Table of contents

A general overview of some recent results obtained using COLTRIMS in diverse areas of physics is given. What began as a tool for ion-atom collisions is now used for a wide array of experimental situations including both charged particle and photonic collisions. The connections are not just technical: many physical and conceptual bridges between these fields exist.

A survey of resonance and threshold phenomena in low-energy electron collisions with molecules and clusters is presented. Following an introduction into the role of resonances to promote vibrational excitation and anion formation through electron attachment, we discuss recent progress in achieving high energy resolution, using optimized conventional setups and photoelectron methods (the latter achieving sub-meV energy widths in attachment studies). Discussing selected cases, we highlight threshold and resonance phenomena in electron scattering and attachment channels, as observed in recent high resolution experiments and characterized by improved theoretical models. The threshold behaviour for dissociative electron attachment proceeding through s-wave and p-wave capture is demonstrated for CCl₄ and Cl₂, respectively. Threshold peaks, vibrational Feshbach and outer-well resonances as well as boomerang-type oscillatory structures are discussed for the polar molecules HF, HCl, and CH₃I. Narrow vibrational Feshbach resonances, observed in cluster anion formation due to electron attachment to molecular clusters of CO₂ and N₂O, and their size-dependent redshifts, illustrating the effects of solvation, are discussed and explained as diffuse weakly-bound electron states. We briefly address the recent observation and the relevance of vibrational resonances in positron-molecule collisions at energies below 0.5 eV, as observed in positron annihilation. We conclude with a brief summary and mention some perspectives for future work.

Atoms exposed to a few oscillation cycles of intense visible or near-infrared light are able to emit a single x-ray burst of sub-femtosecond duration (1 fs = 10^-15 s). Precise temporal control of this energetic photon emission can be achieved by full control of the hyperfast field oscillations in the laser pulses driving the emission process. Sub-femtosecond x-ray pulses along with intense, synchronized, waveform-controlled few-cycle laser pulses led to the development of a new measuring apparatus, which has been dubbed a light-field-controlled streak camera. It measures the time-momentum distribution of electrons ejected from atoms following an impulsive excitation by a sub-femtosecond x-ray pulse. From the time-momentum distribution of ejected primary (photo) and secondary (Auger) electrons the excitation dynamics (i.e. characteristics of the exciting x-ray pulse) and the subsequent relaxation of the electron shell of the excited atom, respectively, can be inferred, currently with a resolution of ~100 attoseconds (1 as = 10^-18 s). The techniques reviewed in this paper offer the potential for advancing time-domain metrology towards the atomic unit of time (24 as).

Optical spectroscopy, and in particular laser spectroscopy, provides numerous possibilities for advanced diagnostics and multi-spectral imaging with real-time capability. We describe applications to the environmental and medical fields and it is noted that the basic approaches are very similar in both areas. Environmental diagnostics discussed includes satellite imagery, long-path absorption atmospheric spectroscopy, differential absorption lidar, gas correlation imaging and fluorescence lidar imaging of vegetation and historical building facades. The medical field is illustrated by photodynamic therapy, laser-induced fluorescence diagnostics, scattering spectroscopy and optical mammography, gas in scattering media absorption spectroscopy and imaging with laser-produced x-rays.

K-shell photodetachment studies of negative ions have been carried out using a collinear photon-ion apparatus coupled with the brightness and spectral resolution of the Advanced Light Source at Lawrence Berkeley National Laboratory. The experimental results, which revealed new phenomena, are pushing the limits of third generation photon sources and are stimulating various enhanced theories.

We present absolute measurements for photoionisation of multiply-charged ions N²⁺ and O³⁺ in the ground and metastable states. Multiconfiguration Dirac Fock calculations were performed for N²⁺ and are tested here by comparison with experimental results. Our experimental and theoretical results are also compared to the theoretical data available in the Opacity Project data base.

Recent progress in the understanding of the photodouble ionisation process in He and a new set of measurements in Ar are reported.

In the case of He we discuss a method which allows to extract the complex amplitudes of the triple differential cross section (TDCS) and their relative phase from the experimental data. This method provides a compact way to compare theory and experiment and does not rely on any assumption on the energy and angular dependence of the TDCS.

The results of the TDCS of the Ar²⁺3p⁴(³P^e) state measured at 20 eV excess energy in equal energy sharing conditions are presented. The data of the present experiment and previous ones of the He²⁺(¹S^e) state in the same kinematic have been represented by an 'exact' parameterisation of the triple differential cross section (Malegat et al 1997 J. Phys. B: At. Mol. Opt. Phys.30 251).

