Site selective and temperature dependent electron attachment

Collisions between electrons and molecules initiate and drive many important chemical reactions associated with radiation physics and chemistry, environmental physics and chemistry, plasma-enhanced chemical vapor deposition, plasma processing of materials for microelectronic devices and many other applications. Moreover, life sciences are a rapidly growing area where the important role of electron driven reactions is only now beginning to be recognized. In addition, electrons and their interaction also drive key reactions in chemical synthesis, in planetary atmospheres and in plasmas used in environmental remediation applications. Moreover, electron attachment and electron ionization studies are of fundamental importance to the understanding of electron-molecule interactions and the mechanisms of formation and the properties of negative and positive ions. Nonetheless, as concluded in recently held workshops (i) sponsored by US government agencies and (ii) carried out within several EU networks, relatively little is known even nowadays about in particular about electron attachment and properties of negative molecular ions. Thus it was recommended that a large scale effort should be mounted to solve these problems in a timely way. The present project is one in an continuing effort from our laboratory to improve knowledge on molecular anions. In the present study we plan to investigate electron attachment to a series of biomolecules, to some selected halohydrocarbons and hydrogen halides. The present study will be based on previous experience and several exploratory studies in our laboratory concerning electron attachment and electron ionization of molecules/clusters. In the course of these studies we have developed various experimental techniques (including a trochoidal monochromator and a hemispherical monochromator both coupled in a crossed beam arrangement to a quadrupole mass spectrometer plus a number of various molecular beams sources custom developed in our lab) allowing us to investigate with high energy resolution and high sensitivity electron attachment as a function of electron energy in the low energy regime. Electron induced decay reactions including kinetic energy release distributions for fragment ions can be studied in a newly constructed three sector field mass spectrometer. A major part of the present project will be devoted to the investigation of biomolecules including nuclear bases (NB), modified nuclear bases (partly and fully deuterated and methylated) and combinations of nuclear bases (NB combined with sugars, Watson Crick pairs, NB clusters). The present project will follow up recent pioneering experiments on nuclear bases and plasmid DNA revealing some rather novel and unexpected phenomena in DEA including for instance site and bond selective dissociative electron attachment. Besides being of an intrinsic and fundamental importance (in terms of the physics and chemistry of DEA), the present experiments with biomolecules are of applied interest to possible elucidate the initial mechanisms of cell damage. There exist attempts to assign SSBs to the initial electron attachment to nucleobases followed by bond rupture of the sugar- phosphate backbone. The present experiments with modified nucleobases and DNA compounds (nucleosides, nucleobase-pairs) can (i) elucidate the molecular mechanism, (ii) identify the most fragile site upon electron attachment (site and bond selectivity of DEA), and (iii) contribute to the development of a model of radiation damage on the nano scale. Clearly, the development of radiation therapy and the need for radiation protection will eventually profit from the detailed knowledge of the processes occurring on a molecular level. Two other subprojects will be concerned with more fundamental questions of the dynamics and mechanism of electron attachment to molecules. For this purpose we will systematically investigate the temperature dependence of electron attachment to HBr and DBr and to some selected halohydrocarbons using a recently developed temperature controlled molecular beam source. The hydrogen halide studies will allow for the first time a direct comparison and possible confirmation with the shapes of the cross-sections predicted with the nonlocal resonance model of Horacek and co-workers as a function of temperature. Having tested this theory with these simple, initially diatomic, systems, will eventually allow extension to theoretical models for DEA which are desired for the larger molecules. The halohydrocarbon XM studies will give us information about the interplay between autodetachment and dissociative attachment reaction as a function of (i) the activation energy and of (ii) the coupling between the initial electronic state of the intermediate ion XM-# and the final antibonding state that leads to formation of X- . For this purpose we will select on the one hand a set of three unsaturated cyclic chloro- hydrocarbon molecules with progressively decreasing coupling between the LUMO state and the antibonding s* (C- Cl - ) state and on the other hand use analogue molecules where hydrogen atoms are replaced by halogen atoms.

Inelastic interaction of low-energy electrons with molecules often leads to the formation of negatively charged ions. This process is of fundamental relevance for several applied research areas, including radiation physics and chemistry, environmental physics and technical plasma physics. Furthermore, electron attachment plays an important role in the synthesis and destruction of molecules in dense interstellar clouds. In 2005 experiments in Innsbruck demonstrated for the first time that low-energy electrons can break selectively individual bonds in complex (bio-)molecules just by appropriate choice of their kinetic energy. Besides the kinetic energy of the electron also the internal energy of the molecule has a strong influence on the formation of negatively charged ions and, in particular, also on the subsequent fragmentation of these anions. In the present project electron attachment to molecules at different temperatures was investigated in detail. Temperatures between 0.37K (inside helium nanodroplets) up to 820K (utilizing heated capillaries) were accessible. At the low end of this temperature range all molecules are in their vibrational ground state whereas at high temperatures vibrationally excited molecules play an important role. For most halogenated hydrocarbon molecules that are of relevance for the destruction of the ozone layer we observed a strong enhancement of the anion production at low electron energies for hot molecules. In the case of the explosive trinitrotoluene fragmentation upon electron attachment was completely quenched inside helium nanodroplets. Specially designed and costums built instruments that were developed over the last years in our institute were utilized for the present investigations. These experimental setups allow investigations with high energy resolution, high sensitivity and good accuracy. Several fundamental results could be obtained in the present project that were published in 67 papers, 44 of those in refereed papers, including the most relevant journals of physics (Phys. Rev. Letters, Phys. Chem. Chem. Phys.) and chemistry (Angew. Chem. Int. Ed., J. Am. Chem. Soc., Chem. Eur. J., ChemPhysChem). The principal investigator and his co-workers were invited to talk at 30 international conferences about the results obtained in this project, in addition numerous further conference contributions were presented. Besides the academic interest the present results provide important input for applied topics such as radiotherapy and material technology.