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Physical review accelerators and beams 24(5), 050101 (2021). doi:10.1103/PhysRevAccelBeams.24.050101, Published by American Physical Society, College Park, MD

We report our plasma rate coefficient and branching ratio measurements for dissociative recombination (DR) of D2Cl+ with electrons. The studies were performed in a merged-beams configuration using the TSR heavy-ion storage ring located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Starting with our absolute merged-beams recombination rate coefficient at a collision energy of approximate to 0 eV, we have extracted the cross section and produced a plasma rate coefficient for a translational temperature of approximate to 8 K. Furthermore, extrapolating our cross-section results using the typical low-energy DR behavior, we have generated a plasma rate coefficient for translational temperatures from 5 to 500 K. We find good agreement between our extrapolated results and previous experimental DR studies on D2Cl+. Additionally, we have investigated the three fragmentation channels for DR of D2Cl+. Here we report on the dissociation geometry of the three-body fragmentation channel, the kinetic energy released for each of the three outgoing channels, the molecular internal excitation for the two outgoing channels that produce molecular fragments, and the fragmentation branching ratios for all three channels. Our results, in combination with those of other groups, indicate that any remaining uncertainties in the DR rate coefficient for H2Cl+ appear unlikely to explain the observed discrepancies between the inferred abundances of HCl and H2Cl+ in molecular clouds and predictions from astrochemical models.

Ultraviolet and visible photodissociation of a vibrationally excited H(3)(+) ion beam, as produced by standard ion sources, was successfully implemented in an ion storage ring with the aim of investigating the decay of the excited molecular levels. A collinear beams configuration was used to measure the photodissociation of H(3)(+) into H(2)(+) + H fragments by transitions into the first excited singlet state with 266 and 532 nm laser beams. A clear signal could be observed up to 5 ms of storage, indicating that enough highly excited rovibrational states survive on the millisecond time scale of the experiment. The decay into H(2)(+) + H shows an effective time constant between about 1 and 1.5 ms. The initial photodissociating states are estimated to lie roughly 1 eV below the dissociation limit of 4.4 eV. The expected low population of these levels gives rise to an effective cross section of several 10(-20) cm(2) for ultraviolet and some 10(-21) cm(2) for visible light. For using multistep resonant dissociation schemes to monitor rotational populations of cold H(3)(+) in low-density environments, these measurements open promising perspectives.

We report on our recent studies of dissociative recombination (DR) employing two different fragment imaging detection techniques at the TSR storage ring in Heidelberg, Germany. Principles of an upgraded 3D optical system and the new energy-sensitive multistrip detector (EMU) are explained together with possible applications in reaction dynamics studies. With the EMU imaging detector we succeeded to observe the branching ratios after DR of deuterated hydronium ions D(3)O(+) at energies of 0-0.5 and 4-21 eV. The branching ratios are almost constant at low energies while above 6 eV both oxygen-producing channels O + D + D + D and O + D(2) + D strongly increase and dominate by about 85% at 11 eV. To demonstrate further capabilities of our fragment imaging detectors, we also summarize some of our additional recent studies on DR of molecular ions important for astrophysics as well as for fundamental unimolecular dynamics.

Time dilation is one of the most fascinating aspects of special relativity as it abolishes the notion of absolute time. It was first observed experimentally by Ives and Stilwell in 1938 using the Doppler effect. Here we report on a method, based on fast optical atomic clocks with large, but different Lorentz boosts, that tests relativistic time dilation with unprecedented precision. The approach combines ion storage and cooling with optical frequency counting using a frequency comb. 7Li+ ions are prepared at 6.4% and 3.0% of the speed of light in a storage ring, and their time is read with an accuracy of 2×10−10 using laser saturation spectroscopy. The comparison of the Doppler shifts yields a time dilation measurement represented by a Mansouri–Sexl parameter , consistent with special relativity. This constrains the existence of a preferred cosmological reference frame and CPT- and Lorentz-violating ‘new’ physics beyond the standard model.

Measurements on the energetic structure of the dissociative recombination rate coefficient in the millielectronvolt range are described for ions produced in the lowest rotational levels by collisional cooling and stored as a fast beam in the magnetic storage ring TSR (Test Storage Ring). The observed resonant structure is consistent with that found previously at the storage ring facility CRYRING in Stockholm, Sweden; theoretical predictions yield good agreement on the overall size of the rate coefficient, but do not reproduce the detailed structure. First studies on the nuclear spin symmetry influencing the lowest level populations show a small effect different from the theoretical predictions. Heating processes in the residual gas and by collisions with energetic electrons, as well as cooling owing to interaction with cold electrons, were observed in long-time storage experiments, using the low-energy dissociative recombination rate coefficient as a probe, and their consistency with the recent cold measurements is discussed.

