mass spectrometry. The most abundant product anions are assigned to CNO(-) and (U-H)(-)/(T-H)(-) and the associated electron transfer mechanisms are discussed. Special emphasis is given to the enhancement of ring breaking pathways in the present experiments, notably CNO(-) formation, compared with free electron attachment measurements.}

{In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J. 2008. 14, 4028) proposed the existence of, a flew class of, radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H-2 molecules we observe various HexHy+ cluster ions, including He2H2+, which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH2+ + He with a low kinetic energy release of 15 4 meV. Surprisingly, another unimolecular reaction is Observed. into HeH+ + HeH (or He + H). The probability of this reaction is ail order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited stated they arc consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He+.}

{We present interaction probability data of low-energy secondary electrons and positrons produced due to the proton impact. The probability distribution functions serve as input data for the Low Energy Particle Track Simulation (LEPTS) approach which allows one to include the effect of low-energy species in medical applications of radiation and in ion-beam cancer therapy, in particular.}

{Electron transfer and dissociative electron attachment to 3-methyluracil (3meU) and 1-methylthymine (1meT) yielding anion formation have been investigated in atom-molecule collision and electron attachment experiments, respectively. The former has been studied in the collision energy range 14-100 eV whereas the latter in the 0-15 eV incident electron energy range. In the present studies, emphasis is given to the reaction channel resulting in the loss of the methyl group from the N-sites with the extra charge located on the pyrimidine ring. This particular reaction channel has neither been approached in the context of dissociative electron attachment nor in atom-molecule collisions yet. Quantum chemical calculations have been performed in order to provide some insight into the dissociation mechanism involved along the N-CH3 bond reaction coordinate. The calculations provide support to the threshold value derived from the electron transfer measurements, allowing for a better understanding of the role of the potassium cation as a stabilising agent in the collision complex. The present comparative study gives insight into the dynamics of the decaying transient anion and more precisely into the competition between dissociation and auto-detachment.}

{Negative ion formation in collisions of potassium atoms with tetrahydrofuran (THF) and D-Ribose (DR) were performed in a crossed molecular beam setup equipped with a time-of-flight (TOF) mass spectrometer. Owing to the role of these molecules as possible representations of the sugar unit in the DNA/RNA, the fragmentation patterns obtained for different collision energies were compared and the role of K+ post-collision interaction with the molecular is evaluated.}

{Helium nanodroplets are doped with SF6, C4F8, CCl4, C6H5Br, CH3I, and I-2. Upon interaction with free electrons a variety of positively and negatively charged cluster ions X+/- He-n are observed where X+/- = F+/-, Cl+/-, Br+/-, I+, I-2(+), or CH3I+. The yield of these ions versus cluster size n drops at characteristic sizes n(s) that range from n(s)=10.2 +/- 0.6 for F+ to n(s) = 22.2 +/- 0.2 for Br-. n(s) values for halide anions are about 70% larger than for the corresponding cations. The steps in the ion yield suggest closure of the first solvation shell. We propose a simple classical model to estimate ionic radii from n(s). Assuming the helium density in the first solvation shell equals the helium bulk density one finds that radii of halide anions in helium are nearly twice as large as in alkali halide crystals, indicating the formation of an anion bubble due to the repulsive forces that derive from the exchange interaction. In spite of the simplicity of our model, anion radii derived from it agree within approximately 10% with values derived from the mobility of halide anions in superfluid bulk helium, and with values computed by quantum Monte Carlo methods for X-Hen cluster anions.}

{In this study we present for the first time site (N-1-H / N-3-H) and bond (N-H / C-H) selectivity of if formation in 1-methylthymine, 3-methyluracil and deuterated thymine (C positions) triggered by potassium molecule collisions. By comparing the H loss of these molecules with H loss in thymine and uracil and setting the energy one can predict site and bond selectivity in these set of molecules.}

{Electron transfer in alkali-molecule collisions to gas phase thymine and uracil yielding H- formation is selectively controlled in the energy range between 5 and 65 eV. By tuning the collision energy, electron transfer from the alkali to methylated thymine (at the N-1 position), methylated uracil (at the N-3 position) and partly deuterated thymine, enables H(-)formation. Such process proceeds not only through the breaking of the (C-H) against (N-H) bonds but also through N-1 against N-3 sites. Such selectivity, as far as bond and site are concerned, is here reported for the first time in collision induced dissociation experiments by alkali-molecule interactions.}

{Acetone in human breath has been established as a biomarker for diabetes. The application of UV spectroscopy for the study of biomedical compounds has been developed over recent years but is often limited by the lack of absolute data for calibration of the instrumentation. By measuring high resolution absolute VUV photoabsorption cross sections of acetone, we are able to provide calibration values at several wavelengths with special attention to 4.661 eV (266 nm). Results are compared with recent published data {[}C Wang, A Mbi, Meas. Sci. Technol., 2007, 18, 2731-2741]. The acetone spectrum is here revisited from a recent contribution {[}Nobre M, et al, Phys Chem Chem Phys, 2008 (in press)] where the absolute cross sectional values are obtained in the 3.7 - 10.8 eV energy region. Future medical units working in close link with synchrotron radiation facilities can make use of the VUV spectra wavelength region to trace acetone in diabetic patients.}

{Here we report the first mass spectrometric study of the synthesis of larger polycyclic aromatic hydrocarbon molecules (PAHs) out of weakly bound triphenylene agglomerates in cold helium droplets upon electron ionization. A highly abundant single reaction product is observed which indicates an extremely efficient and selective process. We propose that the synthesis of large molecules like fullerenes in interstellar clouds may proceed via similar reactions. Future work on different PAHs like benz{[}a]anthracene or chrysene will give more insight into this fascinating synthesis process.}

{The electronic state spectroscopy of pyrimidine C4H4N2 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. rst 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 B-1(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 (B-2(2)), 10.41 eV (B-2(1)) and 11.1 eV ((2)A(1) + (2)A(2)) with new members reported for the. rst time and classi. ed according to the magnitude of the quantum defects (delta). Additionally, the absolute di. erential cross section for inelastic electron scattering has been measured for the most intense band from 6.9 to 7.8 eV assigned to (1)pi pi{*} (3(1)A(1) + 2(1)B(2)).}

{We present negative ion formation driven by electron transfer in atom (K) molecule (amino acids) collisions, probing the influence of side chains in the decomposition mechanism.}

{The electronic state spectroscopy of acetone (CH3)(2)CO has been investigated using high-resolution VUV photoabsorption spectroscopy in the energy range 3.7-10.8 eV. New vibronic structure has been observed, notably in the low energy absorption band assigned to the 1(1)A(1) -> 1(1)A(2) (n(y) -> pi{*}) transition. The local absorption maximum at 7.85 eV has been tentatively attributed to the 4(1) A(1) (pi -> pi{*}) transition. Six Rydberg series converging to the lowest ionisation energy (9.708 eV) have been assigned as well as a newly-resolved ns Rydberg series converging to the first ionic excited state (12.590 eV). Rydberg orbitals of each series have been classified according to the magnitude of the quantum defect (delta) and are extended to higher quantum numbers than in the previous analyses.}