For bulk liquid helium the bottom of the conduction band (V0) is above the vacuum level. In this case the surface of the liquid represents an electronic surface barrier for an electron to be injected into the liquid. Here we study the electronic conduction band for doped helium droplets of different sizes. Utilizing an electron monochromator, the onset of the (H2O)2– ion yield corresponding to V0 is determined for helium droplets doped with the water dimer. While for larger droplets the onset approaches the well-known bulk value of about 1 eV, the barrier does not continuously decrease with smaller droplet size. A minimum value of V0 = 0.76 ± 0.10 eV is observed, which corresponds to a droplet size of Nmin = 1600 ± 900. For droplet sizes below Nmin, a peak at ∼0 eV appears, which is well-known from neat H2O clusters. Hence, we interpret Nmin as the smallest droplet size in which the electronic band structure is formed in liquid helium droplets.

Mass spectroscopic investigations on tetrahydrofuran (THF, C4H8O), a common model molecule of the DNA-backbone, have been carried out. We irradiated isolated THF and (hydrated) THF clusters with low energy electrons (electron energy ~70 eV) in order to study electron ionization and ionic fragmentation. For elucidation of fragmentation pathways, deuterated TDF (C4D8O) was investigated as well. One major observation is that the cluster environment shows overall a protective behavior on THF. However, also new fragmentation channels open in the cluster. In this context, we were able to solve a discrepancy in the literature about the fragment ion peak at mass 55 u in the electron ionization mass spectrum of THF. We ascribe this ion yield to the fragmentation of ionized THF clusters.

Imidazole [C3H4N2] is ubiquitous in nature as an important biological building block of amino acids, purine nucleobases or antibiotics. In the present study, dissociative electron attachment to imidazole shows low energy shape resonances at 1.52 and 2.29 eV leading to the most abundant dehydrogenated anion [imidazole − H]− through dehydrogenation at the N1 position. All the other anions formed exhibit core excited resonances observed dominantly at similar electron energies of ∼7 and 11 eV, suggesting an initial formation through two temporary negative ion states. Among these anions, multiple dehydrogenation reactions are observed resulting in the loss of 2 up to 4 hydrogens, thus, leading to a complete dehydrogenation of the imidazole molecule, an interesting prototype of complex unimolecular decay induced by the attachment of a single electron. Additionally, the quantum chemical calculations reveal that these multiple dehydrogenation reactions are responsible for the remarkable one electron-induced gas-phase chemistry leading to the opening of the ring. The formation of the observed anions is likely driven by the high positive electron affinity of cyanocarbon molecules supported by quantum chemical calculations. The formation of H− showed additional resonance at about 5 eV and dipolar dissociation above ∼14 eV.

We observed the bare W2+ metal cation upon electron ionization of the weakly bound W(CO)6 dimer. This metal cation can be only observed due to the fast conversion of the weak cluster bond into a strong covalent bond between the metal moieties.

Aminoacetonitrile (NH2CH2CN, AAN) is a molecule relevant for interstellar chemistry and the chemical evolution of life. It is a very important molecule in the Strecker diagram explaining the formation of amino acids. In the present investigation, dissociative electron attachment to NH2CN was studied in a crossed electron–molecular beams experiment in the electron energy range from about 0 to 17 eV. In this electron energy range, the following six anionic species were detected: C2H3N2–, C2H2N2–, C2H2N–, C2HN–, CN–, and NH2–. Possible reaction channels for all the measured negative ions are discussed, and the experimental results are compared with calculated thermochemical thresholds of the observed anions. Similar to other nitrile and aminonitrile compounds, the main anions detected were the negatively charged nitrile group, the dehydrogenated parent molecule, and the amino group. No parent anion was observed. Low anion yields were observed indicating that AAN is less prone to electron capture. Therefore, AAN can be considered to exhibit a relatively long lifetime under typical conditions in outer space.

Low-energy electrons (0–8 eV) effectively decompose 4-nitroimidazole (4NI) and the two methylated isomers 1-methyl-5-nitroimidazole and 1-methyl-4-nitroimidazole via dissociative electron attachment (DEA). The involved unimolecular decompositions range from simple bond cleavages (loss of H•, formation of NO2–) to complex reactions possibly leading to a complete degradation of the target molecule (formation of CN–, etc.). At energies below 2 eV, the entire rich chemistry induced by DEA is completely quenched by methylation, as demonstrated in a previous communication (Tanzer, K.; Feketeová, L.; Puschnigg, B.; Scheier, P.; Illenberger. E.; Denifl, S. Angew. Chem., Int. Ed.2014, 53, 12240). The observation that in 4NI neutral radicals and radical anions are formed via DEA at high efficiency already at threshold (0 eV) may have significant implications for the development of nitroimidazole-based radiosensitizers in tumor radiation therapy.

