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.

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.

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.

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.

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.

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.

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

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.

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 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 breakup dynamics of H-3(+) and D-3(+) following dissociative recombination is studied using the combination of two-dimensional imaging and storage ring techniques. The vibrational distributions of the molecular H-2 and D-2 fragments produced in the two body fragmentation channel were measured, as well as the kinematical correlation between the hydrogen or deuterium atoms produced in the three body channel. For the latter, we find predominantly linear dissociation geometries. The data also show that the initial molecular ions H-3(+) and D-3(+), which were stored and electron cooled for up to 40 s prior to recombination, are still rotationally hot.

The diffusion and synchronization in an ion-trap resonator was studied. The motion of an ion bunch trapped between two electrostatic mirrors in an ion-trap resonator was analyzed by using numerical simulations. Results revealed that under certain conditions it was possible to achieve either synchronization of the ion motion or enhanced diffusion that lead to rapid debunching.

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.

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.

The vibrational population in stored H₂⁺ ions interacting with a beam of cold electrons was directly measured to analyze the development of this population as a function of the electron-ion interaction time. A comparison of the data with a simple model showed that the current theoretical understanding of the interaction of low-energy electrons with H₂⁺ is incomplete. Changes of the vibrational population occurred much faster than theoretically predicted.

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(+).

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.

Using an alternative technique to trap keV ion beams, the kinetic-energy release after dissociative charge transfer of vibrationally cold [Formula Presented] was measured. The results indicate that rapid radiative vibrational cooling takes place during the trapping, thus allowing a direct observation of the [Formula Presented] metastable state of [Formula Presented] which has been a subject of controversy in previous dissociative recombination experiments.

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.

A new type of ion trap is used to measure the radiative lifetime of the NO⁺(a ³Σ⁺) metastable state. The ion trap is designed to store ion beams with an energy of a few keV and is well suited for the study of metastable states. The measured value for the radiative lifetime is τ_r = 760±30 ms, in good agreement with the last experimental values of Calamai and Yoshino [J. Chem. Phys. 101, 9480 (1994)], and with the theoretical value of Kuo et al. [J. Chem. Phys. 92, 4849 (1990)].

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 lifetimes of the metastable [Formula Presented] level of [Formula Presented] as well as the lifetime of the average of the [Formula Presented] and [Formula Presented] levels, have been measured using a new type of ion trap that stores keV ion beams using electrostatic fields only. The use of a pure electrostatic field avoids the complication of magnetic-field-induced mixing effects, which can interfere with the measurement of the spontaneous decay. The measured lifetime for the [Formula Presented] state, after correction for decay induced by blackbody radiation, is [Formula Presented] This value is consistent with previous experiments, and in excellent agreement with the most recent theoretical calculations. The average lifetime of the [Formula Presented] and [Formula Presented] is [Formula Presented] which is about 20% lower than the weighted theoretical value.

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.

We present an experimental and theoretical study of the branching ratios in the dissociative recombination of HD⁺ with low energy electrons. The results give direct insight into the dynamics of the avoided curve crossing process between the dissociative state and the Rydberg series of the neutral molecule. Excellent agreement between the experimental results and the theory, based on a Landau-Zener formulation of the crossing process, is obtained.

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.

We present results for the total cross section for the dissociative recombination and dissociative excitation of HD+. Using the technique of two-dimensional imaging, the final states of the atomic fragments is determined at various electron energy. Cross section for the dissociative recombination of the heavier molecules CH+ and OH+ are also shown.

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.

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 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.