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No abstract available

[en] The threshold energy and the threshold behaviour for electron impact ionization of CH₄ were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron-molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum. The values of the ionization energies (IEs) and the threshold behaviour were determined using a fitting procedure involving a convolution of the cross section and the electron energy distribution function. At 293 K the following IEs were obtained: IE(CH₄⁺/CH₄) = 12.65 ± 0.4 eV, IE₁(CH₃⁺/CH₄) = 13.58 ± 0.1 eV (ion pair) and IE₂(CH₃⁺/CH₄) = 14.34 ± 0.1 eV. At 693 K a red shift in these IEs of about 0.14 eV was observed for both CH₄⁺/CH₄ and CH₃⁺/CH₄ processes, which reflects the change in the internal energy of CH₄ with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH₂⁺/CH₄) = 15.1 ± 0.1 eV, IE(CH⁺/CH₄) = 19.8 ± 0.1 eV and IE(C⁺/CH₄) = 20.5 ± 0.2 eV

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[en] Electron attachment to gas phase nitrobenzene, all three isomers of mononitrotoluene and 2,4,6-trinitrotoluene is studied by means of two crossed electron-molecular beam experiments. We point out the formation of long-lived metastable parent anions and the most abundant anions produced via dissociative electron attachment (DEA). The experimental results are supported by quantum-chemical calculations, to determine the electronic configuration of selected molecular orbitals or the electrostatic potential mapped on an isosurface of the total electron density to find preferential sites of electron attachment.

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No abstract available

[en] Mass spectrometric analysis of metastable decay reactions is devoted to the measurements of the kinetic energy release distribution (KERD) for the decay of singly charged rare gas dimer ions Xe₂⁺ and Kr₂⁺, the doubly charged acetylene parent ion C₂H₂²⁺ and the singly and doubly charged SF₆ fragment ions, like for example SF₃⁺, SF₃²⁺ and SF₄²⁺. The KERDs are obtained either from high-resolution mass analysed ion kinetic energy spectra or the measurement of ion beam profiles using a specially improved mass spectrometric system. Due to the high energy resolution measurements and theoretical studies based on ab initio calculations of potential energy curves it is possible to assign the reaction products of the rare gas dimer decays to electronic transitions in the excited parent ion. The C₂H₂²⁺ and also the SF₄²⁺ ions are investigated because of obscurities in the production of their fragment ions. The unusual shape of the SF₃⁺ ionization cross section indicates that at sufficiently high electron energies the fragmentation channel of doubly charged SF₄²⁺ contributes significantly to the ion yield. Additional measurements of the corresponding appearance energies confirm the existence of this second production channel

[en] In the present study, we investigate site selectivity of (multiple) hydrogen loss induced by free electron attachment to purine derivatives. Like for the pyrimidines here efficient hydrogen release is also observed upon low-energy electron capture. By utilizing partially deuterated or methylated derivatives of adenine, we show that the formation of the dehydrogenated parent anion (M-H)^- below 1.4 eV can be ascribed predominately to hydrogen loss from the N9 site. However, the (M-H)^- ion yield shows a strong influence on the functional groups attached to the opposite site of the N9 position. By means of quantum chemical calculations, we ascribe this result to a change in the electrostatic potential distribution for the different compounds. We also present site selectivity for the formation of (M-2H)^- and (M-3H)^- anions formed upon electron attachment to adenine.

[en] The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed