Table of contents

Dissociative recombination between electrons and molecular ions is an elementary reaction in electron-induced chemistry attracting strong attention across discipline boundaries, from fundamental questions of intramolecular dynamics to astrophysics, plasma science, as well as atmospheric and planetary physics. The process is explored on the level of atomic quantum dynamics both experimentally and theoretically, employing cold collisions at temperatures down to 10 Kelvin involving small molecules or also very large systems ranging up to biomolecules. Dissociative recombination (DR) and related processes, such as dissociative excitation, collisional cooling of vibrations and rotations, photodissociation via high-lying electronic states, resonant electron attachment, and electron-induced processes in large molecules and clusters, are studied by a variety of experimental methods, including stored and trapped molecular ions, plasma techniques such as stationary and flowing afterglow, and laser spectroscopic diagnostic of molecular excitations.

The Sixth International Conference on Dissociative Recombination: Theory, Experiments and Applications (DR2004) was organized by the Research Group on Atomic and Molecular Physics with Stored Ions at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany, and held near Heidelberg in the town of Mosbach in July 2004. It was attended by about 90 scientists working in atomic and molecular physics, astrophysics, plasma- and biophysics. International Conferences on Dissociative Recombination and related processes were held before at Lake Louise, Alberta, Canada (1988), Saint Jacut, Brittany, France (1992), Ein Gedi, Israel (1995), Nässlingen, Stockholm Archipelago, Sweden (1999), and last within a symposium at the American Chemical Society meeting in Chicago, USA (2001). The presentations of this conference document a strong development of theoretical ideas towards the understanding of DR in particular in polyatomic systems. Strong attention was given to the elementary triatomic benchmark system H₃⁺, characterized by ambitious, complementary experimental projects. Interaction of experiment and theory improves in particular the understanding of non-adiabatic molecular interactions involving electronic continuum states. New experimental techniques focus on a detailed control of the internal molecular excitation on the level of single quantum states, which gives increasing importance to laser interactions and ion storage at cryogenic temperatures.

Apart from its place in the series of "DR conferences", this meeting is also the final assembly of the EU Research Training Network "Electron Transfer Reactions" (ETR) which in the period from 2000 to 2004 helped to establish many invaluable links between 15 experimental and theoretical institutes active in the field of DR and related processes. We express our gratitude to the EU for the support through the Research Training Network Programme, which has made possible the attendance of many students and young researchers. Furthermore, generous financial support for this conference was provided by the Max-Planck Institute for Nuclear Physics in Heidelberg. The efficient support of the conference center "Alte Mälzerei", operated by the city of Mosbach, is gratefully acknowledged. Finally we warmly thank the staff and the students of the Max-Planck Institute for Nuclear Physics for the dedicated help during the conference.

A general overview of the molecular content in interstellar clouds is reported. We concentrate on the role of dissociative recombination on interstellar chemistry. Recent observations are emphasized both in diffuse and dense clouds environments. Specific suggestions of theoretical and/or experimental studies on branching ratios involved in dissociative recombination experiments are highlighted.

Molecular processes first took place in the Universe in the recombination era as the expanding Universe cooled adiabatically and recombined when the cooling radiation field ran out of photons energetic enough to cause photoionization. The formation of neutral helium heralded the dawn of chemistry as the neutral atoms participated in processes of radiative association to form molecular ions. Dissociative recombination of the molecular ions produced neutral atoms and accelerated the conversion of the ionized plasma into a neutral gas. The subsequent chemistry involved hydrogen, deuterium, helium and lithium created earlier in a brief period of nucleosynthesis. With the continued expansion the Universe became cold and dark and chemistry came to a temporary end until the formation and gravitational collapse of the first distinct cosmological objects. Heavy elements were made, a new source of radiation – starlight – appeared and a richer chemistry was initiated.

In this review, the roles of dissociative recombination reactions and other ionic processes in the exotic environment of interstellar space are elucidated through a discussion of the interstellar medium and the chemistry that occurs in it. Both recent progress in and current challenges to interstellar ion chemistry are emphasized.

