Bond and site selective dissociative electron attachment

Laboratory measurements are proposed to investigate dissociative electron attachment (DEA) to DNA bases and nitrotoluene and their derivatives. For most of the biologically relevant molecules studied so far, neutral H loss and the formation of H- are important DEA channels. Very recent studies on the H- loss from several organic molecules, including the phyrimidine bases uracil and thymine, demonstrate that by tuning the electron energy it is possible to selectively break a specific bond. Isotopically labeled molecules and methylated derivates enable experimentally the assignment of resonances to bond breaking at specific molecular sites. In the presently proposed studies it is planned to investigate site selectivity of neutral H and H - loss from the other DNA bases, i.e., adenine, cytosine and guanine. For NO 2 - , the most abundant fragment anion formed via DEA to various nitrotoluene molecules, site selectivity has been described in the literature, i.e., the energy and intensity of the resonances depend strongly on the position of the NO 2 group relative to the methyl group. For several heavy fragment anions formed via DEA to various nitrotoluene molecules we discovered similar site selectivity. Again labeling with isotopes and functional groups is planned to assign the resonances in the attachment cross sections to bond rupture at a specific site. A pulsed hemispherical electron monochromator will be built. The interaction region between electrons and neutral target molecules will be field-free when electrons are present which is essential for high electron energy resolution. However, a strong extraction field in the time when the electron beam is off can be used to collect all ions with uniform efficiency, even if they are formed with a high kinetic energy. Furthermore, it is planned to utilize this pulsed ion source to measure the kinetic energy that is released in the DEA process. In the case of one atomic fragment, such as the H- formation, the kinetic energy of the anion determines the internal energy that remains in the heavy fragment. In extension to simple gas phase experiments it is planned to study free electron attachment to clusters of the molecules mentioned above or to molecules embedded in a well defined solvent layer. For this purpose two recently constructed neutral beam sources will be utilized, i.e., a helium cluster source and a spray source. Thereby the signature of site selective DEA channels on secondary reaction products resulting from intra-cluster anion molecule reactions will be studied. In the case of biomolecules clusters and complexes with water will bridge the gap between gas phase studies and the investigation of low-energy electron induced DNA strand breaks. Furthermore, free electron interactions with ultra-cold targets of water and biomolecules, formed via pick-up of superfluid He droplets, can give important insight into the formation, liberation and destruction of complex organic molecules in dense interstellar clouds.

Site selective control of chemical reactions is an ultimate goal for the synthesis of complex molecules and several approaches have been proposed. Lasers (coherent control), soft x-ray beams as well as scanning tunneling microscopes have been utilized to break individual bonds. Low energy electron collision on molecules resulting in dissociative electron attachment (DEA) is another technique that can lead to site-selective fragmentation of molecules. In case of the DNA base thymine cleavage of each of the four different hydrogen atoms could be demonstrated with 100% selectivity, simply by choosing the appropriate electron energy. The research funded by this project was extending our knowledge about site selective cleavage of bonds in fairly complex molecules by electrons. We investigated for the first time the effect of functional groups and isomeric structure on DEA to DNA bases and amino acids. In addition, we discovered that molecular ions lose recollection of their formation mechanism, i.e., deprotonation by MALDI and dehydrogenation via free electron attachment at the same site leads to the same fragmentation pattern. Parallel to these experiments on isolated molecules we continued to explore anion formation upon electron irradiation of doped helium nanodroplets. In case of water clusters we discovered that pickup of individual water molecules into a superfluid He droplet leads to neutral isomers that readily can attach an electron for all cluster sizes, quite opposite to water clusters formed by conventional techniques. In superfluid He droplets as well as in the interstellar medium collision complexes completely relax prior the next attachment of a molecule. From these results we conclude that ice formed in cold molecular clouds in the interstellar medium will exhibit structures similar to the clusters formed in He droplets, but different from all conventionally formed ices. Motivated by this idea we developed a new instrument to explore ion formation via electron bombardment of doped He nanodroplets in more detail (higher mass resolution and higher sensitivity). We discovered a new ionization mechanism, where an electronically excited species is dragged by polarization forces to a preformed ion. This kind of energy harvesting is of relevance in all condensed systems where long-lived metastable species can move directly or via exciton hopping. With this new instrument we also discovered that cold fullerenes are readily covered with molecules and atoms. Physisorption of hydrogen or other simple molecules will also happen in the cold interstellar medium and may explain why fullerenes have been observed in hot clouds by astronomers only.