Atomic photoionization provides an experimental situation where the outer electron is ejected to large distance without confinement condition. The application of an electric field during photoionization allows confining the electron flux along one coordinate. Therefore, photoionization in an electric field provides a unique opportunity where quantum properties of the wavefunction are projected into the macroscopic world, permitting a direct view of the square modulus of the electronic wavefunction. We present here the experimental observations of this effect, denominated photoionization microscopy. The analysis is carried out in the framework of the semiclassical model that allows a qualitative interpretation of the experimental results in terms of interferences among various classical trajectories.

An overview of recent experimental and theoretical studies of angular correlations in cascades of Auger transitions photoinduced in rare-gas atoms is given. In particular, the resonant Auger transitions from the photoexcited Ne 1s⁻¹3p, Ar 2p⁻¹4s, Kr 3d⁻¹5p, and Xe 4d⁻¹6p states to the states of ns⁻²n'l' and ns⁻¹np⁻¹n'l' configurations with a following Auger (Coster–Kronig) transition have been studied by means of angle-resolved electron spectroscopy including coincidence measurements of two emitted electrons. A theoretical interpretation of experimental results is given in the framework of the MCDF approach. In general the multi-configuration calculations agree very well with experiment.

Photoionization is one of the basic processes of light-matter interaction, which may be used to transfer at least part of quantum information from photons to electrons in the continuum in such a way that the final state electron carries composite information of the initial electron and of the incident photon. We examine how photoionization may change entanglement of electrons. Our result shows that entanglement in one degree of freedom is affected by a transition in another degree of freedom through the coupling of the two degrees of freedom.

An important breakthrough has been achieved recently in the description of (e,2e) and (γ,2e) processes with the development of new ab-initio theories: the external complex scaling theory (ECS), the time dependent close coupling theory (TDCC), and the hyperspherical -matrix theory with semiclassical outgoing waves (HM-SOW). The principles of these various theories are summarized, their relations are considered, and their achievements are discussed with respect to the available experimental data regarding electron impact ionization of H and photo double ionization of He. Possible directions for future work are outlined.

Our recent studies on nuclear dynamics of core-excited and core-ionized molecules probed by a multiple coincidence momentum imaging technique are reviewed. This technique consists of a time-of-flight spectrometer with a multi-hit two-dimensional position sensitive detector and a supersonic jet. One can probe the symmetries and the nuclear motions of polyatomic molecules using this technique. As a specific example, we illustrate how to resolve the symmetries of the CO₂ C 1s⁻¹2π_u core-excited states, which split into two states, A₁ and B₁, due to D_∞h → C_2v symmetry lowering by the static Renner-Teller effect, and how to probe the nuclear motion caused in these states. We discuss also symmetry lowering D_3h → C_3v and D_3h → C_2v initiated by the B 1s → 4a₂'' excitation and F 1s ionization, respectively, in the BF₃ molecule and nuclear motions in these core-excited and ionized states.

A study highlighting several aspects of resonant spectroscopies where fragmentation processes are investigated is described. We focus primarily on understanding the dynamical processes taking place in core-excited states in molecules. An example where fast dissociation and molecular alignment are discussed is given for the ammonia molecule.

We review some of our work on quantum control employing adaptively shaped femtosecond laser pulses. In this technique, a learning algorithm uses direct experimental feedback signals to optimize user-defined objectives in chemical reaction control. Laser pulses are modified in frequency-domain pulse shapers, which apart from phase-and-intensity modulation can be used also to modify the polarization state of light as a function of time. We present experimental examples on automated femtosecond pulse compression, control of gas-phase molecular femtochemistry, and control of photophysical objectives under liquid-phase conditions.

The two-body Coulomb explosion of acetonitrile, CH₃CN²⁺ → CH_3-n⁺ + H_nCN⁺ (n = 0–2), in intense laser fields (0.15 PW/cm², 70 fs) is studied by the coincidence momentum imaging technique. It is found that the fragment ions for n = 0 are ejected mostly in the direction of the laser polarization vector with ⟨ cos² θ ⟩ = 0.68, where θ is the angle between the fragment recoil direction and the laser polarization vector, while the angle distribution becomes more isotropic as n increases, i.e., ⟨ cos² θ ⟩ = 0.49 for n = 1 and ⟨ cos² θ ⟩ = 0.37 for n = 2. From this characteristic correlation between the anisotropy in the fragment ejection and the hydrogen migration, the Coulomb explosion dynamics competing with the hydrogen atom transfer from the methyl group to the nitrile group is investigated.

