We have measured the spectrum of laser photodissociation of OH+ molecular ions to O + H+ and O+ + H fragments for photon energies of 38 100-40 900 cm-1. The OH+ ions were stored as a fast beam (5.50 MeV) in the storage ring TSR for several seconds to achieve rovibrational cooling into the lowest rotations N'' = 0-11 of the vibrational ground state X3Σ-(v'' = 0), close to room temperature (≈300 K). The many resonances in the spectra reveal the energies, widths, and O/O+ branching ratios of 44 predissociating quasibound levels (Feshbach resonances) that lie between the fine-structure states of the O fragment and belong to the last, near-threshold vibrational states v' = 9 and 10 of the A3Π electronic state. For the A3Π0,1 substates, isolated levels with v' = 11 are observed and attributed to double-well distortions of these curves due to nonadiabatic interactions. Another five isolated levels are assigned to the v' = 0 and 1 states of the shallow 15Σ- electronic state, borrowing oscillator strength from nearby A3Π levels. Together, the near-threshold levels deliver a new value D0 = 40 253.8(1.1) cm-1 for the dissociation energy of OH+. Through a two-step photodissociation process, 72 levels from the lower bound states A3Π(v' = 7-8) appear as well and are rotationally analyzed. The level energies are used to construct improved A3Π and 15Σ- Born-Oppenheimer potentials. The totality of the spectral data (energies, widths, intensities, and branching ratios) can provide tight constraints for the potentials and nonadiabatic interactions assumed in future coupled-channel calculations of OH+ photodissociation or of the related charge-exchange reaction O + H+ → O+ + H.

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.

The dissociative recombination (DR) of molecular ions is a key process for the formation of complex molecules in dense interstellar clouds.1,2 Among all species, H 3 + attracts particular interest owing to its key role in astrochemistry.3 Moreover, the low energy DR rate for H 3 + has been a controversial issue for many years, and there are still unexplained differences reaching orders of magnitude among the experimental and theoretical results.3

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.

D. Strasser,1 L. Lammich,2 S. Krohn,1 M. Lange,2 H. Kreckel,2 J. Levin,2 D. Schwalm,2 Z. Vager,1 R. Wester,2 A. Wolf,2 and D. Zajfman1,* 1Department of Particle Physics, Weizmann Institute of Science, Rehovot, 76100, Israel 2Max-Planck-Institut für Kernphysik and Physikalisches Institut der Universität Heidelberg, D-69029 Heidelberg, Germany (Received 10 July 2000) Fragmentation patterns for dissociative recombination of the triatomic hydrogen molecular ion H 1 3 in the vibrational ground state have been measured using the storage ring technique and molecular fragment imaging. A broad distribution of vibrational states in the H2 fragment after two-body dissociation and a large predominance of nearly linear momentum geometries after three-body dissociation are found. The fragmentation results are directly contrasted with Coulomb explosion imaging data on the initial H 1 3 geometry, compared to existing wave-packet calculations, and considered in the light of a simple physical picture.

A statistical model serving to estimate the branching ratios in the dissociative recombination of polyatomic molecular ions is described. Simple phase-space assumptions are employed separately for the electronic capture step and the subsequent dissociation and yield predictions in good agreement with existing data on H-3(+). Also the vibrational state populations of molecular fragments can be obtained and for H-3(+) are found to agree well with recent measurements.

The dissociative recombination of LiH+ ions with low-energy electrons is observed at a storage ring and the final states are analyzed using fragment imaging and field ionization techniques. The rate coefficient is found to be larger than its estimated value used in astrophysical models. Mostly the highest energetically possible Rydberg states of the lithium atom are populated by the reaction, indicating a common trend for molecular recombination via the noncrossing mode.

Fragmentation patterns for dissociative recombination of the triatomic hydrogen molecular ion H(3)(+) in the vibrational ground state have been measured using the storage ring technique and molecular fragment imaging. A broad distribution of vibrational states in the H(2) fragment after two-body dissociation and a large predominance of nearly linear momentum geometries after three-body dissociation are found. The fragmentation results are directly contrasted with Coulomb explosion imaging data on the initial H(3)(+) geometry, compared to existing wave-packet calculations, and considered in the light of a simple physical picture.

Using the Coulomb explosion imaging method, the change of the relative population for the first six vibrational states of H-2(+) during the interaction with low-kinetic-energy electrons has been measured. A model based on rate coefficients for dissociative recombination and superelastic collision processes is developed to explain the time dependence of the relative vibrational populations. Using this model, we demonstrate that superelastic collisions with rate coefficients of (1-4) x 10(-6) cm(3) s(-1) (about an order of magnitude higher than available theoretical predictions) can explain the observed electron-induced vibrational deexcitation of H-2(+).