Stored fast ion beams in atomic and molecular collision experiments are discussed with an emphasis on electron–ion interactions at low relative energies. Recent progress was obtained in electron collision spectroscopy using an electron-cooled stored ion beam and a separate electron target in the same storage ring; from a cryogenic photocathode, electron beams with internal temperatures of 5 to 10 K were produced. Results are presented for dielectronic recombination resonances, resolving the hyperfine structure of stored lithiumlike scandium ions and obtaining precise results for the fine structure splitting of these ions, and for ro-vibrational resonances in the recombination of electrons with hydrogen molecular ions, revealing sharp structures down to 2 meV. An overview of the cryogenic storage ring (CSR) project in Heidelberg is given.

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.

We have investigated electron capture by NH+ resulting in dissociative recombination (DR). The impact energies studied of ~4–12 eV extend over the range below the two lowest predicted NH+ dissociative states in the Franck–Condon (FC) region of the ion. Our focus has been on the final state populations of the resulting N and H atoms. The neutral DR fragments are detected downstream of a merged electron and ion beam interaction zone in the TSR storage ring, which is located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Transverse fragment distances were measured on a recently developed high count-rate imaging detector. The distance distributions enabled a detailed tracking of the final state populations as a function of the electron collision energy. These can be correlated with doubly excited neutral states in the FC region of the ion. At low electron energy of ~5 eV, the atomic product final levels are nitrogen Rydberg states together with ground-state hydrogen. In a small electron energy interval near 7 eV, a significant part of the final state population forms hydrogen Rydberg atoms with nitrogen atoms in the first excited (2D) term, showing the effect of Rydberg doubly excited states below the predicted 2 2Π ionic potential. The distance distributions above ~10 eV are compatible with nitrogen Rydberg states correlating to the doubly excited Rydberg state manifold below the ionic 2 4 Σ − level.

In order to improve the geometrical characterization of molecular ions using the Coulomb Explosion Imaging method, we discuss the need to include wake effects in a full description of the interactions felt by the swift dissociated atomic ions moving through thin foils. Comparison of simulations and experimental results for the structure of diatomic (HD+) and triatomic (CH2+) molecular ions is significantly improved when including the polarization effects induced in the target by the passage of swift ions. This work was supported by the Spanish Ministerio de Ciencia y Tecnología (projects HA2001-0052 and BFM2003-04457-C02-01/02), the DAAD in the framework of the Acciones Integradas Program 2002/03 and the German-Israel Foundation for Scientific Research (GIF) under Contract No. I-707-55.7/2001. SHA thanks the Fundación Cajamurcia for a Postdoctoral Research Grant.

The laser-spectroscopic frequency measurement of Doppler-shifted optical lines in the forward and backward directions of a fast ion beam permits a sensitive test of the relativistic Doppler formula and, hence, the relativistic time-dilation factor γSR = 1/[Formula: see text]. An experiment on 7Li+, stored at a velocity of v = 0.065c in the Heidelberg heavy-ion storage ring TSR, has confirmed time dilation with unprecedented accuracy limiting deviations to below 2.2 × 10–7. Ongoing improvements on the experimental setup will further tighten this limit.PACS Nos.: 03.30.+p, 06.30.Ft, and 42.62.Fi

In studies of the rate coefficient of the dissociative recombination of H3+ and its isotopomers, the rovibrational excitation of the molecular ions was found to play an important role, in particular when employing the technique of heavy-ion storage rings. The dependence of the DR rate on rotational excitation was investigated in recent experiments at the Test Storage Ring TSR in Heidelberg through time-resolved measurements on D2H+ and H3+ over long storage times. For both molecules, an influence of rotational excitation on the DR rate was observed. The level of excitation in turn was found to be dominated by radiative coupling to the surrounding 300 K background for D2H+. In the case of H3+, a strong influence of electron collisions on the excitation level was found, whereas an additional influence of collisions with residual gas in the storage ring cannot be excluded.