All aspects of attosecond technology rely on electron wavepackets formed by ionization and controlled by strong laser fields. When the electron wavepacket is formed by tunnel ionization in linearly polarized light, attosecond electron or optical pulses can be produced, both of which will play significant rolls in attosecond spectroscopy. When the electron wavepacket is formed by an attosecond x-ray pulse, the x-ray pulse can be fully characterized by using a strong laser field. If an atomic, molecular or nuclear dynamic processes form correlated wavepackets, the decay dynamics can be measured with attosecond precision.

The evolution of the electric field of laser pulses consisting of a few optical cycles depends on the so-called absolute phase. Strong-field photoionization not only provides means to measure the absolute phase. Rather phase-controlled few-cycle pulses allow investigating photoionization on the attosecond time scale as the sub-cycle ionization dynamics becomes manifest in the photoelectron spectra. Few-cycle pulses thus can serve as a time-domain microscope for investigation of electronic transitions.

A theoretical investigation of the behaviour of molecular hydrogen exposed to strong laser fields is presented. Two opposing limits are considered explicitly. In the low-frequency, strong-field regime the quasi-static approximation is used to investigate the ionisation rate together with three strong-field effects: bond softening, bond hardening, and enhanced ionisation. In the high-frequency, low-intensity regime multi-photon ionisation is treated within lowest-order perturbation theory. In this limit, the possibility of a scheme of coherent phase control is investigated, and the occurrence of a phase lag between the ionisation and dissociation rate is discussed. The result is of interest for corresponding experiments that discovered such a phase lag in hydrogen iodide.

We discuss how moderately intense, polarized laser fields can align molecules to given axes fixed in space. One dimensional adiabatic alignment is illustrated by experiments on iodine molecules with nanosecond (ns) laser pulses that are long compared to the rotational period. Similarly, 1-dimensional nonadiabatic alignment is illustrated through experiments on iodobenzene with picosecond (ps) pulses that are much shorter than the rotational periods. Applications of aligned molecules, including control of photodissociation and strong field ionization, are discussed.

The role of electron correlation in the formation of double-K-shell vacancies in Li-like Be⁺, B²⁺, C³⁺ and O⁵⁺ ions has been investigated using high-resolution zero-degree projectile spectroscopy. Using the collision velocity dependence for the cross sections of the observed doubly-K-shell vacant states, the contribution of the e-e interaction was inferred and categorized in terms of shake and dielectronic models. The relative strength of the e-e interaction was examined as a function of the atomic number of the Li-like ion.

This is a report about a novel experiment on double excitation of Helium by absorption of synchrotron radiation close to the double ionization threshold at 79 eV. Partial cross-sections for single-ionization from 78.15 eV up to 78.9 eV (photon energy) with an energy resolution of 3 meV have been measured. The technique described in this article allows for an angular and energy resolved measurement of low energy electrons with a high detection efficiency due to an angular acceptance of 4π solid angle.

Recent progress in the understanding of electron–electron correlations in triply excited states in lithium is reviewed. The possibility of scanning through states of different symmetry by laser-induced transitions among the triply excited states themselves is discussed. Configuration interaction calculations with complex-rotation in a discretized B splines basis is proposed as an ab initio method for determination of resonance energies, widths, Fano parameters and dipole matrix elements. An analytical model based on the physical picture of the three electrons being intrinsically arranged in an equilateral triangle with the coplanar nucleus in the centre is discussed and shown to predict configuration mixing fractions accurately and to suggest a classification scheme. As an application of the model, the prediction of single-electron angular distributions is discussed.

We have measured complete three-dimensional images of fully differential cross sections for single ionization of He by ion impact for a broad range of perturbations (projectile charge to velocity ratio). Qualitative discrepancies to a state-of-the-art calculation are found. Our results demonstrate that even for very small perturbations single ionization is not nearly as well understood as was assumed previously.

We review recent work carried out in Belfast which handles the few-electron dynamics of atomic and molecular systems exposed to high intensity laser fields. The design and application of quantitatively accurate computational methods is discussed. The work is illustrated by results for single and double ionization of helium, for ionization of the hydrogen molecular ion and for single- and double-ionization of the hydrogen molecule.

