Elementary processes induced by the collision of electrons and ions at defined energy with thin molecular films.

In a common project between the Institut für lonenphysik, Universittit Innsbruck and the Institut für Physikalische Chemie, FU Berlin we want to investigate reactions which occur in collisions of electrons and ions (cations and anions) of well defined energy and high energy resolution in the energy range from close to 0 up to about 100 eV with thin molecular films. As surface targets we will use fluorinated self-assembled monolayers (F-SAM) as well as homogeneous and binary films. The latter consist in general of volatile molecules and are condensed cryogenically in well defined amounts and composition on cold metal surfaces. We intend to measure and study in detail and in particular as a function of the projectile`s energy the electron stimulated desorption (ESD) of ions (in Berlin) and also the product ions which are backscattered from the film surface (in Innsbruck). In both laboratories the adsorbates will be analysed via thermal desorption spectroscopy (TDS). In addition the degree of charging of the adsorbate as well as its composition after electron beam exposure with a well defined electron energy will be studied in Berlin via infrared-reflection-absorption-spectroscopy (IRAS). The aim of the common studies is the identification and investigation of reactions in the static surface target induced by different projectiles (electrons, anions, cations) of comparable collision energy, with the perspective to shed light into the elementary mechanisms of the underlying complex energy-, charge- and particle- transfer processes, as well as to use such processes for the controlled synthesis of molecules and also for the modification of surfaces.

During the investigation of the stability and binding energies of rare gas cluster ions we observed an unexpected metastable dissociation of free dimer ions, i.e. ions that consist of only two atoms. In contrast to larger clusters it is not possible that energy that is necessary for the dissociation is distributed among several vibrational degrees of freedom. Furthermore, the low appearance energy that we measured for this decay process, i.e. the minimum electron energy that is needed to generate a metastable dimer ion, excludes the activation of electronically excited atoms. Based on potential energy curves for the dimer ions that were obtained by high level quantum chemical calculations and a reasonable thermal energy distribution of the neutral precursor clusters we derived via the population and lifetime of excited states of the dimer ion the kinetic energy distribution of the fragment ions. In the case of argon and neon we achieved perfect agreement with highly precisely measured energy distributions. In addition, it turns out that the presently developed technique is a sensitive probe for potential energy curves and can be used to evaluate the quality of high level quantum theoretical calculations. A close cycle cryostat that was granted in the present project was used to develop a cluster source that can generate clusters and nano-droplets of helium. In the meantime we determined the magic numbers, i.e. cluster sizes that are particularly abundant in the mass spectra, binding energies and appearance energies of helium cluster ions. For this purpose we utilized two instruments that are especially dedicated to measure these properties. In the future it is intended to utilize this helium cluster source to pick up hot gas phase biomulecules in liquid He-droplets and cool them down by vaporization of helium atoms. Afterwards the interaction of electrons with these ultra cold biomolecules will be investigated. Parallel to these investigations of cluster ions a recently obtained second-hand, high resolution mass spectrometer was commissioned for the needs in Innsbruck. A new inlet for biomolecules was built and the hardware and software to control the instrument via PC and Labview were developed. In the meantime first measurements demonstrated the high energy and mass resolution of this mass spectrometer that will be needed for future investigations of molecules of biological relevance.