Cross sections and branching ratios for the dissociative recombination (DR) reactions of the astrophysically important ions HN₂⁺, HCO⁺, DOCO⁺, and SO₂⁺ at reactant kinetic energies from 1 to 1000 meV have been measured using the CRYRING ion storage ring facility at the Manne Siegbahn Laboratory, Stockholm University. Whereas the break-up of the N-N bond leading to NH + N is the major pathway in the DR of HN₂⁺, the analogous reaction in HCO⁺ almost exclusively leads to H and CO. In the DR of both DOCO⁺ and SO₂⁺ three-body break-up was observed. Inclusion of the newly measured branching ratios into a standard model on dark interstellar clouds leads to an improvement of the predictions of such models, especially concerning the abundances of nitrogen compounds. The impact of these newly found branching ratios and reaction rates on the chemistry of different astronomical environments like dark clouds, circumstellar envelopes and planetary ionospheres, is discussed.

Because dissociative recombination (DR) reactions of molecular ions are often highly exothermic, in the thermospheres of the Earth and planets DR may be a source of translationally and internally excited fragments. DR is important, therefore, for thermospheric neutral heating; if the excited fragments radiate to space, however, DR may be also a source of thermospheric cooling. DR may produce metastable fragments, which may live long enough to participate in reactions that are not available to ground state species. It is rare, however, for DR to be a significant source of minor species in their ground states. An exception appears to be the DR of CO⁺₂, which has recently been found to produce C + O₂ about 9% of the time by Seiersen et al.. Because of the significant rearrangement of bonds that must take place, the branching ratio for the latter channel has been assumed to be negligible, and DR of CO⁺₂ has been assumed to produce mainly CO + O. In order to test the effect of including the branching ratio of CO⁺₂ DR that produces C + O₂ on the ambient densities of thermal and escaping C in planetary thermospheres,we have we have constructed revised models of the thermospheres/ionospheres of Mars and Venus. Because of space limitations, we discuss here only the high solar activity models. For Mars, we find that DR of CO⁺₂ is the most important source of thermal C, but that the production rate of escaping C is not increased. There are important differences between the thermospheres of Venus and Mars, and we find that the inclusion of the C + O₂ channel in the Venus models increases the production rate of atomic carbon in the Venus thermosphere by only 10–16%. At high altitudes on Venus, C⁺ is mostly produced by photoionization and electron-impact ionization of C, with some contribution from the charge transfer reaction, O⁺ + C → C⁺ + O. We compare our computed C density altitude profiles to those inferred by Paxton from Pioneer Venus Orbiter Ultraviolet Spectrometer limb scans of the atomic carbon emission features at 1561 and 1657 Å. Since the most important loss process for C is reaction with O₂, this allows us to to constrain the abundance of O₂ in the Venus thermosphere. We then compute density profiles of C⁺ and compare them to those measured by the Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) (e.g., Taylor et al. [3]) to determine the rate coefficient for the charge transfer reaction of O⁺ to C.

Two major techniques have been used to study electron-ion recombination, afterglows and storage rings. These have differing strengths and limitations for the determination of recombination rate constants/cross-sections, including those as a function of temperature/energy, and for the identification and quantification of the neutral products, both rovibronically excited and ground state. Here, the afterglow techniques will be reviewed (the storage ring is considered by Larsson) with emphasis on new developments. In this vein, there will be discussion of (i) rate constants for larger species, (ii) temperature dependencies of rate constants including those for some isomers, and (iii) electronically excited states; all these connect with other papers in these proceedings. New developments are discussed; in particular a means of quantifying the neutral product distributions, and possible new directions are suggested.

This paper discusses the application of the merged-beam technique to the study of dissociative recombination. After a description of the experimental method, attention is turned to H₂⁺ and its isotopomers. The apparent perfect agreement between experiment and theory which seemed to prevail in the early 1990s has had to give way to an agreement which is satisfactory but far from perfect. The consistent set of data for HD⁺ from merged electron-ion beams in four different ion storage rings is very satisfying, while it is difficult to reconcile single-pass data for H₂⁺ with those from an ion storage ring.

