Publications

The dynamics of ions in an electrostatic ion beam trap in the presence of an external time-dependent field is studied with a recently developed particle-in-cell simulation technique. The simulation technique, capable of accounting for space-charge effects, has reproduced all the experimental results on the bunch dynamics in the radio frequency mode. With simulation, the motion of ions is visualized in phase space and it is shown that the ion-ion interaction strongly affects the distribution of ions in phase space in the presence of an rf driving voltage.

The epoch of first star formation in the early Universe was dominated by simple atomic and molecular species consisting mainly of two elements: hydrogen and helium. Gaining insight into this constitutive era requires a thorough understanding of molecular reactivity under primordial conditions. We used a cryogenic ion storage ring combined with a merged electron beam to measure state-specific rate coefficients of dissociative recombination, a process by which electrons destroy molecular ions. We found a pronounced decrease of the electron recombination rates for the lowest rotational states of the helium hydride ion (HeH+), compared with previous measurements at room temperature. The reduced destruction of cold HeH+ translates into an enhanced abundance of this primordial molecule at redshifts of first star and galaxy formation.

A Velocity Map Imaging (VMI) spectrometer has been designed and integrated with an electrostatic ion beam trap to study delayed electron emission from trapped polyatomic anions upon photodetachment. The VMI spectrometer is small in size and can record a wide range of photoelectron energies, with variable magnification. Delayed electron emission can be recorded in our experimental setup for any time duration after the photoexcitation of the polyatomic anions. Experiments were carried out with trapped O- and C-5(-) ions to demonstrate the capability of the spectrometer. Delayed electron emissions from C-5(-) as well as prompt photoelectrons from O- were detected by the VMI spectrometer upon photoexcitation. The design and performance of the spectrometer are presented in detail.

Ions in an ion bunch trapped inside an Electrostatic Ion Beam Trap (EIBT) exhibit collective oscillations within the bunch under the influence of an external driving force. These internal oscillations have been measured explicitly using a new method with a particle detector outside the EIBT. In this approach, the evolving ion bunch is monitored along the entire trap length, in contrast to the localized single point measurements that are often carried out in other techniques. In the present study, quadrupole oscillations have been measured for the first time in an EIBT along with the dipole oscillations that were measured previously. The frequency of the quadrupole oscillation is found to be about twice the dipole oscillation frequency. This is in agreement with the prediction of a theoretical model.

We demonstrate the ability to control the molecular dissociation rate using femtosecond pulses shaped with third-order dispersion (TOD). Explicitly, a significant 50% enhancement in the dissociation yield for the low lying vibrational levels (v similar to 6) of an H-2(+) ion-beam target was measured as a function of TOD. The underlying mechanism responsible for this enhanced dissociation was theoretically identified as non-adiabatic alignment induced by the pre-pulses situated on the leading edge of pulses shaped with negative TOD. This control scheme is expected to work in other molecules as it does not rely on specific characteristics of our test-case H-2(+) molecule.

Absolute rate coefficients were determined for the recurrent fluorescence (RF) process in C₆^- anions at excitation energies above the adiabatic electron attachment energy. The RF process is an important ingredient in understanding the formation and stabilization of anions in the interstellar medium.

The stability of cationic SF ₅ ⁺(SF ₆) _n-1 clusters was investigated by measuring their blackbody-induced radiative dissociation (BIRD) rates. The clusters were produced in a supersonic expansion ion source and stored in an electrostatic ion-beam trap at room temperature, where their abundances and lifetimes were measured. Using the "master equation" approach, relative binding energies of an SF ₆ unit in the clusters could be extracted from the storage-time dependence of the survival probabilities. The results allow for a deeper insight into the effect of a localized charge on the structure and stability of SF ₆-based clusters.

