Interatomic Coulombic decay of clusters upon electron impact

Interatomic (or Intermolecular) Coulombic Decay (ICD) was discovered to be an important phenomenon in the interaction of photons or energetic alpha-particles with weakly bonded systems like clusters. ICD represents in clusters a relaxation mechanism of a single electronic vacancy. Electronic energy deposited in an atom or molecule by initial ionization or excitation may subsequently be transferred to a neighbour. The latter becomes ionized, i.e. electronic energy is converted into kinetic energy of positive ions and an electron in the continuum. While detailed theoretical descriptions of ICD have been developed and moreover, a plethora of experimental studies using photo- ionization have been carried out in the meantime, no systematic investigations on electron induced ICD exist. We will investigate here if ICD is induced in clusters via inner (valence) shell excitation or ionization by electron impact. The clusters investigated will be rare gas clusters and subsequently, molecular clusters. ICD induced Coulomb explosion of clusters will be monitored utilizing mass spectrometric techniques, in which the kinetic energy of cluster fragment ions will be determined as a function of the initial electron energy. Another process studied in this project will be Interatomic Coulombic Electron Capture (ICEC). In this reaction the excess energy deposited by an electron, which is attached to an atom of a cluster, is released by removal of an electron from a neighbour in the cluster. While theory providing absolute ICEC cross sections as the function of the initial electron energy has been developed for this process recently, the experimental verification is eagerly awaited. The latter is intended in the present project for related systems studied by theory, like for example alkaline earth metal dications surrounded by few water molecules. It was also theoretically predicted that the ICEC cross section should linearly increase with the number of neighbouring water molecules, which will be experimentally tested here. Similar, but akin to ICD, is fragmentation of dopants sequentially ionized in helium droplets, which will be another research objective in the present project. In this case creation of multiple vacancies in doped helium droplets induced by, for example, two metastable helium atoms will lead to charge separation processes investigated here. Comparison with ICD and ICEC processes will proof the strong chemical diversity of electron collision processes. Moreover, the present studies will also have strong implications in electron collision physics since new fundamental pathways of electron induced reactions in matter will be elucidated.

It is known that low-energy electrons effectively and selectively cleave chemical bonds in matter. This is also the case for biological tissue and therefore low-energy electrons are responsible in significant extents for the radiation damage by radioactivity and x- rays. Low-energy electrons are formed by highly energetic radiation passing matter and cause damage of biological material on the molecular level. However, the exact reaction pathways leading to the formation of these low-energy electrons as well as the damage of biological tissue by these particles is not fully clarified yet. Within this project we experimentally confirmed a fundamental reaction pathway by low-energy electrons. This was possible by the utilization of a reaction microscope, which collects all charged particles after the reaction. The studied reaction was the so called Interatomic Coulombic Decay, which is possible for neighboring atoms or molecules. A free low-energy electron removes a bound electron from the atom and further deposits excess energy in the ionized atom. This excess energy is transferred to the neighboring atom, where another electron is emitted. Since two atoms have lost an electron, they start to strongly repel each other and move in opposite directions. This experiment showed that a single electron produces by this reaction two low- energy electrons and two ions, which will subsequently induce damage in matter. In the second subproject reactions initiated by electron capture were investigated. In the course of the present experiments a suggested pathway to electron induced double strand breaks in hydrated DNA was experimentally confirmed for the first time. Experiments with hydrated building blocks of DNA showed that the hydration is essential for this reaction mechanism. Electron capture leads to the generation an OH- radical at the initial site of electron capture. This radical leads to the formation of the first strand break. In addition, the captured electron is released again and this electron completes the double strand break after attaching to the opposite strand. Based on these results significant knowledge concerning the action of low-energy electrons on the molecular level was found.