The dissociative recombination (DR) of the helium dimer 3He4He+ has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg at relative energies up to 40 eV. The vibrational level population in the stored ion beam was shown to be non-thermal with a fraction of 0.1–1% in higher vibrational states, resulting mainly from vibrational excitation in collisions with the residual gas species. The temporal evolution of the DR rate during storage showed evidence for an electron induced rotational de-excitation from the vibrational ground state. After haracterizing the evolution of the rovibrational population of the stored ions, the zero energy DR rate coefficient was extracted from the measurement to be αv=0 DR (0) = (7.3±2.1)×10−10 cm3/s, and the DR reaction from the vibrational ground state was seen to proceed mainly to the 1s2s 1S and 1s2p3P atomic limits. For v ≥ 3, the DR reaction has a rate coefficient ≥ 2×10−7 cm3/s and leads primarily to atomic fragments with n ≥ 3. The energy dependent rate coefficient displays several prominent structures.

We have investigated dissociative recombination (DR) of NH$^+$ with electrons using a merged beams configuration at the TSR heavy-ion storage ring located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We present our measured absolute merged beams recombination rate coefficient for collision energies from 0 to 12 eV. From these data we have extracted a cross section which we have transformed to a plasma rate coefficient for the collisional plasma temperature range from $T_{\rm pl} = 10$ to $18000$ K. We show that the NH$^+$ DR rate coefficient data in current astrochemical models are underestimated by up to a factor of $\sim 9$. Our new data will result in predicted NH$^+$ abundances lower than calculated by present models. This is in agreement with the sensitivity limits of all observations attempting to detect NH$^+$ in interstellar clouds., Comment: July 1st 2014: Accepted for publication in ApJ

The dissociative recombination (DR) of is a key process in interstellar chemistry. More than 30 experimental studies of the DR process have been published in the literature. The DR rate coefficient results obtained from these measurements, however, have not always been consistent. The outcome seems to depend on the experimental method, on the exact measurement procedure and sometimes even on the interpretation of the experimental data. In the past two decades, heavy-ion storage rings have become the working horse for DR measurements, as they provide a direct measurement of the DR products. Furthermore, storage ring measurements yield energy-resolved rate coefficients with unprecedented resolution that allow for detailed comparison with theory. DR results from different storage ring facilities have shown a remarkable consistency throughout the years and they provide additional information on break-up dynamics and internal excitation. In this study, we will review the storage ring DR measurements that have been carried out for .

An overview of recent developments is given for storage ring experiments on dissociative recombination using fast merged ion and electron beams. Dynamical aspects of the process are outlined, demonstrating that fast-beam reaction studies can advance the understanding of basic mechanisms holding generally for the fragmentation of electronically excited molecular systems. Experiments yield the collision energy dependence of the process, including rich resonant structure, product branching ratios of the fragmentation, and fragment momenta which also reveal the internal excitation of the fragments. Using cold electron beams and imaging detectors capable of fragment mass recognition at high readout speed, recent detailed fast-beam fragmentation experiments address the dissociative recombination of multielectron diatomic and polyatomic ions. Some of their results are presented together with an outlook on experiments at upcoming storage ring facilities.

High-resolution dissociative recombination rate coefficients of rotationally cool and hot ${\mathrm{H}}_{3}^{+}$ in the vibrational ground state have been measured with a 22-pole trap setup and a Penning ion source, respectively, at the ion storage-ring TSR. The experimental results are compared with theoretical calculations to explore the dependence of the rate coefficient on ion temperature and to study the contributions of different symmetries to probe the rich predicted resonance spectrum. The kinetic energy release was investigated by fragment imaging to derive internal temperatures of the stored parent ions under differing experimental conditions. A systematic experimental assessment of heating effects is performed which, together with a survey of other recent storage-ring data, suggests that the present rotationally cool rate-coefficient measurement was performed at 380${}_{\ensuremath{-}130}^{+50}$ K and that this is the lowest rotational temperature so far realized in storage-ring rate-coefficient measurements on ${\mathrm{H}}_{3}^{+}$. This partially supports the theoretical suggestion that temperatures higher than assumed in earlier experiments are the main cause for the large gap between the experimental and the theoretical rate coefficients. For the rotationally hot rate-coefficient measurement a temperature of below 3250 K is derived. From these higher-temperature results it is found that increasing the rotational ion temperature in the calculations cannot fully close the gap between the theoretical and the experimental rate coefficients.