The angular distributions of the products of dissociative recombination of diatomic molecular ions are derived. The distributions are determined by the dominant partial waves of the captured electrons. For a single dominant wave, the neutral product distribution is described by the same spherical harmonic that describes the electron partial wave. The derived distributions take into account both direct and indirect dissociative recombination and are essential for modeling the projected distributions measured in storage ring experiments. All prior storage ring experiments have assumed that the product angular distributions can be described by spherical harmonics with angular momenta ≤ 1 and isotropic product distributions for "zero" eV electron capture. These assumptions are tested here by deriving the projected distributions for the dissociative recombination of CH⁺. For CH⁺, it is shown that the "zero" eV storage ring experimental results cannot be explained by only isotropic dissociation. Instead, an anisotropic model product distribution is required. The anisotropic model yields a quantum yield of 1.0 for C(¹D) instead of the incorrect value of 0.79 derived previously using an isotropic model. The quantum yields of all prior storage ring experiments on other molecular ions at "zero" eV electron energy must be reassessed in view of these findings.

The photo-excitation or photo-ionization of a polyatomic molecule is typically accompanied by a strong excitation of the vibrational modes. In particular when a conical intersection of the electronic potential energy surfaces involved lies within or close to the Frank-Condon zone, the nuclear motion becomes very complicated, often chaotic, and the spectra become irregular and dense.

An accurate simulation of the dynamics of such excited molecules requires firstly that the multi-dimensional and multi-state potential energy surface – or a reliable model thereof – can be determined. Secondly, the multi-dimensional quantum dynamics have to be solved. This is a very difficult task, because of the high dimensionality of the problem (6 to 30 degrees of freedom, say). The multi-configuration time-dependent Hartree (MCTDH) method has proven to be very useful for the study of such problems. In fact, an accurate treatment of the quantal dynamics of molecules like the allene cation (C₃ H⁺₄, 15D), the butatriene cation (C₄ H⁺₄, 18D), or the pyrazine molecule (C₄N₂H₄, 24D) in their full dimensionality, is – up to date – only possible with MCTDH. (The acronym n D denotes the dimensionality.)

The construction of the vibronic model Hamiltonian and the MCTDH method will be briefly discussed. After this, the excited state dynamics of the butatriene and pyrazine molecules will be discussed.

We report recent developments in the theory of dissociative recombination of the H₃⁺ ion. The theory is able to explain the surprisingly large dissociative recombination rate observed in storage-ring experiments. We have found that the high rate is caused by Jahn-Teller coupling between the electronic and vibrational degrees of freedom. We compare two theoretical models: A simplified model and an improved one. The two models each produce a theoretical rate that is consistent with the storage-ring observations. The final theoretical dissociative recombination rate presented here accounts for several experimental details that are often not considered.

After more that 30 years of experimental and theoretical work it now appears that theory and experiment on H₃⁺ recombination have finally converged. Since the storage ring results come very close to the latest theoretical calculations one might conclude that "the case is closed". However, some troublesome issues remain and should not be dismissed too quickly. For instance, some afterglow measurements showed faster decays at early afterglow times. It is now clear that the fast recombining species cannot be vibrationally excited ions since the same rates were found for spectroscopically identified H₃⁺ ions in ν=0. On the other hand, there is no convincing explanation for the very small rates obtained in the Prague experiments at low H₂ densities. It is also puzzling that some measurements find nearly the same recombination coefficients for H₃⁺ and D₃⁺, while others indicate that H₃⁺ recombines much faster. It has often been stated that vibrational excitation of the H₃⁺ ions tends to enhance recombination, but the evidence for that is far from solid; some observations suggest that the opposite is just as likely.

We present dissociative recombination measurements, using the CRYRING ion storage ring, of H₃⁺ ions produced in a supersonic expansion discharge source. Before and after the CRYRING measurements, the ion source was characterized in Berkeley using infrared cavity ringdown spectroscopy, and was found to exhibit a typical rotational temperature of ∼30 K. Our measurement of the dissociative recombination cross section using this ion source revealed resonances that had not been observed clearly in previous experiments that used rotationally hot ion sources. Based on the present measurements, we infer a thermal dissociative recombination rate coefficient for ions at interstellar temperatures of ∼ 2.6 × 10⁻⁷ cm³s⁻¹. Our results are in general agreement with theoretical calculations of the dissociative recombination cross section by Kokoouline and Greene. We will review the enigma of the abundance of H₃⁺ in the diffuse interstellar medium, and discuss the impact of these experiments, especially in the context of the recent observation of H₃⁺ towards ζ Persei.

