Interactions of neutral and singly charged keV atomic particles with gas-phase adenine molecules

KeV atomic particles traversing biological matter are subject to charge exchange and screening effects which dynamically change this particle's effective charge. The understanding of the collision cascade along the track thus requires a detailed knowledge of the interaction dynamics of radiobiologically relevant molecules, such as DNA building blocks or water, not only with ionic but also with neutral species. We have studied collisions of keV H+, He+, and C+ ions and H-0, He-0, and C-0 atoms with the DNA base adenine by means of high resolution time-of-flight spectrometry. For H-0 and H+ we find qualitatively very similar fragmentation patterns, while for carbon, strong differences are observed when comparing C-0 and C+ impact. For collisions with He-0 and He+ projectiles, a pronounced delayed fragmentation channel is observed, which has not been reported before. (C) 2007 American Institute of Physics

Ion induced fragmentation of biomolecular systems at low collision energies

In this paper, we present results of different collision experiments between multiply charged ions at low collision energies (in the keV-region) and biomolecular systems. This kind of interaction allows to remove electrons form the biomolecule without transferring a large amount of vibrational excitation energy. Nevertheless, following the ionization of the target, fragmentation of biomolecular species may occur. It is the main objective of this work to study the physical processes involved in the dissociation of highly electronically excited systems. In order to elucidate the intrinsic properties of certain biomolecules (porphyrins and amino acids) we have performed experiments in the gas phase with isolated systems. The obtained results demonstrate the high stability of porphyrins after electron removal. Furthermore, a dependence of the fragmentation pattern produced by multiply charged ions on the isomeric structure of the alanine molecule has been shown. By considering the presence of other surrounding biomolecules (clusters of nucleobases), a strong influence of the environment of the biomolecule on the fragmentation channels and their modification, has been clearly proven. This result is explained, in the thymine and uracil case, by the formation of hydrogen bonds between O and H atoms, which is known to favor planar cluster geometries.</p

Electron-correlation effects in appearance-potential spectra of Ni

Spin-resolved and temperature-dependent appearance-potential spectra of ferromagnetic Nickel are measured and analyzed theoretically. The Lander self-convolution model which relates the line shape to the unoccupied part of the local density of states turns out to be insufficient. Electron correlations and orbitally resolved transition-matrix elements are shown to be essential for a quantitative agreement between experiment and theory.Comment: LaTeX, 6 pages, 2 eps figures included, Phys. Rev. B (in press

Mn12_{12}-Acetate Complexes Studied as Single Molecules

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn12 coordination clusters of general formula [Mn$^{III}$$_{8}$Mn$^{IV}$$_{4}$O$_{12}$(RCOO)$_{16}$(H$_{2}$O)$_{4}$] had been exemplified by bulk samples of the archetypal [Mn$^{III}$$_{8}$Mn$^{IV}$$_{4}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn12 system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{15}$(CH$_{3}$CN)]$^{+}$ (1) by gas phase X-ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{15}$(CH$_{3}$CN)]$^{+}$ (1), [[Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$] (2), [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$] (3), and the complex in bulk geometry [Mn$^{III}$$_{8}$Mn$^{IV}$$_{4}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$] (5). The found magnetic fingerprints – experiment and theory alike – are of a remarkable robustness: The Mn$^{IV}$$_{4}$ core bears almost no magnetic anisotropy while the surrounding MnIII8 ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn$_{12}$ core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects

Intramolecular hydrogen transfer in DNA induced by site selective resonant core excitation

We present experimental evidence for soft X ray induced intramolecular hydrogen transfer in the protonated synthetic tri oligonucleotide d FUAG in the gas phase FU fluorouracil . The trinucleotide cations were stored in a cryogenic ion trap and exposed to monochromatic synchrotron radiation. Photoionization and photofragmentation product ion yields were recorded as a function of photon energy. Predominanly glycosidic bond cleavage leading to formation of nucleobase related fragments is observed. In most cases, glycosidic bond cleavage is accompanied by single or double hydrogen transfer. The combination of absorption site sensitive soft X ray spectroscopy with fragment specific mass spectrometry allows to directly relate X ray absorption site and fragmentation site. We observe pronounced resonant features in the competition between single and double hydrogen transfer towards nucleobases. A direct comparison of experimental data with time dependent density functional theory calculations, using short range corrected hybrid functionals, reveal that these hydrogen transfer processes are universal and not limited to population of particular excited states localized at the nucleobases. Instead, hydrogen transfer can occur upon X ray absorption in any nucleobase and in the DNA backbone. Resonances seem to occur because of site selective suppression of hydrogen transfer channels. Furthermore, non covalent interactions of the optimized ground state geometries were investigated to identify intramolecular hydrogen bonds along which hydrogen transfer is most likel