Atomic site sensitive processes in low energy ion-dimer collisions

Electron capture processes for low energy Ar9+ ions colliding on Ar2 dimer targets are investigated, focusing attention on charge sharing as a function of molecule orientation and impact parameter. A preference in charge-asymmetric dissociation channels is observed, with a strong correlation between the projectile scattering angle and the molecular ion orientation. The measurements provide here clear evidences that projectiles distinguish each atom in the target and, that electron capture from near-site atom is favored. Monte Carlo calculations based on the classical over-the-barrier model, with dimer targets represented as two independent atoms, are compared to the data. They give a new insight into the dynamics of the collision by providing, for the di erent electron capture channels, the two-dimensional probability maps p(~b), where ~b is the impact parameter vector in the molecular frame

Anion emission from water molecules colliding with positive ions: Identification of binary and many-body processes

It is shown that negative ions are ejected from gas-phase water molecules when bombarded with positive ions at keV energies typical of solar-wind velocities. This finding is relevant for studies of planetary and cometary atmospheres, as well as for radiolysis and radiobiology. Emission of both H- and heavier (O- and OH-) anions, with a larger yield for H-, was observed in 6.6-keV 16O+ + H2O collisions. The ex-perimental setup allowed separate identification of anions formed in collisions with many-body dynamics from those created in hard, binary collisions. Most of the ani-ons are emitted with low kinetic energy due to many-body processes. Model calcu-lations show that both nucleus-nucleus interactions and electronic excitations con-tribute to the observed large anion emission yield.Comment: 5 pages, 4 figure

Near-infrared photoabsorption by C(60) dianions in a storage ring

We present a detailed study of the electronic structure and the stability of C(60) dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least similar to 1.5 eV. For C(60)(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C(60)(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C(60) to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11 500 cm(-1) to 13 500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C(60)(-) may also be a transition to a cone state

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

Interatomic Coulombic Decay as a New Source of Low Energy Electrons in slow Ion-Dimer Collisions

We provide the experimental evidence that the single electron capture process in slow collisions between O$^{3+}$ ions and neon dimer targets leads to an unexpected production of low-energy electrons. This production results from the interatomic Coulombic decay process, subsequent to inner shell single electron capture from one site of the neon dimer. Although pure one-electron capture from inner shell is expected to be negligible in the low collision energy regime investigated here, the electron production due to this process overtakes by one order of magnitude the emission of Auger electrons by the scattered projectiles after double-electron capture. This feature is specific to low charge states of the projectile: similar studies with Xe$^{20+}$ and Ar$^{9+}$ projectiles show no evidence of inner shell single-electron capture. The dependence of the process on the projectile charge state is interpreted using simple calculations based on the classical over the barrier model

Fragmentation of α- and β-alanine molecules by ions at Bragg-peak energies

The interaction of keV He(+), He(2+), and O(5+) ions with isolated alpha and beta isomers of the amino acid alanine was studied by means of high resolution coincidence time-of-flight mass spectrometry. We observed a strong isomer dependence of characteristic fragmentation channels which manifests in strongly altered branching ratios. Despite the ultrashort initial perturbation by the incoming ion, evidence for molecular rearrangement leading to the formation of H(3)(+) was found. The measured kinetic energies of ionic alanine fragments can be sufficient to induce secondary damage to DNA in a biological environment. (C) 2008 American Institute of Physics

Anion and cation emission from water molecules after collisions with 6.6-keV 16 O+ ions

DOI: https://doi.org/10.1103/PhysRevA.100.032713arXiv link: http://arxiv.org/abs/1910.00657International audienceAnion and cation emission following water dissociation was studied for 6.6-keV $^{16}$O$^{+}$ + H$_{2}$O collisions. Absolute cross sections for the emission of all positively and negatively charged fragments, differential in both energy and observation angle, were measured. The fragments formed in hard, binary collisions appearing in peaks were distinguishable from those created in soft collisions with many-body dynamics that result in a broad energy spectrum. A striking feature is that anions and cations are emitted with similar energy and angular distributions, with a nearly constant ratio of about 1:100 for H$^{-}$ to H$^{+}$. Model calculations were performed at different levels of complexity. Four-body scattering simulations reproduce the measured fragment distributions if adequate kinetic-energy release of the target is taken into account. Providing even further insight into the underlying processes, predictions of a thermodynamic model indicate that transfer ionization at small impact parameters is the dominant mechanism for H$^{+}$ creation. The present findings confirm our earlier observation that in molecular fragmentation induced by slow, singly charged ions, the charge states of the emitted hydrogen fragments follow a simple statistical distribution independent of the way they are formed