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 recombination of H⁺₃ ions with electrons has been studied in afterglow plasma in three different experiments. In two experiments, using the Variable Temperature Stationary Afterglow (VT-AISA) and the Variable Temperature Flowing Afterglow (VT-FALP) techniques, a decay of the electron number density was measured by an electrostatic Langmuir probe to determine the recombination rate coefficient. In the third experiment a near infrared Cavity Ring-Down Absorption Spectrometer (CRDS) was used to monitor the decay of the H⁺₃ (v = 0) ion density during the afterglow. Measurements were carried out in helium buffer gas with small admixtures of argon and hydrogen at total pressures ranging from 150 up to 1200 Pa and at buffer gas temperatures ranging from 100 up to 330 K. In the experiments the partial number density of hydrogen was varied from 5 × 10¹⁰ up to 1 × 10¹⁶ cm⁻³ and for this broad range of hydrogen number densities effective recombination rate coefficients were obtained, which varied over three orders of magnitude from 2 × 10⁻⁹ cm³s⁻¹ at [H₂] = 5 × 10¹⁰ cm⁻³ up to 3 × 10⁻⁶ cm³s⁻¹ at [H₂] = 1 × 10¹⁶ cm⁻³. Using our experimental results we discuss possible mechanisms of recombination in hydrogen plasma in a very broad range of several parameters: buffer gas pressure, temperature, electron number density, hydrogen number density and internal excitation of recombining ions.

Charge-exchange neutralization of H₃⁺ with Cs allows preparation of the low-lying Rydberg states of H₃. These states are predissociated by the repulsive ground state and may play roles as intermediates in the dissociative recombination of H₃⁺ + e⁻. Translational spectroscopy and measurements of product momentum partitioning in three-body dissociative charge exchange of fast (12 keV) H₃⁺ and D₃⁺ with Cs yields insights into the nuclear motion during dissociation for the three lowest-lying 2s²A₁', 2p²A₂'' and 3p²E' bound Rydberg states of H₃ and the two 2s²A₁' and 2p²A₂'' states for D₃. This data provide an empirical benchmark for the refinement of theoretical models involving non-adiabatic interactions and dynamics for H₃.

We report a study of the recombination of H₃⁺ (v = 0) ions with thermal electrons at 330 and 100 K. A near infrared cavity ring-down absorption spectrometer (CRDS) working on the v₂ = 3 ← 0 transition of H₃⁺ (λ = 1382 nm) has been used to monitor the H₃⁺ (v = 0) ion number density in decaying afterglow plasma. The plasma was created in helium gas with small admixtures of argon and hydrogen by pulses of microwaves. The measurements were carried out for hydrogen number densities ranging from 10¹³ up to 10¹⁶ cm⁻³. The total pressure in the discharge tube was 4–10 mbar. The temperature of the recombining ions was determined from the Doppler broadening of absorption lines. The obtained effective recombination rate coefficients are α (H₃⁺ (v = 0)) = (0.8 ± 0.3) × 10⁻⁷ cm³s⁻¹ and α(H₃⁺ (v = 0)) = (2.3 ± 1.1) × 10⁻⁷ cm³s⁻¹ at 330 and 100 K, respectively. The influence of the formation of H₅⁺ at higher pressures and lower temperature is also discussed.

Ion storage and trapping techniques in connection with efficient internal state control and diagnostic for molecular ions offer the potential of providing rate coefficients for the dissociative recombination of H₃⁺ for well-defined initial rotational states, required for understanding the role of this ion in the interstellar medium and other cold environments. Information on the vibrational and rotational excitation in stored H₃⁺ ion beams, as obtained from experiments at the ion storage ring TSR, is reviewed. In addition, the arrangement of the TSR injector trap, using buffer gas cooling in a cryogenic radiofrequency multipole structure to inject pulses of internally cold H₃⁺ ions into the TSR via a high-energy accelerator, is outlined. An account is given of tests towards the in-situ diagnostic of rotational level populations, where laser transitions between low-lying rovibrational levels could be detected in dilute H₃⁺ ion ensembles using chemical probing in the radiofrequency multipole ion trap.

