Experimental data are presented from three different heavy-ion storage rings (ASTRID in Aarhus, CRYRING in Stockholm, and TSR in Heidelberg) to assess the reliability of this experimental tool for the extraction of absolute rate coefficients and cross sections for dissociative recombination (DR). The DR reaction between HD. and electrons has been studied between 0 and 30 eV on a dense energy grid. HD+ displays two characteristic local maxima in the DR rate around 9 and 16 eV. These maxima influence the data analysis at smaller collision energies. We conclude that resonant structures in the DR cross sections are reproduced among the experiments within the collision energy resolution. The absolute cross sections agree within the systematic experimental errors of 20% related to the measurement of the ion currents. Absolute thermal rate coefficients for HD. ions are given for an electron temperature range of 50-300 K. Results for the DR cross section and the thermal rate coefficients are compared to recent theoretical calculations including rotational effects, finding satisfactory agreement.

Vibrational relaxation and isomerization of internally excited deuterated formyl and isoformyl cations has been investigated on the time scale of 2 ms to 12 s using the nearly interaction-free environment of an ion storage ring. De-excitation of the v2 bending modes of DCO+ and DOC+ due to spontaneous radiative transitions was observed as a function of the storage time by measuring their foil-induced Coulomb explosion using three-dimensional coincident fragment imaging. No isomerization of low-lying vibrational levels of DOC+ ions was observed on the time scales considered. By comparing the Coulomb explosion data to molecular bond angle distributions obtained from vibrational wave function calculations, the time evolution of the mean v2 population is deduced for both isomers. The stored DOC+ ions are found to thermalize with the 300 K black-body radiation, while relaxation of the DCO+ bending vibrations was found to require considerably longer times, in agreement with a predicted very small transition moment of the v2=1 level.

The vibrational relaxation of H-3(+) molecules from a conventional plasma ion source is studied performing Coulomb explosion imaging on the ions extracted from a storage ring after variable times of storage. Storage for 2 s is found sufficient for radiative relaxation of the breathing excitation and the fragment velocity distribution in the breathing coordinate then agrees well with simulations based on the calculated ground-state wave function. The radiative decay of the two lowest pure breathing levels (1,0(0)) and (2,0(0)) is seen to be considerably faster than expected from rotationless calculations. Assuming a high rotational excitation of the H-3(+) ions, as suggested already in earlier experiments, the theoretical transition probabilities of the University College London line list for H-3(+) [L. Neale, S. Miller, and J. Tennyson, Astrophys. J. 464, 516 (1996)] can explain the increase of the vibrational cooling rates and reproduce the observed decay curve for the lowest breathing-excited level, confirming the absolute transition probabilities of these line tables. The observations give evidence for a quasistable population of high-lying rotational levels in the stored ion beam, relevant for the interpretation of storage ring measurements on the rate coefficients for dissociative recombination of H-3(+) ions with low-energy electrons.

Thin diamond-like carbon (DLC) foils were tested as stripping targets for molecular structure studies using the Coulomb explosion imaging method. The multiple scattering of MeV atomic and molecular ion beams penetrating DLC foils was measured and compared to Formvar foils which have been used in CEI setups so far. The DLC targets were found to be of similar thickness (0.7–0.9 μg/cm2) as the Formvar foils but of higher efficiency for CEI measurements as they exhibit less pinholes. Other advantages of DLC foils are: smaller linear thickness due to their higher density, higher homogeneity and better control of the production process. The production of targets with even smaller thicknesses is anticipated.

An experimental scheme, which combines Coulomb explosion imaging (CEI) with storage of fast molecular ions, has been introduced recently at the TSR heavy ion storage ring facility in Heidelberg. CEI is an experimental technique that provides direct observation of the nuclear conformations within small molecules. The combination of CEI with the storage ring technique enables the control of the internal excitation of the measured molecules, which is an essential condition to the interpretation of CEI results in terms of "structure" assigned to specific molecular states. This structure is measured as a function of storage time, thus enabling one to study processes of slow intramolecular dynamics such as isomerization, metastable states, etc. Moreover in this scheme, CEI can be used as a diagnostic tool for the intramolecular excitation, while other molecular interactions (e.g. with. electrons or photons) are investigated. In this report, the CEI principle and the new experimental setup are described with an emphasis on the new prospects for studies in molecular physics. CEI measurements of stored CH2+ and NH2+ molecular ions are presented. The study of the angular distribution in these molecules as a function of their vibrational relaxation to the ground state, reveals unexpected behavior near the linear conformation which is inconsistent with the current adiabatic theories.