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.

We have designed a velocity map imaging (VMI) setup and integrated it with an Electrostatic Ion Beam Trap (EIBT) to study photon induced prompt and delayed electron emission from trapped cluster anions. The operational details are discussed here. The performance of the VMI is also reported for photoionization of trapped O- ions.

Sidebands are observed in the Fourier transform of the pickup signal of an RF-bunched beam oscillating in an electrostatic ion beam trap. We show that the sidebands are dominated by a collective longitudinal oscillation of the center of the bunch around the synchronous ion as the bunch travels within the storage device. We present evidence for a linear dependence of the sideband frequency and for a seemingly chaotic behavior of the amplitude of the sideband peaks on the number of stored ions.

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.

Aluminum cluster anions were stored in an electrostatic ion storage ring and irradiated by a laser pulse. Neutral as well as charged decay products emitted 20 mu s after the excitation were identified and recorded using the daughter ion mass spectrometry method. Besides delayed electron detachment, only monomer emission was observed for Al-4(-). The branching ratio for the monomer channel was determined to be 8 +/- 2%, while the dimer channel was found to be

H₂⁺ photodissociation, induced by intense short laser pulses, was measured by a full 3D imaging system. We have conducted a series of experiments, in which we systematically changed the linear chirp, using a pulse shaper, and observed the kinetic energy release spectra(KER). Distinct differences in the KER spectra are observed both in peak positions and angular distribution for laser pulses with similar duration and intensity but opposite chirp sign.

The fragmentation of the hydronium cation H3 O+ after photoabsorption at 13.5 nm has been investigated with a crossed-photon-and-ion-beams experiment making pulsing and trapping techniques available for fragment momentum imaging at the intense Free-electron LASer in Hamburg. The observed photofragmentation patterns demonstrate that the photolysis of H3 O+ proceeds by valence ionization into H3 O2+ which subsequently fragments to mainly OH+2 H+ and H2 O+ + H+ with a branching ratio of up to 0.6:1 and with different degrees of excitation of the molecular fragment. The cross section for fragmentation into OH+2 H+ is found to be (0.37±0.18) × 10-18 cm2, while the total photoabsorption cross section is estimated to be greater than 0.95× 10-18 cm2. The data suggest that ionization mainly occurs from the 3 a1 and 1e valence orbitals and that initial ionization from 3 a1 mainly leads to fragmentation into H2 O+ (A A2 1) + H+ while initial ionization from the 1e orbital predominantly populates the H2 O+ (B B2 2) + H+ and OH (X Π2) +2 H+ channels. The results are of significance for astrophysical models of gas clouds in the vicinity of hot radiating objects and for models of the chemistry of planetary and lunar ionospheres under solar irradiation.

The dissociative recombination (DR) of ⁴He₂⁺ has been investigated at the heavy-ion storage ring TSR in Heidelberg. Rate coefficients were measured up to collision energies of 40 eV. Vibrational level populations were monitored using the Coulomb explosion imaging technique showing relaxation to the v 0 level (>95%) through collisions with cold electrons within 50s. Low-energy DR rate coefficients are derived for v 0, 1 and ≥2 which show a strong v-dependence. A low-energy super-elastic collision (SEC) rate coefficient of α^v10_SEC (E _d 0)1.8×10^-7cm³s^-1 was found.

Angular fragment distributions from the dissociative recombination (DR) of HD+ were measured with well directed monochromatic low-energy electrons over a dense grid of collision energies from 7 to 35 meV, where pronounced rovibrational Feshbach resonances occur. Significant higher-order anisotropies are found in the distributions, whose size varies along energy in a partial correlation with the relative DR rate from fast-rotating molecules. This may indicate a breakdown of the nonrotation assumption so far applied to predict angular DR fragment distributions.

We describe a bent electrostatic ion beam trap in which cluster ions of several keV kinetic energy can be stored on a V-shaped trajectory by means of an electrostatic deflector placed between two electrostatic mirrors. While maintaining all the advantages of its linear counterpart [Zajfman, Phys. Rev. A 55, R1577 (1997); Dahan, Rev. Sci. Instrum. 69, 76 (1998)], such as long storage times, straight segments, and a field-free region for merged or crossed beam experiments, the bent trap allows for simultaneous measurement of charged and neutral fragments and determination of the average kinetic energy released in the fragmentation. These unique properties of the bent trap are illustrated by first results concerning the competition between delayed fragmentation and ionization of Aln- clusters after irradiation by a short laser pulse.

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).

Molecular photofragmentation has been studied by event imaging on HeH+ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n≥2)+H+ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4±0.7)×10-18cm2, releasing energies of 10-20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.

