Using a crossed electron-molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CHCl3+e→Cl− at electron energies from about 0 to 2 eV and in a target gas temperature range of about 300–430 K. The energy resolution and working conditions of this newly constructed crossed beams machine have been characterized using CCl4 as a test and calbrant gas. Utilizing the improved energy resolution of the present experimental setup (which allows measurements with FWHM energy spreads down to below 5 meV) it was possible to determine the accurate shape and magnitude of the cross section function in the low-energy range. This leads to the conclusion that between an electron energy of about 20 and 130 meV the reaction proceeds via deBroglie s-wave capture, whereas at higher energy (above about 0.4 eV) autodetachment plays a significant role. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, the number of peaks and in the energy position of these peaks. Finally, by analyzing the measured strong temperature dependence of the cross section close to zero electron energy the activation barrier for this dissociative attachment was determined to be 110±20 meV in good agreement with thermochemical data from swarm experiments performed under thermal equilibrium. Taking into account the present results it is also possible to discuss the mechanism for the existence of the second peak.

Using high sensitivity two sector field mass spectrometric techniques (in particular MIKE scans) we have studied quantitatively (and systematically) the energetics of the superasymmetric spontaneous decay reactions (involving either C2+ or C4+ loss) of triply, quadruply, quintuply, and sextuply charged carbon clusters ions Cnz+ in the size range from n=36 up to n=70. From the kinetic energy release data determined, the apparent intercharge distance has been derived using different models including the simple point charges model, the movable charges model and the charged conducting sphere model. As in earlier but less extensive studies the intercharge distance obtained is for all three models used larger than the cage radius of the respective precursor fullerene ion. It is shown that this and other experimental results are only compatible with the recently suggested auto charge transfer (ACT) reaction as the decay mechanism responsible for the superasymmetric charge separation reactions, whereas two other conceivable decay mechanisms (ball-chain-propagation and decay of charged conducting liquid sphere) are not consistent with all of the experimental fingerprints observed.

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The production of O 2 and O 2 2 by dissociative electron attachment to ozone is reported for incident electron energies between 0 and 10 eV. A previously unobserved sharp structure is observed in the formation of O 2 ions at zero incident energy. This large additional cross section peak has important consequences for the role of ozone in the anion formation processes in the ionosphere.[ S0031-9007(99) PACS numbers: 34.80.Ht, 94.10.Fa The discovery of the "ozone hole" above Antarctica by Farmer and co-workers in 1985 has led to a major international research program to study the chemical reactions of ozone responsible for such dramatic ozone loss [1]. The catalytic destruction of ozone by halogen free radicals is now largely understood, and the major mechanisms by which ozone is destroyed in the stratosphere have been identified [2]. The role of ozone in the D region in the ionosphere is, however, less well established. The physics and chemistry of the D layer of the ionosphere are dominated by ion-molecule reactions and, in particular, by the formation rates of CO 3 2 , HCO 3 2 , and NO 3 2 anions [3]. Current models of the D layer [3] assume that the major negative ion formation process in this region arises from the exothermic nondissociative three-body electron attachment to molecular oxygen [4], i.e.,with CO 3 2 , HCO 3 2 , and NO 3 2 anions being formed subsequently in a complicated sequence of reactions involving neutral constituents [5]. The dissociative electron attachment reaction to molecular oxygen is not considered in these models since the cross section is significant only at electron energies above about 4 eV [6].Conversely, electron impact dissociative attachment to ozone may also form significant concentrations of oxygen anions at about thermal energies. Two possible dissociative attachment channels first observed by CurranThe product oxygen anions may then be subsequently lost either by associative detachment with atomic or molecular oxygen or by charge transfer reactions with ozone. The O 3 2 anions produced in these charge transfer reactions may then undergo further reactions with H, CO 2 , and NO 2 to form HCO 3 2 ,CO 3 2 , or NO 3 2 . Current ionospheric models [3] have used the rate coefficients estimated from the early experiments of Curran [7] and Stelman et al. [8] (see also [9]) to compare the anion formation rate from electron attachment to ozone via the exothermic reaction (2a) with the three-body attachment to molecular oxygen via reaction (1). They concluded that the electron attachment process (2a) is only a minor contribution to the total oxygen anion production. Nevertheless, several groups [10][11][12][13][14] have recently reported new results on the mechanism of dissociative electron attachment to ozone and ozone clusters [15] which suggest that the role of dissociative electron attachment to ozone in the ionosphere should be reevaluated.Similarly the role of low energy electron attachment in those electrical discharges used to produce ozone in commercial apparatus (so-call...

Using two different crossed-beams high-resolution electron attachment instruments (employing either a trochoidal electron monochromator or a hemispherical electron monochromator) we have determined the cross section curve for H − production from H 2 via the 4 eV resonance at two different temperatures. These relative partial cross sections have been calibrated by comparing present values for the 14 eV resonance with absolute total cross sections determined previously. Taking into account the experimental energy distribution and the rotational excitation and its influence on the cross section shape we obtain very good agreement with theoretical predictions in terms of both the shape and magnitude of this resonance peak.