H. T. Schmidt

Complete Branching Ratios for the Dissociative Recombination of H2O+, H3O+, and CH3+

Dissociative recombination of the polyatomic ions H2O+, H3O+, and CH+3 with electrons has been measured at the heavy-ion storage ring ASTRID. Complete branching ratios for all the possible product channels have been determined at zero relative energy using an energy-sensitive detector masked by grids with known transmissions. In the dissociative recombination of H3O+, water molecules are produced with a probability of 33%, whereas the production of atomic oxygen is negligible. Atomic carbon is, on the other hand, produced with a branching ratio of 30% in the dissociative recombination of CH+3. For all three molecular ions, the three-particle breakup is a major process. Relative cross sections for dissociative recombination of H3O+ and for dissociative excitation of H3O+ have been measured for relative electron energies up to 40 eV. Implications for the modeling of the chemistry of interstellar molecular clouds are discussed.

Resonance structure in the electron-impact detachment cross section of caused by the formation of

Electrons have been scattered on in the energy range from 0 - 35 eV. A resonance structure in the detachment channel was observed at an energy of about 10 eV. To explain the structure, a potential curve was constructed from the isoelectronic ground-state potential curve and the repulsive Coulomb potential, and it is argued that the structure may be related to the population of a state. The state is predicted to decay primarily by electron emission since a large potential barrier prevents dissociation into .

Dissociative recombination for vibrationally excited ions: the effect of laser photodissociation on the initial vibrational distribution

We report on measurements of dissociative recombination of ions populated in a range of initial vibrational levels with free electrons in the energy range of 0 - 20 eV. A photodis-sociation technique was applied to modify the vibrational distribution, and the influence on the dissociative-recombination (DR) spectrum was studied. To estimate the vibrational distribution in the experiment, the results of a simple model calculation describing a part of the measured spectrum are presented. From the comparison between the experimental data and the model calculation, we conclude that when laser photodissociation is not applied, our result is consistent with a Franck - Condon vibrational distribution from the ion source. When laser photodissociation is applied, a dramatic change of the DR spectrum is observed, which is consistent with the expected effect, based on theoretical photodissociation cross sections.

Dielectronic recombination with Delta n=0 and Delta n=1 core excitations and radiative recombination for C-like F3+ ions

A merged-beams technique has been used to measure the rate coefficient ( nu sigma ) for radiative recombination (RR) as well as dielectronic recombination (DR) with C-like F3+ ions at relative energies up to 74 eV. The RR measurement is compared to a theory based on the hydrogenic approximation with an effective charge Zeff. The data indicate that Zeff is larger than the full ionic charge. This is believed to be due to incomplete screening. DR with Delta n=0 core excitations as well as DR with Delta n=1 core excitations is observed. Structures associated with all three terms of the ground-state configuration of F3+ are found in our spectra, whereas no sign is seen of the metastable 1s22s2p3 5S state.

Status of DESIREE

DESIREE [1] is a double electrostatic storage ring in a cold (13 K) environment with an excellent vacuum. Very long storage times in both rings have been achieved, which has enabled the preparation of beams in a single quantum state. The status of DESIREE is presented with particular emphasis on measurements of stored beam currents in the sub-nA range. We also discuss the ongoing work towards stochastic cooling of very slow beams.

Fragmentation dynamics of complex molecules and their clusters

Complex molecular systems such as large molecules or clusters are characterised by a large number of degrees of freedom. Energies well in excess of individual thresholds for fragmentation can be stored for long times and metastable excited states become important. We will concentrate in this paper on the study of the response of such nanoscale systems, i.e. we will study excitation and fragmentation mechanisms induced by highly charged ion radiation, reflecting dynamic energy and charge flow processes. We will illustrate these relaxation processes for different molecular systems from Polycyclic Aromatic Hydrocarbons, water or biomolecule targets and their clusters in collision with multiply charged ions. We will emphasize that slow multiply charged ions provide an efficient way to study the stability of complex systems. Indeed, such ions are known to remove several electrons at large impact parameters resulting in a fast and gentle ionization.

Even-odd effects in the ionization cross sections of [C{sub 60}]{sub 2} and [C{sub 60}C{sub 70}] dimers

We report strong even-odd effects in multiple ionization yields of van der Waals dimers in slow

RADIATIVE RECOMBINATION WITH HIGHLY CHARGED SI6+ AND SI11+ IONS

{\mathrm{Xe}}^{30+}+[{\mathrm{C}}_{60}{]}_{2}([{\mathrm{C}}_{60}{\mathrm{C}}_{70}])\ensuremath{\rightarrow}\ensuremath{\cdots}+{[{\mathrm{C}}_{60}{]}_{2}}^{r+}({[{\mathrm{C}}_{60}{\mathrm{C}}_{70}]}^{r+})

Highly Charged Clusters of Fullerenes : Charge Mobility and Appearance Sizes

electron-transfer collisions as functions of

Static over-the-barrier model for electron transfer between metallic spherical objects

r\ensuremath{\le}7