A cryogenic electrostatic ion storage ring CSR is under development at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany. Cooling of the ultrahigh vacuum chamber is envisaged to lead to extremely low pressures as demonstrated by cryogenic ion traps. The ring will apply electron cooling with electron beams of a few eV up to 200 eV. Through long storage times of 1000 s as well as through the low wall temperature, internal cooling of infrared-active molecular ions to their rotational ground state will be possible and their collisions with merged collinear beams of electrons and neutral atoms can be detected with high energy resolution. In addition storage of slow highly charged ions is foreseen. Using a fixed in-ring gas target and a reaction microscope, collisions of the stored ions at a spead of the order of the atomic unit can be kinematically reconstructed. The layout and the cryogenic concept are introduced.

The lifetimes of three isotopologs of the molecular hydrogen anion have been measured using an electrostatic ion-beam trap. Much longer lifetimes (up to ∼2 ms for D2-) than predicted by the most recent calculation are found, and it is proposed that more than one electronic state contributes to the overall lifetimes of these species.

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.

A novel cryogenic electrostatic storage ring is planned to be built at the Max-Planck Institute for Nuclear Physics in Heidelberg. The machine is expected to operate at low temperatures (similar to 2 K) and to store beams with kinetic energies between 20 to 300 keV. An electron target based on cooled photocathode technology will serve as a major tool for the study of reactions between molecular ions and electrons. Moreover, atomic beams can be merged and crossed with the stored ion beams allowing for atom molecular-ion collision studies at very low up to high relative energies. The proposed experimental program, centered around the physics of cold molecular ions, is shortly outlined.

The motion of an ion bunch trapped between two electrostatic mirrors is described and the two main regimes, called dispersive and self-bunching, are characterized. Use of the delta-kick cooling method is demonstrated for the dispersive mode.

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 recent development of electrostatic devices which allow us to store keV ion beams has launched several new kinds of investigations. We review the basic ideas behind the development of the electrostatic ion storage ring and the electrostatic ion beam trap techniques. The various experiments performed with atomic and molecular ion beams, ranging from the measurement of lifetimes of metastable atomic states up to biological applications and single component plasma studies are discussed.

The conventional way of trapping ions is based on the uses of RF or magnetic fields, like in Paul or Penning traps. In such traps the ions are stored with approximately zero kinetic energy. In many applications a well-defined ion beam is needed especially in collision experiment, where the initial direction of a beam is critical for reaction product measurements and a well defined field free region is required at the collision place. In the last few years we have developed a new type of electrostatic ion trap for ion beams of a few keV per charge, with no mass limit. The ions are injected through a stack of electrodes, which are used as an electrostatic mirror. The ions are confined in a region of few tens of centimeters by two electrostatic mirrors, located on opposite sides. The stability criterion of such a trap can be demonstrated to be similar to the one existing for optical resonator. The dynamics of such trapped ion beam was studied for various potentials on the electrostatic mirrors. Two modes of operation where found. In the first mode a self bunching effect was observed where the ion-ion Coulomb interaction generates a single bunch with constant length along the whole trapping time. This mode of operation can be used for Fourier mass spectrometry. A second mode, where the Coulomb interaction enhances the correlation between the ion position and momentum, enables phase space manipulation of the stored ion beam.

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 [Formula Presented] and electrons has been studied between 0 and 30 eV on a dense energy grid. [Formula Presented] 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 [Formula Presented] ions are given for an electron temperature range of 50300 K. Results for the DR cross section and the thermal rate coefficients are compared to recent theoretical calculations including rotational effects, finding satisfactory agreement.

We describe a new mass spectrometric technique that is based on the use of a linear electrostatic ion trap and a newly discovered self-bunching phenomenon. Ions are stored in the trap and their oscillation frequencies are determined by Fourier transform of their oscillation times. Using this system, we demonstrate that it is possible to simultaneously trap several masses and obtain their mass spectra with high resolution. The instrument is compared to time-of-flight mass, as well as to ion cyclotron resonance mass spectrometers.

The self bunching effect in an ion trap resonator was analyzed. Using a resonatorlike electrostatic ion trap, it was found that the width of a packet of ions oscillating between two mirrors could be kept constant. The effect was exhibited for specific values of the potential profile in the mirror regions for which the trap dispersion was positive.

We demonstrate that the dissociative recombination of [Formula presented] with low-energy electrons depends on the rotational energy of the molecular ion such that highly excited ions have a larger rate coefficient than colder ones. Observations on an ion beam continuously interacting with electrons at low relative velocity indicate that excited rotational levels are preferentially depleted which, in competition with radiative heating due to blackbody radiation, provides an opportunity for controlling the rotational temperature of stored molecules.