We have performed measurements of the dissociative electron recombination (DR) of ${\mathrm{H}}_{3}^{+}$ at the ion storage ring TSR utilizing a supersonic expansion ion source. The ion source has been characterized by continuous wave cavity ring-down spectroscopy. We present high-resolution DR rate coefficients for different nuclear spin modifications of ${\mathrm{H}}_{3}^{+}$ combined with precise fragment imaging studies of the internal excitation of the ${\mathrm{H}}_{3}^{+}$ ions inside the storage ring. The measurements resolve changes in the energy dependence between the ortho-${\mathrm{H}}_{3}^{+}$ and para-${\mathrm{H}}_{3}^{+}$ rate coefficients at low center-of-mass collision energies. Analysis of the imaging data indicates that the stored ${\mathrm{H}}_{3}^{+}$ ions may have higher rotational temperatures than previously assumed, most likely due to collisional heating during the extraction of the ions from the ion source. Simulations of the ion extraction shed light on possible origins of the heating process and how to avoid it in future experiments.

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 3 eV 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.

International audience; Rotational cooling of HD+ by superelastic collisions (SEC) with electrons was observed at the Heidelberg test storage ring by merging a beam of rotationally hot HD+ ions with an electron beam at zero relative energy. Neutral fragments resulting from DR events were recorded at different electron densities using a high resolution imaging detector and a large-area, energy sensitive detector. The data allowed to deduce the time dependence of the population of three groups of rotational angular momentum states J built on the vibrational ground state of the ion together with the corresponding DR rate coefficients. The latter are found to be (statistical uncertainties only) langlealpharangle0,1,2 = 3.8(1), langlealpharangle3,4 = 4.0(2), and langlealpharangle5,6,7 = 9.0(1.3) in units of 10-8 cm3/s, in reasonable agreement with the average values derived within the MQDT approach. The time evolution of the population curves clearly reveals that rotational cooling by SEC takes place, which can be well described by using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclear rotation approximation. We verify the DeltaJ = -2 coefficients, which are predicted to be dominant as opposed to the DeltaJ = -1 coefficients and to amount to (1 - 2) · 10-6 cm3/s, to within 30%.

We report on an energy-sensitive imaging detector for studying the fragmentation of polyatomic molecules in the dissociative recombination of fast molecular ions with electrons. The system is based on a large area (10 cm x 10 cm) position-sensitive, double-sided Si-strip detector with 128 horizontal and 128 vertical strips, whose pulse height information is read out individually. The setup allows to uniquely identify fragment masses and is thus capable of measuring branching ratios between different fragmentation channels, kinetic energy releases, as well as breakup geometries, as a function of the relative ion-electron energy. The properties of the detection system, which has been installed at the TSR storage ring facility of the Max-Planck Institute for Nuclear Physics in Heidelberg, is illustrated by an investigation of the dissociative recombination of the deuterated triatomic hydrogen cation D2H+. A huge isotope effect is observed when comparing the relative branching ratio between the D2+H and the HD+D channel; the ratio 2B(D2+H)/B(HD+D), which is measured to be 1.27 +/- 0.05 at relative electron-ion energies around 0 eV, is found to increase to 3.7 +/- 0.5 at ~5 eV., 11 pages, 12 figures, submitted to Physical Review A

As a step towards in situ probing of the rotational-state distribution of H3+ ions in a stored beam, we have realized their photodissociation in a storage ring by a parallel ultraviolet laser beam. The decay of the photodissociation signal of H3+ for 4.9 eV photons into the channel H+2 + H was identified and showed a time constant of about 2 ms, typical for vibrational levels of H3+ over a wide range of binding energies. The photodissociation products of H3+ at this photon energy were further investigated for a single pass ion beam by kinetic energy release and charge-exchange measurements.

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 DeltaJ=-2 SEC rate coefficients, which are predicted to be dominant as opposed to the DeltaJ=-1 rates and to amount to (1-2)x10;{-6} cm;{3} s;{-1} for initial angular momentum states with J< or =7, to within 30%.