Using event-by-event fragment momentum spectroscopy in a storage-ring merged-beams experiment, we find laboratory evidence that in the dissociative recombination (DR) of HCNH⁺ with cold electrons the energetic isomer HNC is produced with a high yield, similar to that of HCN. With a newly implemented mass-sensitive fragment imaging detector, we analyze the kinetic energy release of the triatomic fragments DCN/DNC from the DR reaction of the isotopologue DCND⁺ with cold (near 10K) electrons. The results show that the internal energy of these fragments is extremely high, far exceeding the isomerization barrier between DNC and DCN. From this laboratory characterization of the DR reaction we conclude that also the triatomic fragment HCN/HNC from the DR of HCNH⁺ will carry a large amount of ro-vibrational excitation and show that this implies an isomeric production ratio in a narrow range near unity.

The competition between dissociation paths of I₂⁺ and NO⁺ molecules was studied using femtosecond laser pulses with different intensities. It was found, both for moderate fields and for strong fields, that the dissociation path strongly prefers the higher energy dissociation path with smaller kinetic energy rather than the lower energy path with higher kinetic energy.

We report the experimental findings of a systematic study of the effect of linear chirp on strong field photodissociation of H₂⁺. For vibrational levels around or above the one photon crossing, the effect manifests itself in terms of a shift in the kinetic energy release (KER) peaks. The peaks shift up for negative chirp whereas they shift down for positive chirp. The measurements are carried out by varying two of the three laser pulse characteristics, energy, pulse peak intensity and linear chirp, while keeping the third constant. The shifts in the KER peaks are found to be intensity dependent for a given value of chirp. However, in the last two cases (i.e., fixed pulsed energy and fixed pulse peak intensity), they are found to be independent of the chirp magnitude. The results are understood on the basis of saturation of photodissociation probabilities for these levels.

Individual product channels in the dissociative recombination of deuterated hydronium ions and cold electrons are studied in an ion storage ring by velocity imaging using spatial and mass-sensitive detection of the neutral reaction fragments. Initial and final molecular excitation are analyzed, finding the outgoing water molecules to carry internal excitation of more than 3eV in 90% of the recombination events. Initial rotation is found to be substantial and in three-body breakup strongly asymmetric energy repartition among the deuterium products is enhanced for hot parent ions.

Merging an HD+ beam with velocity matched electrons in a heavy ion storage ring we observed rapid cooling of the rotational excitations of the HD+ ions by superelastic collisions (SEC) with the electrons. The cooling process is well described using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclei rotation approximation. We verify the ΔJ=-2 SEC rate coefficients, which are predicted to be dominant as opposed to the ΔJ=-1 rates and to amount to (1-2)×10-6cm3s-1 for initial angular momentum states with J≤7, to within 30%.

Rate coefficients for dissociative recombination (DR), dissociative excitation (DE), and vibrational excitation between the helium dimer ion He2+4 and electrons from a few meV up to 40 eV were measured using fast (3.8 and 8.3 MeV) ion beams stored for up to 85 s. Vibrational relaxation to greater than 95% in the v=0 level was achieved by collisions with cold electrons over 50 s. Low-energy, strongly v -dependent DR rate coefficients are given for v=0, 1, and ≥2. The rate coefficients at higher energies for v=0, with DR and DE given on an absolute scale, are compared to results from recent wave-packet calculations on the fast dissociation dynamics of the doubly excited helium dimer, where the three processes occur as competing reaction channels. Also given are rate coefficients for vibrationally superelastic electron collisions at near 10 meV average energy and the approximate vibrational excitation cross section for fast collisions with the residual gas.

A three-dimensional imaging technique installed at the Heidelberg Test Storage Ring (TSR) has been used to investigate the three-body breakup channels occurring in the dissociative recombination process of the methylene ion CH2+. By selecting dissociation planes perpendicular to the molecular beam direction the dissociation kinematics could be measured with unprecedented momentum resolution. Release energies, the relative branching ratio, and the kinematical correlations between the three fragments were determined for the two energetically allowed channels: C(P-3)+H(S-2)+H(S-2) and C(D-1)+H(S-2)+H(S-2).

