[en] Absolute ionization cross sections have been measured for the processes N₂+e→N₂⁺+2e, N₂+e→N₂⁺⁺+3e, O₂+e→O₂⁺+2e, and O₂+e→O₂⁺⁺+3e as a function of electron energy up to 170 eV in a crossed molecule--electron beam apparatus. The ionization cross section curve for producing the singly charged parent ions N₂⁺ and O₂⁺ is found to have at least one inflection of the type first observed by Lawrence for mercury. Using nth root extrapolation the following minimum ionization potentials were derived from the low energy cross section behavior: N₂⁺(X ²Σ/sub g/⁺) = 15.6 +- 0.1 eV, N₂⁺⁺(³PI/sub u/) = 42.9 +- 0.3 eV, O₂⁺(X ²PI/sub g/) = 12.1 +- 0.1 eV, and O₂⁺⁺(X ¹Σ/sub g/⁺) = 35.6 +- 0.3 eV. From distinct breaks in the single ionization cross section curve the following excited states were observed: N₂⁺(B ²Σ⁺/sub u/) = 18.8 +- 0.2 eV, O₂⁺(a ⁴PI/sub u/) = 16.0 +- 0.2 eV, and O₂⁺(²Σ/sub g/^-) = 21.0 +- 0.2 eV. In the double ionization cases, a square law was found from threshold up to some 20 eV above threshold. All results are compared with previous measurements if available

[en] Unimolecular and collision-induced dissociation of Ar₂⁺ produced by electron-impact ionization of Ar₂ were studied quantitatively with a double-focusing mass spectrometer. The occurrence of the metastable dissociation process Ar₂ /sup +asterisk/→Ar⁺+Ar is interpreted qualitatively in terms of potential-energy curves calculated recently using quantum-mechanical methods

[en] Kr clusters produced in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry. Mass resolved spectra (with n up to 180) show two homologous series consisting of Kr⁺_n and Kr²⁺_n ions. The distribution of Kr⁺_n ions shows distinct magic number effects, the observed abundance anomalies being very similar to the ones observed in Ar and Xe. This confirms the superior stability of closed-shell and -subshell icosahedral structures. Moreover, we have found evidence for the occurrence of Kr³⁺_n and Kr⁴⁺_n ions. It was possible to determine appearance sizes of these multiply charged cluster ions (yielding n₂=69, n₃=156, and n₄=264), and to study the electron energy dependence of singly and doubly charged cluster ions (yielding a linear threshold law). These results are discussed in view of various theoretical considerations and previous results where available

[en] Size distributions of fragments from multiply charged clusters have been determined. On a time scale of 0.1 ms with respect to ionization, triply charged CO₂ clusters of size nroughly-equal114 are found to fission extremely asymmetrically, their doubly charged fragments carrying 92% of the mass. For doubly charged CO₂ clusters, however, evaporation of neutral monomers is the only channel for delayed dissociation. A model, based on the liquid-drop approximation, can quantitatively account for the lower size limits of multiply charged clusters as well as for the distribution of fission fragments from (CO₂)/sub roughly-equal114/ ³⁺. .AE

[en] Cation and anion formation is studied experimentally upon inelastic low energy electron interactions with 5-bromo-uridine (BrUrd) using a crossed electron/neutral beams set-up combined with a quadrupole mass spectrometer. The BrUrd molecule belongs to the class of radiosensitizers that increase the sensitivity of DNA (or RNA) to ionizing radiation. In the case of positive ion formation the ionization efficiency curves are measured near the threshold and the corresponding appearance energies are determined using a non-linear least square fitting procedure. The present studies on halogenated uridine show clearly that the effect of radiosensitization is predominantly the result of an increased cross-section for negative ion formation. The anion yields are investigated in the electron energy range from about 0 to 14 eV. The results confirm that also the nucleoside BrUrd is very sensitive upon low energy dissociative electron attachment with Br^- as the dominant reaction product. From the comparison of the present results for negative and positive ions information concerning the underlying mechanism of radiosensitizers is deduced

[en] 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. (authors)

[en] In the present study we investigate free electron attachment to the amino acid valine. Mass spectra and anion efficiency curves are measured in the electron energy range from about zero eV to about 15 eV and the anionic fragments are analyzed with a double focusing mass spectrometer. The high sensitivity of the present setup allows the detection of 10 fragment anions that have not been reported before and the high mass resolution of our sector field mass spectrometer allows us the separation and identification of isobaric anions. Thus the isobaric ion pairs, CN^-/C₂H₂^-, and O^-/NH₂^-, can be identified and assigned to individual resonances. For some of the heavier fragment anions formed we have studied collision induced dissociation to collect more information on the structures of these anions. (authors)

[en] Creating suitable chemical categories and developing read-across methods, supported by quantum mechanical calculations, can be an effective solution to solving key problems related to current scarcity of data on the toxicity of various nanoparticles. This study has demonstrated that by applying a nano-read-across, the cytotoxicity of nano-sized metal oxides could be estimated with a similar level of accuracy as provided by quantitative structure-activity relationship for nanomaterials (nano-QSAR model(s)). The method presented is a suitable computational tool for the preliminary hazard assessment of nanomaterials. It also could be used for the identification of nanomaterials that may pose potential negative impact to human health and the environment. Such approaches are especially necessary when there is paucity of relevant and reliable data points to develop and validate nano-QSAR models. (paper)

[en] New measurements of the cross-section for electron impact ionization of the molecular ion C₂H₂⁺ have been carried out recently. These data differ significantly from earlier data, because cross-sections corresponding to all the possible dissociative ionization processes were determined. The new data in conjunction with the significant discrepancies between the earlier data and the results of various calculations, which disagreed among themselves by a factor of 3, motivated a renewed attempt to apply the semi-classical Deutsch-Mark (DM) formalism to the calculation of the absolute electron-impact ionization cross-section of this molecular ion. A quantum chemical molecular orbital population analysis for both the neutral molecule and the ion revealed that in the case of C₂H₂⁺ the singly occupied molecular orbital (i.e. the 'missing' electron) is highly localized near the site of a C atom in the molecule. This information is explicitly incorporated in our formalism. The results obtained by taking the ionic character directly into account are in excellent agreement with the recent experimental data. (authors)

[en] Ionization and fragmentation of water and uracil molecules was studied both by electron and proton impact. A special coincidence technique allows on an event by event basis the investigation of product ions formed upon the collision of protons with neutral molecules including the identification of the charge state of the projectile. This enables the characterization of the ionization processes occurring, i.e. direct ionization, single electron capture or double electron capture for 0, 1 or 2 electrons that are transferred from the target to the projectile, respectively. For uracil the fragmentation patterns obtained by electron and proton impact ionization reveal close similarities and indicate a comparable amount of excitation for the two different ionization mechanisms at high enough projectile energies. (authors)