The method of laser induced reaction (LIR) is used to obtain an IR spectrum of bare CH₅⁺ in the range of 250 to 3200 cm⁻¹. The experimental spectrum compares rather favorable to theoretical predictions based on molecular dynamics simulations except for the very low frequency range below 500 cm⁻¹. An equation relating the experimental LIR signal to the absorption coefficient and the rate of reaction of the excited species as well as a simple model for the reaction rate coefficient of the laser excited molecules is derived. A variety of LIR schemes are exemplified and their value for IR spectroscopy of molecular ions is discussed.

The study of dissociative recombination is complicated by the high energy and large number of channels that are open. Theory in this area has not kept up with experiment with few molecules studied. We will present our work on the study of the 'direct' dissociative recombination process, where one or more resonant states exist. The resonance parameters are taken from accurate electron scattering calculations and used as input to the dynamics calculations. We will illustrate the method with the study of DR in the HCN⁺ and HNC⁺ systems.

We report on ab initio calculations relevant for dissociative recombination of HCO⁺. From accurate quantum chemistry calculations, it is found that the electron collision is driven by capture into Rydberg states. The Renner-Teller effect is not important for higher Rydberg states. From calculated potentials, the effective quantum numbers are fitted in three dimensions. The model of the electron collision is simplified using an adiabatic approximation, where one of the Jacobi coordinates is treated adiabatically. Then the electron collision is described using two different theoretical methods. Preliminary results on autoionization widths for Rydberg states and dissociative recombination cross section are given.

The vibrationally and rotationally specific cross section is investigated for the dissociative recombination (DR) of H⁺₂ and HeH⁺ using the multi-channel quantum defect theory. In order to give the zero energy cross section, a convolution formula is introduced. A systematics in the vibrational dependence of DR is made clear. The rotational motion plays an important role in the DR at energies lower than 0.1 eV, causing large enhancement in some cases and rotational sensitivity. The state-specific investigation indicates a possibility of a non-thermal distribution of the initial rotational states in storage-ring experiments on HD⁺.

Preliminary results which will be used for calculations of dissociative recombination (DR) of electrons with the CO²⁺ dication are presented. The measurements of CO²⁺ dissociative recombination rates, the first for any multiply charged target, were obtained at the ASTRID heavy ion storage ring. The present study starts from the potential energy curves for the first 7 electronic states of CO²⁺ and results for low-energy e⁻ – CO²⁺ scattering that were obtained in recent R-matrix calculations (Vinci and Tennyson 2004 J. Phys. B 37 2011). Meanwhile, in order to apply an MQDT-type approach that has been previously used for NO⁺, we concentrate on partial the resonance series converging to the ¹Δ target state in the CO⁺²Σ⁺ symmetry. The quasi-discrete vibrational spectrum of the CO²⁺ ground electronic state is explored.

The dissociative recombination (DR) of the helium dimer ³He ⁴He⁺ 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 characterizing the evolution of the rovibrational population of the stored ions, the zero energy DR rate coefficient was extracted from the measurement to be α^v=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 1s2s¹S 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.

Resonant dissociative recombination of the He₂⁺ molecular ion is an important process in the ionosphere as well as in laboratory plasmas. Recently, rate coefficients have been measured for the dissociative recombination of He⁺₂ using the ion storage ring TSR in Heidelberg, Germany. Previously, calculations have been reported for the cross section in the low energy region. We will report results in the 0 to 15 eV energy region, where the cross section is dominated by a series of resonances converging to the first excited state of the molecular ion. We will also discuss the resonances converging to the next series of excited states. The resonance parameters for this system are obtained from electron scattering calculations using the Complex Kohn variational method. These resonance parameters are used as input to a time-dependent wave packet calculation of the dissociation dynamics. The calculated cross sections and rates will be reported and compared to available experiment.

