Inelastic interaction of electrons with size selected neutral clusters and with molecular ions

The experimental and theoretical investigation of clusters of atoms or molecules has become an important, extremely lively and rapidly growing area of chemical physics in the past years. Today it is possible to produce clusters of a large variety thus allowing the investigation of almost every material of interest. Nevertheless, one sort of cluster target still has eluded widespread investigation despite the enormous importance of these studies, i.e., the study of mass selected neutral cluster beams due to the great difficulties in generating such target beams. One aim of the present project is the generation of mass resolved neutral cluster beams and the quantitative study of the interaction of these beams with electrons (including the measurement of ionization cross sections, appearance energies, and attachment cross sections). The second line of research in this research project - somehow related with the first - is the generation of quantitative data concerning electron induced ionization and dissociative ionization of mass selected molecular ions and mass selected cluster ions and the elucidation of the underlying reaction mechanisms, dynamics and energetics. Both goals are on the one side of fundamental importance, but at the same time have also a direct link to applied research, as some of these data are needed in laboratory and industrial plasma physics and chemistry. The neutral and ionized targets to be investigated will be selected according to these goals and will include atmospheric gases, fusion plasma edge impurities, carbon, metals and PAH`s. The present investigations involve the innovative use and partial extension of an existing ion-source/mass spectrometer system employing novel measuring techniques. Based on construction work during the PhD thesis of the applicant a three sector field mass spectrometer is now available which will allow to investigate electron interaction (using newly constructed high performance electron guns) with mass selected ion beams at different time windows after primary ionization with high sensitivity. This enables us to measure accurate (i) kinetic energy release distributions for spontaneous and electron-induced decay reactions of positive and negative ions as a function of time, and to measure (ii) partial ionization cross sections and appearance energies for electron dissociation and electron ionization of mass selected positive and negative ions. Moreover, in a further extension of existing techniques neutral mass selected cluster beams will be (i) produced in this apparatus using one high power electron gun to detach electrons from mass selected cluster anions (alternatively photodetachment will be employed) and (ii) then investigated (electron ionization and electron attachment) using a second electron gun.

We investigated successfully the generation of quantitative data concerning electron induced ionization and dissociation of mass selected molecular ions and mass selected cluster ions and the elucidation of the underlying reaction dynamics and energetics, by improving the existing three sector field mass spectrometer in many steps. Due to the improved accuracy and sensitivity we have now the possibility to assign decay products of small mass selected molecular ions to specific transitions of the excited parent ion. This enables us to check independently from spectroscopic methods the quality of calculated potential energy curves. The relevance of our results can be recognized by the fact that our first investigations were published in Physical Review Letters. We found that argon dimer ions show on the s time scale only a radiative decay followed by fragmentation, whereas the neon dimer ions decay in the same time window also by a non-radiative decay caused by electronic predissociation. Using the high resolution mass spectrometer for the selection of parent ions we could extend our previous experiments on neon and krypton cluster ions carried out with a two sector field spectrometer. Those experiments were limited to relatively small cluster ions, because the "mass" selection by the magnetic field alone does not permit selection of individual isotopomers for elements like Kr that have many naturally occurring isotopes, except for very small cluster sizes. The possible superposition of different reaction products from different parent ion masses makes it nearly impossible to analyze accurately the fragment ion peak, especially because an additional difficulty arises from the fact that larger cluster ions have different isotopic compositions even if they have the same mass to charge ratio. Due to different compositions of the selected parent ion the evaporated monomers can have different masses leading to different partial fragment peaks. This leads to an additional modification of the shape of the fragment peak, but if all isotopomer probabilities are taken into account in the analysis, it is possible to calculate the various positions in the fragment ion spectrum and the relative abundances of the various contributions and thus disentangle the different contributions. Thus we could determine the average kinetic energy release for all decay reactions of a given cluster ion and in a further step calculate the binding energies of these cluster ions by applying finite heat bath theory. Due to the fact that we investigated very successfully the decay of the C60 ions for many years in order to determine the correct binding energy of the C60 we have now the expertise to apply the same technique also to other systems. . Furthermore we have now the possibility to measure also systems, which are not only produced in the ion source but also in decay reactions of larger ions. If we select parent ions with the magnetic and the first electrostatic sector of our three sector field machine we have no interference in the measured spectrum with coinciding decay reactions from the first field free region. The fragment ion scan can be performed on an absolutely clean mass peak selected by the high resolution mass spectrometer. We analysed the fragmentation of C3 H5 ++ and one can nicely observe the problem of coincidences with decay reactions happening in ff1 when the mass selection is only done by the magnetic field. However nocoincidences were observed when we selected the parent ions with two sector fields. The fact that both fragment ions from a coulomb explosion can be detected (both are charged) enabled us to determine the overall kinetic energy release of the decay reaction by analysing two fragment ion peak shapes. We found perfect agreement of the two results which demonstrates the reliability of our data analysis. It was also interesting to compare the experimentally determined KER with calculations based on the potential energy curves of the C3 H5 ++, because it turned out that all of the available energy goes to kinetic energy of the departing fragments.