The dissociative recombination of the lowest rotational states of H3+ has been investigated at the storage ring TSR using a cryogenic 22-pole radiofrequency ion trap as injector. The H3+ was cooled with buffer gas at ~15 K to the lowest rotational levels, (J,G)=(1,0) and (1,1), which belong to the ortho and para proton-spin symmetry, respectively. The rate coefficients and dissociation dynamics of H3+(J,G) populations produced with normal- and para-H2 were measured and compared to the rate and dynamics of a hot H3+ beam from a Penning source. The production of cold H3+ rotational populations was separately studied by rovibrational laser spectroscopy using chemical probing with argon around 55 K. First results indicate a ~20% relative increase of the para contribution when using para-H2 as parent gas. The H3+ rate coefficient observed for the para-H2 source gas, however, is quite similar to the H3+ rate for the normal-H2 source gas. The recombination dynamics confirm that for both source gases, only small populations of rotationally excited levels are present. The distribution of 3-body fragmentation geometries displays a broad part of various triangular shapes with an enhancement of ~12% for events with symmetric near-linear configurations. No large dependences on internal state or collision energy are found., 10 pages, 9 figures, to be published in Journal of Physics: Conference Proceedings

Rate coefficients for dissociative recombination (DR), dissociative excitation (DE), and vibrational excitation between the helium dimer ion He-4(2)+ 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.

On a dense energy grid reaching up to 75 meV electron collision energy the fragmentation angle and the kinetic energy release of neutral dissociative recombination fragments have been studied in a twin merged beam experiment. The anisotropy described by Legendre polynomials and the extracted rotational state contributions were found to vary on a likewise narrow energy scale as the rotationally averaged rate coefficient. For the first time angular dependences higher than 2$^{nd}$ order could be deduced. Moreover, a slight anisotropy at zero collision energy was observed which is caused by the flattened velocity distribution of the electron beam., Comment: 8 pages, 4 figures; The Article will be published in the proceedings of DR 2007, a symposium on Dissociative Recombination held in Ameland, The Netherlands (18.-23. July 2008); Reference 19 has been published meanwhile in S. Novotny, PRL 100, 193201 (2008)

Angular fragment distributions from the dissociative recombination (DR) of HDwere 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 anisotro- pies 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. DOI: 10.1103/PhysRevLett.100.193201 PACS numbers: 34.80.Lx, 34.80.Ht Positively charged molecules can be fragmented effi- ciently by low-energy electron impact at collision energies down to the sub-eV range in the process of dissociative recombination (DR) (1). This reaction attracts strong in- terest in the modeling of the chemical networks in cold and dilute ionized media such as interstellar clouds (2), atmos- pheric layers (3), and industrial plasmas (4). It is driven by the formation of intermediate neutral compound states, in which the incident electron is captured following resonant energy exchange with the cation, and which then dissociate into neutral product channels reached via excited potential surfaces. Rovibrational Feshbach resonances were recognized to play an important role at low electron energies in the DR (5) as well as in the related process of dissociative electron attachment (DEA), an efficient destruction channel of neu- tral molecules by low-energy electron impact (6,7). They reflect the energy exchange of the incident electron with the nuclear motion, in contrast to the one within the elec- tronic system alone. Thus, in the direct mechanism (5 )o f DR purely electronic energy exchange forms a neutral, doubly excited potential surface unstable against dissocia- tion. In the indirect mechanism (5) a rovibrationally ex- cited neutral Rydberg state is formed with the initial bound electronic state of the ion as a core and then predissociated by often the same doubly excited surface. These mecha

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 ${\text{CH}}_{2}^{+}$. 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: $\text{C}(^{3}P)+\text{H}(^{2}S)+\text{H}(^{2}S)$ and $\text{C}(^{1}D)+\text{H}(^{2}S)+\text{H}(^{2}S)$.

The energy-resolved rate coefficient for the dissociative recombination (DR) of $\mathrm{H}_{3}{}^{+}$ with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to $500\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ by using a cryogenic GaAs photocathode. This and a previous cold-$\mathrm{H}_{3}{}^{+}$ measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate $\mathrm{H}_{3}{}^{+}$ in the two lowest, $(J,G)=(1,1)$ and $(1,0)$ rotational states prevailing also in cold interstellar matter. The use of para-${\mathrm{H}}_{2}$ in the ion source, expected to enhance para-$\mathrm{H}_{3}{}^{+}$ in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

This work has been supported in part by the German-Israel Foundation for Scientific Research (GIF) under Contract No. I-707-55.7/2001, the Spanish Ministerio de Ciencia y Tecnologia (Project Nos. BFM2003-04457-C02-01/02 and HA2001-0052), the DAAD in the framework of the Acciones Integrados Program 2002/03, and the European Community within the Research Training Network “Electron Transfer Reactions.” One of the authors (S.H.A.) thanks the Fundacion Cajamurcia for a Postdoctoral Grant.