The performance of an electron energy loss spectrometer with hemispherical energy selectors has been improved in terms of low energy capacity, resolution, response function correction at low energies, sensitivity, and extension of the angular range to 180°. The extended capacity permitted a more detailed observation of threshold peaks and near threshold structures in halogen halides, observation of near threshold structures in CO₂, N₂O and CS₂ and the observation of selectivity in the excitation of Fermi-coupled vibrations by the ²Π_u shape resonance in CO₂.

Recent progress in the measurement of absolute collision cross sections for electron molecule scattering is discussed in the context of measurements which have relevance to atmospheric phenomena and technological applications. Cross sections for low energy (< 5 eV) scattering from NO highlight the role of resonant processes in the excitation of this important atmospheric gas, while for both C₂F₄ and C₄F₈, which are used in plasma processing of semiconductors, we present a range of experimental data. In each case we make comparison of the measured cross sections with results derived from the analysis of transport data from electron swarm experiments.

We present in this paper MQDT calculations of the dissociative recombination process of H₂⁺, HeH⁺ and LiH⁺ with very slow electrons. The process is induced by non-adiabatic interactions. Based on a recently derived (Stolyarov and Child 2001 Phys. Rev. A 63 052510) generalised Hellman–Feynman theorem, regular radial coupling effects can be treated explicitly. Our treatment uses a bound-continuum electronic matrix of the radial coupling between a Rydberg dissociative state and the lε electronic continuum with the same lλ to calculate the cross sections for the dissociative recombination.

The dissociative recombination of polyatomic molecular ions has been thought to normally be dominated by the crossing between a dissociative neutral surface and an ionic ground state surface. For some molecules such as H₃⁺, however, the low energy dissociative recombination (DR) rate is observed to be high despite the absence of such a crossing. We show how the Jahn–Teller coupling mechanism can produce a surprisingly high DR rate in H₃⁺ by first exciting a Rydberg state attached to an ionic vibrationally excited level, which subsequently dissociates. The resulting DR rate coefficients are in good agreement with storage ring experiments that have utilized a rotationally cold ionic target.

Electron energy-loss spectra of H₂, D₂ and N₂ tagged with emissive neutral-dissociation, i.e. coincident electron-energy-loss spectra, were measured in the range where doubly excited states of these molecules lie. The doubly excited states, in particular forbidden ones with dissociative character, were revealed for the first time in electron-molecule collisions. It has turned out that this method, the coincident electron-energy-loss spectroscopy, is a key tool for investigating doubly excited molecules.

The reaction of molecular ions with electrons via dissociative recombination is the most important neutralising process in plasmas cold enough to contain molecules. For diatomic ions, the basic features and principles describing reaction process are now reasonably well understood and characterised. For poly-atomic ions, however, the recombination process is much less well understood. To help address this, experiments carried out at ion storage rings have shown that the recombination of poly-atomic molecular ions tend to lead to a high-degree of fragmentation, involving multiple bond breaking and formation. Furthermore, tri-atomic molecular ions are observed to pre-dominantly break up into their constituent atoms upon recombination with free electrons. These observations have been investigated further at ion storage rings using particle-imaging techniques, and this technique will be discussed together with data obtained from studies of H₂O⁺, NH₂⁺ and CH₂⁺. Finally, how this technique could be used to study more complex polyatomic ions will also be discussed, taking D₅O₂⁺ as an example.

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 Nature422 500).

Electron–hydrogen fully differential ionisation cross sections are studied using the convergent close-coupling (CCC) method that utilises box-based eigenstates. Convergence as a function of the box size parameter R₀ is presented for the coplanar case of 15.6 eV incident electrons with two 1 eV outgoing electrons.

The phenomenal output of high-resolution infrared, visible, EUV, and x-ray spectra from NASA and ESA satellites, and the marked improvement in ground-based observations, has led to a parallel need for accurate atomic data. This need is in large part being addressed, thanks to the equally remarkable development of laboratory facilities. The astrophysics observations include measurements at the Sun, stars, the interstellar medium, and the recent surprising discovery of x-ray emissions from comets. The required database includes cross sections for electron-impact excitation of highly charged ions (HCIs), direct and indirect electron-ion recombination, photoionization, single and multiple charge exchange of HCIs with neutral gases, and x-ray emission. In addition, the observed photon emissions result from a balance between collisional excitation and radiative decay. Hence accurate lifetimes, branching fractions, and Einstein A and B coefficients are required. Some recent spacecraft observations will be reviewed, the underlying atomic physics summarized, and the need for relevant atomic data in HCIs discussed.