Keywords: DENSE INTERSTELLAR CLOUDS; H2O+; IONS; H3O+; OH

The motion of an ion bunch trapped between two electrostatic mirrors in an ion trap resonator has been studied. Under certain conditions the ion motion becomes correlated and either synchronization of the ion motion or enhanced diffusion occurs.

The relaxation of excited H₃⁺ molecules in the essentially collision-free environment of an ion storage ring is addressed. Using the coulomb explosion imaging (CEI) method, the observed time dependence of the vibrational excitation is compared with theoretically predicted rovibrational emission rates, and this comparison turns out to give indications also about the rotational excitation of the investigated molecules. The results show that complete relaxation can be achieved within ∼2 s for the vibrational degrees of freedom, including the infrared-inactive breathing mode.

The combination of two-dimensional imaging and storage ring techniques was used to study the breakup dynamics of H₃⁺ and D₃⁺ following dissociative recombination. The vibrational distributions of the molecular H₂ and D₂ fragments produced in the two body fragmentation channel and the kinematical correlation between the hydrogen or deuterium atoms were measured. The linear dissociation energies were found for the kinematical correlation between the hydrogen or deuterium atoms produced in the three body channel.

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.

A statistical model was used to estimate the branching ratios in the dessociative recombination (DR) of polyatomic molecular ions. The model was based on statistical assumptions about the redistribution of energy among the dissociating nuclei and about the population of electronic potential surfaces. The vibrational state populations of molecular fragments were found to be obtained and for H₃⁺, were in good agreement with the measurements.

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.

We report on the application of an electrostatic ion beam trap as a mass spectrometer. The instrument is analogous to an optical resonator; ions are trapped between focusing mirrors. The storage time is limited by the residual gas pressure and reaches up to several seconds, resulting in long ion flight paths. The oscillation of ion bunches between the mirrors is monitored by nondestructive image charge detection in a field-free region and mass spectra are obtained via Fourier transform. The principle of operation is demonstrated by measuring the mass spectrum of trapped Ar⁺ and Xe⁺ particles, produced by a standard electron impact ion source. Also, mass spectra of heavier PEG_nNa⁺ and bradykinin ions from a pulsed MALDI ion source were obtained. The long ion flight path, combined with mass-independent charge detection, makes this system particularly interesting for the investigation of large molecules.

A method for measuring the lifetime of metastable states with much higher precision was developed. For demonstration purposes, the method was used to measure the lifetime of the ¹S₀ metastable state of Xe²⁺. A value of 4.46±0.08 ms was obtained.

The radiative lifetime of the [Formula Presented] metastable state of [Formula Presented] has been measured with a different type of electrostatic ion trap. The ion trap stores ion beams of a few keV using purely electrostatic fields and provides a large field-free region where the stored ions oscillate. A photomultiplier working in the single-photon counting mode was used to observe the 380 nm photons arising from the magnetic dipole [Formula Presented] transition [Formula Presented] of [Formula Presented] The present measurement yields a value of [Formula Presented] which is in good agreement with previous measurements, but is almost four times more accurate.

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].

The lifetime of the metastable [Formula Presented] state is measured using an electrostatic linear ion trap which stores keV-ion beams. Trapping using electrostatic fields avoids the complication of magnetic-field-induced mixing effects which can interfere with the measurement of the spontaneous decay. The result is found to be [Formula Presented] which is a factor of 40 better in accuracy than the previous result determined in a heavy-ion storage ring. The measured lifetime is found to be 30% longer than the most recent theoretical value.

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.

The relative dissociative recombination rate coefficients for specific vibrational states of [Formula Presented] 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 311 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.

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.

Dissociative recombination of the polyatomic ions H₂O⁺, H₃O⁺, and CH₃⁺ 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 CH₃⁺. 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.

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.

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.

The dissociative recombination of vibrationally cold CD+ with electrons in the energy range of 0.01 eV to 60 eV has been measured using the Test Storage Ring in a merged beam arrangement. New resonance structures are observed with prominent peaks at 0.8 eV, 8.6 eV, and 11.7 eV. While the second and third resonance are readily attributed to direct recombination processes through Rydberg levels with dissociating molecular ion cores, the origin of the first resonance structure is not well understood, however, its small width of 0.45 eV is an indication of an indirect recombination process.

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.

The combination of laser photodetachment of C₄^- and the Coulomb Explosion Imaging method was applied for the investigation of the structure of several C₄ isomers and was correlated with their measured electron affinities.

The structure of protonated acetylene (C2H3+), including correlations, is measured and analyzed by an advanced Coulomb explosion imaging method. In the data analysis, it is essential to include large-amplitude motions of the nuclei within the molecule and many-body correlation features for revealing the correct structure. We find that the nuclear conformations in this molecule differ markedly from theoretical predictions and previous experimental findings.

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.