The fragmentation of LiH₂ ^- anions after electron impact was investigated at the heavy-ion storage ring TSR. The main reaction channel was found to be electron detachment followed by a breakup into LiH + H. In the first ms after production of the molecular ions in a cesium sputtering ion source, additional contributions were observed in the Li + H₂ and Li^- + H₂ channels, hinting at an initial population of a short-lived state of the anion. To gain a better understanding of the mechanisms underlying the observed behavior of the system, ab initio calculations of relevant potential energy surfaces were performed at selected geometries. The experimental findings are discussed in the light of these calculations.

The dissociative recombination (DR) of He3 He+4 has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg by observing neutral products from electron-ion collisions in a merged beams configuration at relative energies from near-zero (thermal electron energy about 10 meV) up to 40 eV. After storage and electron cooling for 35 s, an effective DR rate coefficient at near-zero energy of 3 10 rquote 9 cm3 rquote 1 is found. The temporal evolution of the neutral product rates and fragment imaging spectra reveals that the populations of vibrational levels in the stored ion beam are nonthermal with fractions of bc0.1 ldblquote 1 in excited levels up to at least v=4, having a significant effect on the observed DR signals. With a pump-probe-type technique using DR fragment imaging while switching the properties of the electron beam, the vibrational excitation of the ions is found to originate mostly from ion collisions with the residual gas. Also, the temporal evolution of the DR signals suggests that a strong electron induced rotational cooling occurs in the vibrational ground state, reaching a rotational temperature near or below 300 K. From the absolute rate coefficient and the shape of the fragment imaging spectrum observed under stationary conditions, the DR rate coefficient from the vibrational ground state is determined; converted to a thermal electron gas at 300 K it amounts to (3.3 b10.9)10 rquote 10 cm3 rquote 1. The corresponding branching ratios from v=0 to the atomic final states are found to be (3.7 b11.2) for 1s2s S3,(37.4 b14.0) for 1s2s S1,(58.6 b15.2) for 1s2p P3, and (2.9 'b13.0) for 1s2p P1. A DR rate coefficient in the range of 'c3 10 rquote 7 cm3 rquote 1 or above is inferred for vibrational levels v=3 and higher. As a function of the collision energy, the measured DR rate coefficient displays a structure around 0.2 eV. At higher energies, it has one smooth peak around 7.3 eV and a highly structured appearance at 15 ldblquote 40 eV. The small size of the observed effective DR rate coefficient at near-zero energy indicates that the electron induced rotational cooling is due to inelastic electron-ion collisions and not due to selective depletion of rotational levels by DR.

We describe a system combining an ion beam trap and a low energy electron target in which the interaction between electrons and vibrationally cold molecular ions and clusters can be studied. The entire system uses only electrostatic fields for both trapping and focusing, thus being able to store particles without a mass limit. Preliminary results for the electron impact neutralization of C-2(-) ions and aluminum clusters are presented. (C) 2005 American Institute of Physics.

An overview of the use of stored ion beams and phase space cooling (electron cooling) is given for the field of molecular physics. Emphasis is given to interactions between molecular ions and electrons studied in the electron cooler: dissociative recombination and, for internally excited molecular ions, electron-induced ro-vibrational cooling. Diagnostic methods for the transverse ion beam properties and for the internal excitation of the molecular ions are discussed, and results for phase space cooling and internal (vibrational) cooling are presented for hydrogen molecular ions. (C) 2004 Elsevier B.V. All rights reserved.

The size dependence of the electron detachment process of negatively charged clusters was investigated. The electron binding energy was found to be the important parameter in the determination of both the threshold behavior and the magnitude of the cross section above threshold. It was found difficult to extend systematic sie-dependent studies of electron-anion collisions to very large clusters due to the limited magnetic rigidity of the heavy ion storage rings. The results suggest that the size-dependent polarizability of the clusters is responsible for the observed behavior.

