Electron driven reactions in doped He nanodroplets
Prof. Paul Scheier
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck
17:15 - 18:15 Tuesday 03 November 2009 TUG P2 Positively and negatively charged cluster ions are formed upon collisions of free electrons with doped He nanodroplets. These ions are analyzed by mass spectrometry and their stability is probed via MS-MS techniques. The main ionization process for cations is charge transfer from an initially formed He+ to the embedded cluster. The large ionization energy of He enables double ionization for several dopants which subsequently leads to Coulomb explosion and characteristic product ion distributions. Surprisingly rich chemistry that requires multiple bond cleavages and rearrangement is observed [1].
Negative-ion mass spectra are recorded at the electron energies where resonances appear. The yield of anions is determined as a function of the electron energy. Recently we have published the first mass spectrometric investigation of negative ions formed via electron attachment to molecules embedded in helium clusters [2]. The molecules chosen for this first study were the nucleobases adenine, thymine and partially methylated or deuterated thymine. The DEA to these molecules in helium droplets (see [2]) exhibits the remarkable site selectivity that we have previously established for these biomolecules in the gas phase [3]. However, low-mass fragments formed via core excited resonances in gas phase are almost completely quenched and lead to the formation of dehydrogenated closed shell anions via loss of a neutral H-atom. Such quenching of relatively slow decay reactions provides a unique method to follow the fragmentation pathway of excited molecules in slow motion.
Acknowledgements: This work has been supported by the FWF, Wien, Austria and the European Commission, Brussels.
References
[1] S. Denifl et al., Angew. Chem. Int. Ed. in print (2009).
[2] S. Denifl et al., Phys. Rev. Lett. 97, 043201 (2006).
[3] S. Ptasinska et al., Phys. Rev. Lett. 95, 093201 (2005).
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