We report data on a study into the vibrational dependence in the dissociative recombination of O⁺₂ (X²Π_g). The experiment has been performed with a merged-beam apparatus in the heavy-ion storage ring, CRYRING. We present the total rate coefficients as function of collision energy for five different vibrational populations of the ion beam and the partial rate coefficients and branching fractions near 0 eV collision energy for the vibrational levels v = 0, 1 and 2. Furthermore, we report on the control over and characterization of vibrational populations. The total rate coefficient is found to be weakly dependent on the vibrational excitation. The rate decreases upon increasing vibrational excitation, though only within an order of magnitude. The partial rate coefficients are strongly dependent on the vibrational level; with the v = 0 oxygen ions recombining the fastest. The partial branching fractions are also found to be strongly dependent on the vibrational level, specifically the O(¹S) quantum yield which increases strongly upon increasing vibrational level.

In several recent dissociative recombination (DR) experiments, the observed DR products depend on the structure, bonding and charge center of the molecular ion. For examples, the dominant product channel observed in the DR of N₂O⁺₂, D⁺₅, and D₅O₂⁺ suggests that the former two ions have the form NO⁺·NO, and D⁺₃·D₂, respectively, whilst the latter is known to have the form D₂O·D⁺·D₂O. We compare and contrast these observations by investigating the DR of one of the simplest such systems, Li⁺·H₂. This system, a weakly bound cluster with the charge center located on the lithium atom, will provide us with an excellent opportunity for investigating the role played by the type of bonds and charge center in the DR process.

Rate constants and product distributions have been measured for a number of hydrocarbon ions at the CRYRING facility at the Stockholm University. Rate constants at 300 K are about 5×10⁻⁷ cm⁻³ s⁻¹. The electron temperature dependences are also roughly constant and follow a power law. The products appear to correlate with reaction exothermicity.

Branching ratios for the recombination of hydrocarbon ions have been measured using the ASTRID storage ring electron cooler. These studies have concentrated upon the competition of channels where the carbon skeleton of the molecule is left intact with other channels where carbon-carbon bonds are broken. A discussion of the effects of initial molecular structure on the resulting dissociation pattern is given.

Molecular ions often play a very important role in plasmas. The electron energy distribution function (eedf) and density (n_e) are influenced by the reactions of molecular ions with electrons. We bring these aspects into focus by studying successively the following situations: We show that the dissociative recombination of Ar⁺₂ allows to understand the measured characteristics of an argon supersonic plasma flow where the electron density is low. Afterwards, we show the dominating role of NO⁺ in the chemistry of a space vehicle atmospheric re-entry air plasma. Finally, by using the Boltzmann equation in order to show the influence of molecular ions such as NO⁺ in air plasmas on the eedf, we comment on the common assumption of Maxwell-Boltzmann equilibrium for this distribution.

A new technique, Flowing Afterglow with Photo Ions – FLAPI, has been developed for measuring the rate coefficient for the recombination of complex ions with electrons. The method is based on the FALP-MS apparatus at the Université de Rennes I. A helium plasma is generated by a microwave discharge in a He buffer gas and downstream a small amount of argon gas is injected to get rid of helium metastables. A very small amount of neutral PAH molecules is added to the afterglow plasma by evaporation from a plate coated with the PAH to be studied. PAH ions are then produced by photoionization of the parent molecule using a pulsed UV laser (157 nm). The laser beam is oriented along the flow-tube and so a constant spatial concentration of photoions is obtained. The electron concentration along the flow-tube is measured by means of a movable Langmuir probe. The decay of the ion concentration in time is measured at a fixed position using a quadrupole mass spectrometer which is triggered by the laser pulse. Anthracene ion recombination has been studied using this technique and we have obtained the preliminary recombination rate coefficient (1.1 ± 0.5) × 10⁻⁶ cm³ s⁻¹.

Experiments are described where two-photon transitions in atomic xenon have been laser-excited from the ¹S₀ ground state to Xe* np (n = 8−11) and Xe* nf (n = 4−8) Rydberg states in an apparatus which combines a time-of-flight mass spectrometer and a dispersive photoelectron spectrometer. Xe* atoms in nf-states, but not in np-states, undergo rapid associative ionization to form Xe₂⁺, followed by dissociative recombination to form atoms predominantly in lower energy 6p and 5d excited states. Orbital selectivity for associative ionization can be explained by a simple qualitative model where the Rydberg nf-electron is radially localized outside the Xe⁺ core for long periods of time due to a larger centrifugal barrier in the effective atomic potential. This enhances the probability that excited Xe atoms in an nf state will encounter a ground state atom at those impact distances that lead to the formation of dimer ions.