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.

When infrared active molecular ions are stored in a heavy ion storage ring, a fast beam of vibrationally and rotationally relaxed molecules can be obtained. This opens up new opportunities for various experimental studies in molecular ion physics, particularly those involved with spectroscopy and dissociation dynamics of excited states. The analysis of such experiments is facilitated by the fact that the information on the initial states is able to identify and, therefore, eliminate them from the assignment problem. Moreover, when the intensity of the measured transitions is monitored as a function of storage time, the dependence of these processes on the degree of internal relaxation is revealed. We demonstrate these general principles by application to near threshold dissociation spectroscopy of the OH+ molecular ion. The structure of quasibound levels near the dissociation limit of OH+ and its dissociation energy are deduced.

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.

Hot CH2 1 molecular ion ensembles were prepared, accelerated, and stored for radiative cooling to room temperature. The structure of the species was measured by the Coulomb explosion imaging method at different stages of cooling. The bending angle distributions were extracted and compared with recent theories as well as a previous Coulomb explosion imaging measurement. The comparison reveals an apparent large nonadiabatic contribution to the low-lying CH2 1 wave functions.@S1050-2947 ~99!02403-8#

The relative dissociative recombination rate coefficients for specific vibrational states of HD+ have been measured. The method is based on using merged electron and molecular ion beams in a heavy-ion storage ring together with molecular fragment imaging techniques which allow us to probe the vibrational-state population of the stored beam as a function of time as well as the final state of the dissociation. The initial vibrational distribution of the stored ion beam (from a Penning ion source) is found to be in good agreement with a Franck-Condon model of electron impact ionization, apart from slightly larger experimental populations found for low vibrational states; its time evolution in the storage ring reflects the predicted vibrational level lifetimes. Dissociative recombination measurements were performed with the electron and ion beams at matched velocities (corresponding to average collision energies of about 10 meV), and at several well-defined collision energies in the range of 3-11 eV. The obtained vibrational-state specific recombination rate coefficients are compared with theoretical calculations and show that, although an overall agreement exists between experiment and theory, large discrepancies occur for certain vibrational states at low electron energy. [S1050-2947(99)09411-1].

Rate coefficients for dissociative recombination of HD+ in selected vibrational states have been measured by a combination of two molecular fragment imaging methods by using the heavy-ion storage ring technique. Recombination fragment imaging yields state-to-state reaction rates. These rates are converted to rate coefficients by using vibrational level populations of the stored ion beam, derived from nuclear coordinate distributions measured on extracted ions. The results show strongly increasing rate coefficients for high vibrational excitation, where additional dissociation routes open up, in agreement with a theoretical calculation. Very low rate coefficients are found for certain, isolated vibrational states.

Isomerization of a highly excited vibrational state of acetylene was studied using the Coulomb explosion imaging technique. A vinylidene isomer was prepared by electron photodetachment of the negative molecular ion and the corresponding distribution function of the nuclear configurations of the molecule was sampled after a time period of 3.5 mu s. The population of the vinylidene isomer was found to be significantly high (similar to 50%), in contrast to the commonly accepted notion of vinylidene as a short-lived isomer. [S0031-9007(98)07417-1].

We present a novel experimental setup which combines the storage of molecular ions with subsequent Coulomb explosion imaging (CEI) for molecular structure studies. A new extraction system has been installed at the heavy ion storage ring TSR which allows slow extraction of ions out of the ring. Attached to the extraction beamline, a new detector system for Coulomb explosion imaging has been set up to analyze the extracted molecular ions. By combining the CEI technique with the storage ring, it is possible to observe the structure of small molecular ions while the vibrational relaxation is in progress or after the ions have reached thermal equilibrium with the environment (300K), where typically only the vibrational ground state is populated. A detailed description of the facility installed at the TSR is given and the first CEI results for HD+, which are in very good agreement with the theoretical expectation, are discussed.

Energy loss in the MeV range of simple clusters impinging on thin carbon targets has been measured using a time-of-flight method. Stopping-power ratios defined as the ratio of the stopping power of the cluster to the sum of the stopping powers of the constituent atoms moving at the same velocity were investigated, Stopping-power ratios close to unity were observed for O-2 and B-3 clusters, while deenhancement effect is observed in the stopping-power ratios of C-3 and C-4. The experimental results are compared both with an existing theoretical model, which takes into account the spatial correlation of the fragments, and with a simple united-atom model, which also includes the charge state evolution of the fragment ions inside the target. [S1050-2947(97)10711-9].