[en] After many years of research the accurate determination and interpretation of threshold energies at which a molecule is ionized by electron impact remains still difficult. The reasons for this are a number of technical obstacles like preparing electrons with a high energy resolution and a complicated physical situation in the reaction complex involving a quantum mechanical many body system. The use of photoionization sometimes appears to be less difficult but nevertheless the values obtained by this technique are not directly comparable to those obtained by electron impact studies. With the use of a newly constructed hemispherical electron monochromator the interaction of electrons under high energy resolution (up to 30 meV) with atoms, molecules and clusters was investigated. In the present study two new techniques have been invoked to obtain more information about the energy resolution of the electrons. Up to now it was only possible to determine the electron energy resolution with the help of s-wave attachment cross sections, e.g. the Cl-/CCl4 resonance at 0 eV. The new techniques allow the investigation at higher energies (at about 12 up to about 58 eV) and by using positive ions. Especially in the case of measuring threshold energies of positive ions the new methods have the advantage that there is no need to change between positive and negative ions. Additionally one gets information about the calibration and the linearity of the energy scale. The value at which the resolution is determined lies also in the range of the threshold. The results show that the resolution is constant over a large electron energy range. Due to low ion signals at the threshold regions the used electron energy resolution was set at about 120 meV for most of the present measurements. In the present work it was for the first time possible to measure accurately the appearance energies for rare gas cluster ions (Ar, Ne) and for hydrogen cluster ions. There are two important observations: a) in the present study the appearance energy of the dimer ion is always equal or lower than the appearance energy of the trimer ion and b) the present appearance energies of larger clusters are shifted to higher values as compared to the photoionization data where available (e.g. in the case of Ar). Taking into account the present accuracy we are able to demonstrate that for rare gas clusters the electron impact ionization process proceeds in a different way than photoionization. Moreover, as there is no simple way to mass select neutral clusters before ionization also the amount of cluster ions that are produced from larger clusters by evaporation can influence the appearance energies. We have studied this phenomenon by changing the stagnation gas temperature (and therefore the cluster size distribution). We clearly observe a dependence that supports this idea. The evaluation of appearance energies was also tested by studying parent and fragment ions of methane and propane. Following this, the appearance energies of the greenhouse gases carbon tetrafluoride and trifluoromethane were measured. Both gases are used in the semiconductor industry and especially for the CHF3 only a few and limited data sets were available up to now. The present measurements improve and complete the tabulated values. The present set-up allows also the study of high resolution electron attachment reactions. This technique was applied to dissociative electron attachment to hydrogen. This process is an important source for large concentrations of H- in low temperature plasmas and the detailed knowledge of the formation is very important for modeling of such plasmas in thermonuclear fusion devices. It was possible to solve discrepancies between earlier experiments and theoretical results concerning the cross section of the electron attachment in the energy range of about 4 eV. With additional results from a trochoidal electron monochromator a detailed study on the influence of rotational levels and the width of the energy distribution was possible. A comparison between the present experimental results and theories allows the selection of theoretical results which should be used for modeling plasmas. In addition to these studies it was possible to analyze in detail results which where obtained earlier with an apparatus to study ion/surface collision processes. Thereby mass selected parent and fragment ions of C60 were collided with a hydrocarbon covered stainless steel surface. Afterwards the secondary products are analyzed with a time of flight mass spectrometer. Fragmentation and pick-up reactions have been studied as a function of the collision energy. Compared to C60+, ions with n = 58 fragment much more strongly. This is attributed to the fact that the initial excitation energy in these projectiles, which are formed from neutral C60 by electron impact ionization, is higher. However, identical values of 6.8 % are derived for the conversion efficiency of translational into internal energy. (author)

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[en] We have determined appearance energies AE(Xⁿ⁺/X) for the formation of multiply charged He, Ne, Ar, Kr and Xe ions up to charge state n=2 (He), n=4 (Ne), n=6 (Ar), n=6 (Kr) and n=8 (Xe) using a recently commissioned high-resolution electron impact ionization mass spectrometer. The data analysis is based on the Marquart-Levenberg algorithm, involving an iterative, non-linear least-squares fitting of the threshold data assuming a 2-function or a 3-function fit based on a Wannier-type power law. This allows us to extract the relevant AEs and corresponding Wannier exponents

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[en] In order to understand and elucidate the role of the radiative and collisional processes in the plasma edge region of fusion reactors, it is essential to have available a detailed and quantitative knowledge on these elementary processes such as cross sections, reaction rate coefficients etc. Much effort has been recently devoted to the experimental determination of absolute partial and total electron impact ionization cross sections of molecules and radicals due to the ever increasing importance of these cross sections in the diagnostics of many applications such as low- and high-temperature plasma physics and chemistry, atmospheric physics and mass spectrometry. In addition supporting calculations have recently been developed in order to allow the analytic quantitative description of these cross section functions for modelling codes using a novel approach to the determination of electron induced appearance energies. In this review we will therefore discuss (i) experimental studies on electron impact ionisation of neutral molecules and electron impact ionisation of molecular ions including the measurement of ionization cross sections and appearance energies and the determination of kinetic energy release distribution for dissociative ionization and (ii) the theoretical determination of electron impact ionisation cross sections for the single ionisation of molecules and the multiple ionisation of atoms. (author)