Dissociative recombination of protonated hydrogen cyanide HCNH⁺ is a very important process in dark interstellar molecular clouds. The dominant mechanism that drives this process, either 'direct' through a resonance, or 'indirect' through Rydbergs is currently an issue of controversy. Only qualitative conclusions for the branching ratio between the HCN and HNC fragments is available. We will report ab initio electron scattering calculations using the complex Kohn variational method for low energy electron scattering from HCNH⁺ using a correlated wave function for the target. Resonance energies and widths were abstracted and their behavior as a function of the internuclear geometry is studied.

We report extensive calculations of energy positions and autoionization widths for the doubly excited states of Ne₂⁺ between the first and second ionization thresholds obtained from electron scattering calculations using the complex Kohn variational method. The dynamics of the dissociative recombination process was investigated using multichannel quantum defect theory. For these preliminary calculations, only the direct mechanism of the dissociative recombination reaction was studied and only the ^1,3Σ_g⁺ dissociative states, which lie closest in energy to the ion at its internuclear separation were included. These states should dominate the low-energy dissociative recombination.

The rate coefficient for the NeH⁺ dissociative recombination (DR) with electrons was recently measured at the ASTRID storage ring in Denmark. The rate coefficient, as a function of the electron energy, is non-negligible at near-zero energy and displays two broad peaks between 10 and 30 eV. Both peaks are due to DR via Rydberg states converging to different dissociation limits of the NeH⁺ ion. The first one is due to the capture of the incoming electron by doubly excited Rydberg states dissociating in Ne (1s²2s²2p⁵3s) + H(n ≥ 2 s, p). This series of Rydberg states converges to the core-excited ion state dissociating to Ne (3s) + H⁺. The second peak is due to the electron capture by Rydberg states dissociating to Ne (1s²2s²2p⁵n ≤ 3 s, p) + H(1s). This series converges to the second ionization limit limit Ne⁺ + H(1s). We will report resonances found in the 10–30 eV energy range by electron scattering calculations using the Complex Kohn Variational method. The resonances, electronic couplings between resonances and the autoionization widths will be used in the time-dependent wave packet calculation describing the dissociation dynamics. The calculated cross sections and dissociation rates will be compared to the experimental ones measured by Mitchell et al.

Electron-biomolecular ion collisions were studied using an electrostatic storage ring with a merging electron beam device. Biomolecular ions produced by an electrospray ion source and accelerated to 20 keV/charge were injected into the ring after being mass-analyzed. The circulating ion beam was then merged with an electron beam. Neutral reaction products in collisions of electrons with ions were detected by a micro-channel plate outside of the ring. Electron-ion collisions were studied for multiply-deprotonated oligonucleotide and peptide anions as well as singly protonated oligonucleotide and peptide cations. For peptide cations, neutrals were resonantly emitted at an electron energy of around 6.5 eV, which was almost independent of the ion masses. This is deduced to come from electron-ion recombination, resulting in the cleavage of a peptide bond. For DNA oligonucleotide cations, resonant neutral particle emission was also observed. In electron and DNA anion collisions, neutrals started to increase from definite threshold energies, where the threshold energies increased in proportion to the ion charge. The same was found for peptide anions. The origin of this phenomenon is discussed.

This communication starts with a comparison of dissociative recombination and dissociative attachment placing emphasis on the role of resonances as reactive intermediates. The main focus is then the mechanism of electron attachment to polar molecules at very low energies (100 meV). The scheme considered consists of two steps: First, an electron is captured in a diffuse dipole-bound state depositing its energy in the vibrational degrees of freedom, in other words, a vibrational Feshbach resonance is formed. Then, owing to the coupling with a valence state, the electron is transferred into a compact valence orbital, and depending on the electron affinities of the valence state and possible dissociation products, as well as on the details of the intramolecular redistribution of vibrational energy, long-lived anions can be generated or dissociation reactions can be initiated. The key property in this context is the electronic coupling strength between the diffuse dipole-bound and the compact valence states. We describe how the coupling strength can be extracted from ab initio data, and present results for Nitromethane, Uracil and Cyanoacetylene.