The dissociative recombination (DR) of He-3 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 x 10(-9) cm(3)s(-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 similar to 0.1-1% 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 +/- 0.9) x 10(-10) cm(3)s(-1). The corresponding branching ratios from v=0 to the atomic final states are found to be (3.7 +/- 1.2) % for 1s2s S-3, (37.4 +/- 4.0) % for 1s2s S-1, (58.6 +/- 5.2) % for 1s2p P-3, and (2.9 +/- 3.0) % for 1s2p P-1. A DR rate coefficient in the range of 2 x 10(-7) cm(3)s(-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 15-40 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 have experimentally investigated dissociative recombination of NH+ with electrons using a merged ion and electron beam configuration in a storage ring. A fast counting and position sensitive imaging detector enabled us to perform fragment imaging measurements over relative electron-ion collision energies from 0 to 12 eV. The results show unprecedented details on product excitation and on the reaction dynamics.

We report on dissociative recombination measurements of several halogen-containing molecular ions using a merged electron and ion beams geometry.

Storage ring studies of low energy electron collisions with molecular ions have been carried out for dissociative recombination (DR) of fluorine-bearing molecules. Here we report on work aiming to improve the understanding of astrochemistry involving HF, a possible spectroscopic tracer of interstellar H2. For CF+ the rate coefficient was obtained for temperatures down to 10 K. For D2F+ the DR fragmentation branching ratios were determined to be 66(3)%, 24(2)%, and 10(2)% for the F+D+D, DF+D, and D2+F channels, respectively. The molecular DR products of this reaction, DF and D2, display an unusually high level of internal excitation, close to their dissociation limit.

The energy dependence of the branching ratios in dissociative recombination of DCO+ with a known small admixture of DOC+ has been measured for collision energies from 0 to 25eV using an energy- and position-sensitive surface barrier detector which is designed for the analysis of multi-fragment events occurring in a molecular fragmentation study. The measurements are compared with theoretical calculations on the direct mechanism of dissociative recombination of HCO+ including the contribution from HOC+ at the experimental abundance fraction. At low collision energies, dissociative recombination of HCO+ is dominated by dissociation into H + CO. For collision energies above 2eV there is a transition into dissociation to HC+O, which can be explained by electron capture into resonant states. Signatures of DOC+ dissociative recombination are found experimentally and confirmed by the calculations for HOC+. Three-body breakup becomes important for collision energies above 6eV.

Dissociative recombination (DR) is an exothermic process leading to neutralisation and chemical fragmentation in many plasma environments, down to the lowest temperatures. We present results of complex investigations on DR of HF+ and CF+ performed at the TSR storage ring. Beyond the rate coefficient down to meV collision energies also dissociation pathways and geometries have been investigated using full 3D fragment imaging technique. Fragment imaging also provides HF+ rotational structure and fine-structure as well as the HF+ dissociation energy with sub-meV precision.

Dielectronic recombination of highly charged atomic ions and dissociative recombination of molecular ions have been investigated using fast ion beams merged with cold intense electron beams (down to

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 αv = 1→0SEC (Ed = 0)≈1.8×10−7cm3s−1 was found.

We present results from our recent studies of the dissociative recombination of the CF+ cation. On one hand, dissociative recombination was measured with 3 MeV CF+ ions in the heavy-ion Test Storage Ring in Heidelberg, using the twin electron beam configuration with an electron cooler and a separated electron target for collision measurements. In this experiment, the low temperatures of the electron beam provided by a photocathode (temperature in co-moving frame below 1 meV) account for a fast kinetic cooling of the heavy-ion beam and a high resolution in the measured rate coefficients. Fragment imaging measurements show a complete switching of the dissociation route by only a small change of the collision energy and the disappearance of neutral Rydberg product states on crossing the DE threshold. On the other hand, extensive calculations of energy positions and autoionization widths for the doubly excited states of CF between the first and second ionization thresholds have been obtained from electron scattering calculations using the complex Kohn variational method, followed by calculations of the dissociative recombination process with the multichannel quantum defect theory. In preliminary computations, only the first dissociative state in each molecular symmetry, which lies closest in energy to the ion potential at its equilibrium internuclear separation, and thus is dominant for the low-energy dissociative recombination, was included. Although only the direct mechanism of dissociative recombination reaction has been considered in this step, the size and the shape of the DR rate coefficient are already well reproduced.

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).