The application of two-dimensional imaging and storage ring techniques to analyze the breakup dynamics of D₂H⁺ and H ₂D⁺ following dissociative recombination with low energetic electrons, was investigated. The correlation between the hydrogen and deuterium atoms produced in the three-body channel were calculated. It was observed that the deuterium atom has a large probability at the center and the three particle tend to dissociate with geometry close to linear. The results show that the remaining average internal excitation energy stored in the rotation of the molecules corresponds to a temperature of less than 70 meV.

Experiments on the dissociative recombination (DR) of low-energy electrons with triatomic hydrogen ions (H₃⁺) using merged beams in storage rings are discussed, focusing on the ro-vibrational relaxation and state diagnostics of the stored ion beam, on studies of the three-body breakup following DR, and on new observations showing a variation of the DR rates during the continuous interaction of the stored ions with a cold electron beam. Linear fragmentation patterns in the three-body DR are discussed in the light of the H₃ electronic potential surfaces. Measured energy dependencies of the H₃⁺ DR rate coefficient are compared to recent theoretical predictions (Kokoouline V and Greene C H 2003 Phys. Rev. A 68 012703) and a recent storage-ring measurement at CRYRING (McCall B J et al 2003 Nature 422 500).

The dynamics of the fragmentation process following the recombination with low energetic free electrons was studied for H-3(+) and D-3(+) using a two-dimensional imaging technique. The kinematical correlation of the atomic fragments in the three-body fragmentation channel, as well as the vibrational excitation distribution of the molecular fragments in the two-body channel, could be determined. In addition, a considerable rotational excitation of the H-3(+) and D-3(+) ions circulating in the storage ring TSR was observed even after storage time of more than 1 min. In contrast, the heteronuclear D2H+ isotopomer possessing an electric dipole moment does not exhibit any internal excitations after 10 s of storage. (C) 2003 Elsevier Science Ltd. All rights reserved.

We report on the possibility to produce homonuclear vibrationally cold molecular ion beams (using D₂⁺ as an example) via the interaction of cold electrons with a stored beam. We demonstrate that cooling of the vibrational degrees of freedom can be achieved within a reasonable time, which is comparable to the beam lifetime. We also present preliminary results on the electron impact vibrational excitation of HD⁺.

Keywords: TEST STORAGE RING; COLD HD+; STATES

We demonstrate compression of the velocity spread of a stored ion bunch in an electrostatic ion-beam trap, using the [Formula Presented]-kick method. An analytical formula for the optimum pulse is derived and is used in the laboratory. A 20% reduction in the asymptotic velocity spread is observed after application of a single cooling pulse to a bunch of 4.2-keV [Formula Presented] ions.

The advantages of using the heavy-ion storage ring technique for measuring rate coefficient of reaction relevant to the evolution of the interstellar medium is presented, with the dissociative recombination of LiH⁺ as a specific example.

The photodissociation spectrum of internally cold CH⁺ ions that populate only a few rotational levels of the ground state X ¹Σ⁺(v″=0) was examined. Internal cooling was achieved by storing the beam in a decay by spontaneous emission. Based on the data, the intensity, number, and asymmetric character of the numerous experimental features were characterized.

As a benchmark system for isomerization and bending vibrational relaxation, the internal relaxation of stored DCO⁺ and DOC⁺ molecular ions was studied on the time scale of several seconds using the technique of foil-induced Coulomb explosion imaging (CEI) combined with the heavy test storage ring (TSR). By measuring asymptotic fragment velocities, the CEI technique allowed to infer information about the distribution of bond angles in the DCO⁺ and DOC⁺ molecules in R space.

The stability and ion loss in an linear electrostatic ion-trap resonator was investigated by using numerical simulations. The decay of a stored 4.2 KeV Ar⁺ beam was monitored for different configurations of the ion trap. Results showed that the ion-loss processes were controlled by collisions both with residual gas species and among the stored ions.