We study non-conventional states of singly negatively charged ions which are supported exclusively due to the presence of an external magnetic field and have no counterparts in the field-free space. Such states experience a significant impact due to the motional effects of an anion as an entity across the magnetic field. We briefly outline the physical insight and the state-of-the-art concerning the magnetically bound anions.

The halfium model is presented and its application to ¹Σ⁺_g ionization channels of H₂ is described. Ab initio calculations of the (2pσ)² doubly excited state of H₂ are presented and compared with previous calculations. The connection with a fully ab initio study of the H₂ dissociative recombination process is outlined.

We report optical-optical-optical triple-resonant spectroscopic structure for BH that corresponds with scattering resonances formed by high-lying Rydberg states that couple strongly to decay channels for electron loss and boron-hydrogen bond cleavage to neutral atoms. Lineshapes and intensities provide information on state-to-state relaxation dynamics and interference between bound-bound and bound-continuum transition moments.

Preliminary results of a high resolution coherent vacuum ultraviolet study of photoion-pair formation in HF/DF are reported. These results include threshold ion-pair production (TIPPS) and total photoion-pair yield spectra, which show sharp resonances due to predissociating Rydberg states. Detailed analysis of the spectra provides precise energies for the ion-pair thresholds, and insight into the mechanism for ion-pair formation.

We investigate the effects of a strong static electric field on the rovibrational spectra of diatomic heteronuclear molecules in a ¹Σ⁺ electronic ground state. Using a hybrid computational technique combining discretization and basis set methods the full rovibrational equation of motion is solved. As a prototype for our computations we take the carbon monoxide molecule. For experimentally accessible field strengths we observe that while low-lying states are not significantly affected by the field, for highly excited states strong orientation and hybridization are achieved. We propose an effective rotor Hamiltonian, including the main properties of each vibrational state, to describe the influence of the electric field on the rovibrational spectra of a molecular system with a small coupling between its rotational and vibrational motions. This effective rotor approach goes significantly beyond the rigid rotor approach and is able to describe the effect of the electric field for highly excited states.

The Interatomic Coulomb Decay (ICD) is a radiationless decay mechanism occurring via electron emission in inner valence ionized weakly bound clusters. In this article the theoretical description of the ICD in the inner valence ionized Ne dimer is presented, using a time-dependent formalism based on nuclear wave packet propagation. The theoretical predictions are compared with the first experimental results, and a very good agreement is found.

Fullerene ionization can under some circumstances be modelled as thermal emission of electrons from a transiently hot electron gas. Application of this idea to experiments involving femtosecond lasers gave theoretical evidence for ion yields that would vary with the pulse energy of the laser to some power, identical to the behavior seen in multi-photon experiments. The reason for this behavior is investigated here. The crucial component is identified as the Poisson statistics for photon absorption and a strong variation of the ion yield with energy.

This contribution describes the use of the RoentDek hexanode delay line detector to detect fragments from the dissociative recombination of O₂⁺ in an experiment at the ion storage ring CRYRING, Manne Siegbahn Laboratory, Stockholm. In this experiment, the fragments have a maximum time- and position-separation of 20 ns and 40 mm, respectively. The position resolution obtained was 0.16 mm (FWHM) on the 67 mm diameter detector. The time resolution obtained from the time-of-arrival difference between the product fragments was about 1 ns. The detector system handles event rates as large as 30 kHz. Techniques for the calibration of the absolute position of particles on the detector are discussed.

An ultra cold electron source was developed for the storage ring TSR in Heidelberg to study electron-ion interactions with high energy resolution. The heart of the source is a GaAs photocathode which emits electrons with energy spreads below 10 meV. Photoemitted electrons are extracted in the space charge mode and then undergo adiabatic magnetic expansion and adiabatic acceleration to obtain an ultra cold electron beam which is overlapped with the stored ion beam in a straight section of the ring. In the first recombination measurements on HD⁺ unprecedented energy resolution for low-energy resonances was found, demonstrating a transverse and longitudinal temperature of the electron beam of about 0.5 meV and 0.02 meV, respectively.

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 (∼2K) 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.