Electron ionization of the DNA nucleobase, adenine, and the tRNA nucleobase, hypoxanthine, was investigated near the threshold region (∼5–20 eV) using a high-resolution hemispherical electron monochromator and a quadrupole mass spectrometer. Ion efficiency curves of the threshold regions and the corresponding appearance energies (AEs) are presented for the parent cations and the five most abundant fragment cations of each molecule. The experimental ionization energies (IEs) of adenine and hypoxanthine were determined to be 8.70 ± 0.3 eV and 8.88 ± 0.5 eV, respectively. Quantum chemical calculations (B3LYP/6-311+G(2d,p)) yielded a vertical IE of 8.08 eV and an adiabatic IE of 8.07 eV for adenine and a vertical IE of 8.51 eV and an adiabatic IE of 8.36 eV for hypoxanthine, and the lowest energy optimized structures of the fragment cations and their respective neutral species were calculated. The enthalpies of the possible reactions from the adenine and hypoxanthine cations were also obtained computationally, which assisted in determining the most likely electron ionization pathways leading to the major fragment cations. Our results suggest that the imidazole ring is more stable than the pyrimidine ring in several of the fragmentation reactions from both adenine and hypoxanthine. This electron ionization study contributes to the understanding of the biological effects of electrons on nucleobases and to the database of the electronic properties of biomolecules, which is necessary for modeling the damage of DNA in living cells that is induced by ionizing radiation.

•We investigated electron attachment to platinum(II) bromide.•Only Br− is observed upon electron capture by the molecule.•Two resonances are ascribed to Br− + PtBr and Br− + Pt + Br dissociation channels, respectively.Dissociative electron attachment to PtBr2 in the gas phase was studied in the low electron energy range from zero up to 10 eV with an energy resolution of 150 meV. The experiments were carried out using a hemispherical electron monochromator coupled with a quadrupole mass spectrometer and pulse counting acquisition system. The only anion observed was Br−. This ion is formed at three resonance electron energies: 0.4 eV, 1.2 eV and 7 eV. By the measurements of the Br− formation at different sample temperatures (in the 350–430 K range) the 0.4 eV resonance was associated with the electron capture by HBr generated in the apparatus at elevated temperatures. In addition, the thermodynamic thresholds for dissociative electron attachment reactions for platinum(II) bromide were calculated and compared with the experimental results.

•Doubly charged CO2 clusters are fingerprinted by detecting their isotopologues.•Contributions from singly charged ions are eliminated by background subtraction.•Smaller doubly charged ions than reported so far are identified.•A sequential process that involves Penning ionization is invoked to explain the appearance of doubly charged ions.Helium nanodroplets are doped with carbon dioxide and ionized by electrons. Doubly charged cluster ions are, for the first time, identified based on their characteristic patterns of isotopologues. Thanks to the high mass resolution, large dynamic range, and a novel method to eliminate contributions from singly charged ions from the mass spectra, we are able to observe doubly charged cluster ions that are smaller than the ones reported in the past. The likely mechanism by which doubly charged ions are formed in doped helium droplets is discussed.Figure optionsDownload full-size imageDownload high-quality image (176 K)Download as PowerPoint slide

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.

Gas phase dissociative electron attachment (DEA) measurements with methyl-dialanine, C7H14N2O3, are performed in a crossed electron-molecular beam experiment at high energy resolution (∼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 ∼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.

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.

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.

Negative ion formation in the three perfluoroethers (PFEs) diglyme (C6F14O3), triglyme (C8F18O4) and crownether (C10F20O5) is studied following electron attachment in the range from ∼0 to 15 eV. All three compounds show intense low energy resonances at subexcitation energies (<3 eV) decomposing into a variety of negatively charged fragments. These fragment ions are generated via dissociative electron attachment (DEA), partly originating from sequential decompositions on the metastable (μs) time scale as observed from the MIKE (metastable induced kinetic energy) scans. Only in perfluorocrownether a signal due to the non-decomposed parent anion is observed. Additional and comparatively weaker resonances are located in the energy range between ∼10 and 17 eV which preferentially decompose into lighter ions. It is suggested that specific features of perfluoropolyethers (PFPEs) relevant in applications, e.g., the strong bonding to surfaces induced by UV radiation of the substrate or degradation of PFPE films in computer hard disc drives can be explained by their pronounced sensitivity towards low energy electrons.Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (127 K)Download as PowerPoint slideHighlights► Studied perfluoroether are sensitive towards subexcitation electrons. ► Fragment anions are formed by loss of neutral CF2, CF3 and CF2OCF3 units. ► Electrons contribute to degradation of perfluoropolyether films on hard discs.