Using a resonator-like electrostatic ion trap, we demonstrate that, under certain condition, it is possible to keep constant the width of a packet of ions oscillating between two mirrors. We show, using one dimensional calculations, that the effect is the result of Coulomb repulsion which, in a counter-intuitive way, keeps the ions together. Preliminary results of the exploitation of this phenomenon for high precision mass spectrometery are given.

The absolute dissociative recombination (DR) rate coefficient of LiH⁺ was experimentally studied. Final-state populations were measured by fragment imaging and by state-selective field ionization. It was found that the rate coefficient is larger than its value used in astrophysical models.

Time-dependent photodetachment spectra for small electronically and vibrationally excited negatively charged carbon clusters C_n^- (n = 2-6) are measured using an electrostatic ion trap. The time dependence demonstrates the presence of metastable electronic states with lifetimes in the range of 10 to 200 ms. Comparison is made with available data and theoretical calculations.

An innovative technique for three-dimensional imaging is presented, which uses the ratio of intensities of a pair of two-dimensional images to extract timing information. The principal advantage of this method is the ability to measure position and time for an almost unlimited number of particles hitting the detector simultaneously. The detector is capable of subnanosecond time resolution and position resolution of about 50 μm. The photodissociation of H⁺₂ is used to demonstrate the capability of the detector.

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 mu g/cm(2)) 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. (C) 2000 Elsevier Science B.V. All rights reserved.

The branching ratios for the dissociative recombination of various vibrationally cold polyatomic molecular ions have been measured using the ASTRID ion storage ring. The results show that many particles are ejected during the recombination process, and that isotopic effects exist when hydrogen atoms are replaced by deuterium.

Hot CH2+ 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+ wave functions. [S1050-2947(99)02403-8].

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.

Sharp thresholds are observed in the dissociative recombination cross section of vibrationally cold HD⁺ in the energy range where new channels H(1s) + D(n) [or D(1s) + H(n)] with n > 2 open. The occurrence of these thresholds, not predicted by current theoretical calculations, contradicts the current assumption that the size of the total cross section can be calculated without accounting for the detailed branching ratios. An indirect signature of ion pair production is also found in the data, suggesting a significant branching into that channel. 1999

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 (300 K), 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. (C) 1998 Elsevier Science B.V. All rights reserved.

Photodissociation of CH+ molecules (CH+ + h nu --> C+ + H) is studied by collinear laser spectroscopy on a rotationally relaxed fast molecular ion beam in a storage ring. We have detected the resonances predicted between the thresholds related to the two fine-structure levels of the C+ fragment for low initial rotations, and found that no heating processes hinder the complete rovibrational thermalization of the fast stored beam in a roam temperature environment.

A new type of multiparticle three-dimensional imaging detector for the measurement of kinetic energy releases in molecular dissociation processes is presented. The detector makes use of the new generation of multianode photomultipliers to produce a timing signal for the simultaneous impact of several particles on the surface of a microchannel plate coupled to a phosphor screen. The detector is capable of subnanosecond time resolution (about 100 ps for the present setup) and position resolution (using a standard charge-coupled-device camera) of about 100 μm. The detector is suitable for ultrahigh-vacuum operation and can work with particles over a range of kinetic energies from keV to MeV.

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 [Formula Presented] and [Formula Presented] clusters, while deenhancement effect is observed in the stopping-power ratios of [Formula Presented] and [Formula Presented] 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.

This chapter presents the basic principles and recent developments in the field of molecular imaging techniques with fast ion beams. It is explained why imaging of molecular dissociation makes it possible to increase our understanding of dissociation processes as well as molecular structure, and enable full control over the parameters (such as internal excitation and kinetic energy release) that dominate the breaking up of molecules. The two most relevant geometries, in which imaging detectors have been used, are described, and their respective advantages and disadvantages are discussed. Examples of imaging for the study of various dissociation processes and molecular geometry are also presented.