Electron attachment to CO2 clusters performed at high energy resolution (0.1 eV) is studied for the first time in the extended electron energy range from threshold (0 eV) to about 10 eV. Dissociative electron attachment (DEA) to single molecules yields O− as the only fragment ion arising from the well known 2Πu shape resonance (ion yield centered at 4.4 eV) and a core excited resonance (at 8.2 eV). On proceeding to CO2 clusters, non-dissociated complexes of the form (CO2)n− including the monomer CO2− are generated as well as solvated fragment ions of the form (CO2)nO−. The non-decomposed complexes appear already within a resonant feature near threshold (0 eV) and also within a broad contribution between 1 and 4 eV which is composed of two resonances observed for example for (CO2)4− at 2.2 eV and 3.1 eV (peak maxima). While the complexes observed around 3.1 eV are generated via the 2Πu resonance as precursor with subsequent intracluster relaxation, the contribution around 2.2 eV can be associated with a resonant scattering feature, recently discovered in single CO2 in the selective excitation of the higher energy member of the well known Fermi dyad [M. Allan, Phys. Rev. Lett., 2001, 87, 0332012]. Formation of (CO2)n− in the threshold region involves vibrational Feshbach resonances (VFRs) as previously discovered via an ultrahigh resolution (1 meV) laser photoelectron attachment method [E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf and H. Hotop, Eur. Phys. J. D, 2000, 12, 125]. The complexes (CO2)nO− clearly arise from DEA at an individual molecule within the cluster involving both the 2Πu and the core excited resonance.

Dissociative electron attachment (DEA) to 1-bromo-2-chlorobenzene (1,2-C6H4BrCl) and 1-bromo-3-chlorobenzene (1,3-C6H4BrCl) is studied in a crossed electron/ molecular beams experiment in the electron energy range between about 0 and 2 eV and in the gas temperature range from 377 to 583 K. For both molecules the two fragment anions Cl− and Br− are formed. The ion yields of both Br− and Cl− show a pronounced temperature effect when the gas temperature is raised from 377 to 583 K. These DEA processes can be well interpreted qualitatively with thermodynamics calculations within adiabatic approximation scheme, in particular, the thermally excited out-of-plane bending and C–Cl/Br bond stretching vibrations may be closely related to the Cl−/Br− branching of the temporary negative anions of 1,2- and 1,3-C6H4BrCl.We have investigated dissociative electron attachment to bromo-chloro-benzenes at different gas temperatures. A pronounced temperature effect of the ion yield of fragment anions is observed.

Gas phase dissociative electron attachment (DEA) measurements with methyl-dialanine, C7H14N2O3, are performed in a crossed electron-molecular beam experiment at high energy resolution (∼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 ∼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 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.

We observed the bare W2+ metal cation upon electron ionization of the weakly bound W(CO)6 dimer. This metal cation can be only observed due to the fast conversion of the weak cluster bond into a strong covalent bond between the metal moieties.

Electron attachment to CO2 clusters performed at high energy resolution (0.1 eV) is studied for the first time in the extended electron energy range from threshold (0 eV) to about 10 eV. Dissociative electron attachment (DEA) to single molecules yields O− as the only fragment ion arising from the well known 2Πu shape resonance (ion yield centered at 4.4 eV) and a core excited resonance (at 8.2 eV). On proceeding to CO2 clusters, non-dissociated complexes of the form (CO2)n− including the monomer CO2− are generated as well as solvated fragment ions of the form (CO2)nO−. The non-decomposed complexes appear already within a resonant feature near threshold (0 eV) and also within a broad contribution between 1 and 4 eV which is composed of two resonances observed for example for (CO2)4− at 2.2 eV and 3.1 eV (peak maxima). While the complexes observed around 3.1 eV are generated via the 2Πu resonance as precursor with subsequent intracluster relaxation, the contribution around 2.2 eV can be associated with a resonant scattering feature, recently discovered in single CO2 in the selective excitation of the higher energy member of the well known Fermi dyad [M. Allan, Phys. Rev. Lett., 2001, 87, 0332012]. Formation of (CO2)n− in the threshold region involves vibrational Feshbach resonances (VFRs) as previously discovered via an ultrahigh resolution (1 meV) laser photoelectron attachment method [E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf and H. Hotop, Eur. Phys. J. D, 2000, 12, 125]. The complexes (CO2)nO− clearly arise from DEA at an individual molecule within the cluster involving both the 2Πu and the core excited resonance.

Imidazole [C3H4N2] is ubiquitous in nature as an important biological building block of amino acids, purine nucleobases or antibiotics. In the present study, dissociative electron attachment to imidazole shows low energy shape resonances at 1.52 and 2.29 eV leading to the most abundant dehydrogenated anion [imidazole − H]− through dehydrogenation at the N1 position. All the other anions formed exhibit core excited resonances observed dominantly at similar electron energies of ∼7 and 11 eV, suggesting an initial formation through two temporary negative ion states. Among these anions, multiple dehydrogenation reactions are observed resulting in the loss of 2 up to 4 hydrogens, thus, leading to a complete dehydrogenation of the imidazole molecule, an interesting prototype of complex unimolecular decay induced by the attachment of a single electron. Additionally, the quantum chemical calculations reveal that these multiple dehydrogenation reactions are responsible for the remarkable one electron-induced gas-phase chemistry leading to the opening of the ring. The formation of the observed anions is likely driven by the high positive electron affinity of cyanocarbon molecules supported by quantum chemical calculations. The formation of H− showed additional resonance at about 5 eV and dipolar dissociation above ∼14 eV.