Dissociative Recombination (DR) is a major process in planetary ionospheres and interstellar clouds. The branching ratio in the dissociative recombination of the astrophysically important species H3O+, H2O+ and CH3+ with low energy electrons is presented. The molecular ions were stored and vibrationally cooled in the ASTRID storage ring of Aarhus Denmark. The recombination was measured by merging the ions with the electron cooler. All the branching ratios were measured directly for the first time. The three body dissociation was found to be a major channel, a fact that was neglected previously in common interstellar chemical models.

The cross section as well as the branching ratios for the dissociative recombination of ground-state CH+ ions with electrons have been measured using the heavy-ion storage-ring technique and two-dimensional fragment imaging. Although the absolute value of the cross section at thermal energies is found to be in very good agreement with the theory, several unpredicted narrow resonances are also present in the data. These structures are interpreted as due to an indirect recombination process via core-excited Rydberg states. The branching-ratio measurement shows that at low electron energy the 2 (2) Pi state, producing carbon fragments C(D-1), is the most important dissociative state, although transitions during the dissociation to other dissociative potential curves are also present. Anisotropy in the angular distribution of the dissociating fragments is visible for some of the final states. Dissociative recombination of ions in the metastable excited a (3) Pi state is also observed, and the lifetime as well as the excitation energy of this state are deduced from the imaging data.

Water is probably the most important molecule for the thermal properties of dense, interstellar gas, and as such plays a decisive role in establishing the rate at which molecular clouds collapse towards star formation. The synthesis of water is thought to be mainly due to dissociative recombination of H₃O⁺with electrons, although experiments have, as yet, provided no evidence that H₂O is a product of the reaction. Here we present the first observation of water synthesis from the recombination of H₃O⁺, with a branching ratio of 33%. This value is in good agreement with existing observations of the [H₂O]/[H₃O⁺] ratio in interstellar clouds.

The Coulomb explosion imaging method is used for the measurement of the vibrational state distribution of He2+, under various conditions of excitation. Application of this method to the measurement of vibrational excitation of molecular ions stored in heavy-ion storage rings is discussed, especially in view of the recent results obtained for the dissociative recombination of molecular ions with electrons using a merged-beam technique in a storage ring.

Radiative relaxation times of rovibrational transitions for the electronic ground state of HD+, O1816O+, and C1312C+ are calculated. These lifetimes are compared with the storage time in a heavy-ion storage ring where molecular ions can be stored, in order to provide vibrationally and rotationally cold ions. The relevance of the results to the measurement of dissociative recombination of cold molecular ions is discussed.

As a first example of a molecular physics experiment in a storage ring we describe measurements with a HD⁺ beam stored in the Heidelberg Test Storage Ring TSR with a lifetime of 5 s. The transverse degree of freedom could be electron cooled within 10 s. Deexcitation of high vibrational states was observed within ≅ 300 ms. With this internally cold ion beam dissociative recombination and dissociative excitation with free electrons at an energy of 0.3-40 eV were measured. The resonances and the threshold behavior of the cross section depend sensitively on the internal excitation. Future applications for molecular physics in a storage ring are discussed.

Absolute cross-sections for photodetachment of negative carbon clusters are reported for C(n)-(n=3,...,8). These measurements are made using different types of ion sources, which create different isomers. These new results indicate that various negative and neutral isomers exist, some with electron affinities as low as approximately 1 eV.

The dissociative recombination cross section between HD+ molecules and electrons at energies between 0.3 and 40 eV has been measured for the first time in an ion storage ring. The ion beam, stored with a mean lifetime of 5 s, was observed to be vibrationally cold. In the measured cross section, theoretically predicted high-energy resonances appeared, which can be attributed to recombination via doubly excited Rydberg states of HD.

A quantitative analysis of the influence of multiple scattering on fast molecules dissociating in solids is made using a Monte Carlo technique. The simulations allow the computation of asymptotic velocities of all of the fragments after Coulomb explosion including the effects of ion-solid interactions. Examples for deducing moleculat structure using this method are given for two well-known molecular ions.