Systematic studies of electron–ion recombination for Li-like, Na-like, and Cu-like ions have been made during the last few years at the CRYRING facility. The main purpose has been to measure recombination spectra with such high resolution and accuracy that stringent tests of advanced atomic calculations of doubly excited states can be made. From the comparison with theory it is possible to obtain energy positions, as well as radiative and nonradiative decay rates for the doubly excited states. In addition, for heavy, highly charged ions it is possible to deduce radiative corrections for energy splittings with very high accuracy.

Spin-resolved (e,2e) measurements provide stringent tests to the theory of electron impact-induced ionization and afford insight into the correlated many-body behaviour of systems of charged particles. Here we describe recent improvements made to our toroidal-sector (e,2e) electron spectrometer and demonstrate its enhanced instrumental sensitivity with new measurements on helium and xenon targets. Through comparison with calculation, these results will lead to an improved understanding of many-electron phenomena.

Accurate, normalized doubly-differential cross-sections, differential cross-section ratios and relative differential cross-sections for electron impact ionization, excitation to the H(n = 3,4) levels and elastic scattering of atomic hydrogen have been measured at incident electron energies ranging from 14.6 to 40 eV for scattering angles of 10° to 130°. The measurements are normalized to the accurate differential cross-section for electron impact excitation of H(n = 2) from the convergent close-coupling theory, which has recently been shown to be accurate within 10%. Our measurements show very good agreement in comparison with the most recent theoretical models viz. the Convergent Close-Coupling (CCC) model and the Exterior Complex Scaling (ECS) model.

We present a new, unprecedented electron impact (e,3–1e) study of the ionization of the 2p-inner shell of argon. For the first time, multi-angular distributions are measured for the coincidence detection of two electrons emitted with the same kinetic energy (205 eV) as the Auger electron issued from the electronic rearrangement of the target. The incident energy is chosen such as to leave the third, unobserved electron with either a small or a large kinetic energy, or else with the same energy of 205 eV. In the latter case, the three final state electrons are fully indistinguishable (or at 'resonance'). A rich structure is observed in the angular correlation diagrams. At the highest energy, the observed pattern may qualitatively be explained by considering the Auger emission as a 2-step process. At lower energies, and in particular when all three outgoing electrons have identical energies, the picture dramatically changes, bearing almost no trace anymore of the 2-step process, and being very likely dominated by strong interference and post collisional interaction effects.

The status of benchmark calculations for electron collisions with atoms, ions and molecules is reviewed. Examples of theoretical results obtained in recent years using different computational techniques are compared with each other and with experiment. Progress toward establishing new benchmarks for electron-ion collisions with ions in the iron peak region of the periodic table is described.

We describe the Relativistic Distorted Wave method (RDW) used to obtain the scattering amplitude for an excitation process where explicit account of the spin of the projectile electron is taken into consideration. From the scattering amplitude we can obtain various scattering parameters, viz. the generalized STU and generalized Stokes parameters which uniquely describe a complete scattering experiment. The RDW method is applied to the excitation of the 6¹P₁ and 6³P₁ states of mercury from the ground 1¹S₀ state using a spin polarized beam of electrons. The calculations are performed at 8, 15 and 40 eV energies using two set of multi-configuration wave functions and the various scattering parameters are determined. Selected results are presented and compared with available experimental and theoretical semi-relativistic distorted wave and R-matrix calculations.

Recent progress in the calculation of electron-atom scattering, with particular reference to electron-hydrogen ionization is presented. There have been substantial developments and improvements computationally, and also perhaps unexpectedly, in the formal theory. We conclude that within the frame of non-relativistic scattering theory electron-hydrogen scattering is practically a solved problem.

Electron attachment (EA) to uracil (U), thymine (T) and cytosine (C) was studied in the electron energy range from about 0 to 12 eV using a high resolution crossed electron/molecule beams technique. The dominant negative ions formed via EA reactions to U, T and C are (U - H)^-, (T - H)^- and (C - H)^-. The respective partial EA cross sections could be determined yielding peak values of σ(1.0 eV) = 3 × 10^-20 m², σ(1.05 eV) = 1.2 × 10^-19 m², σ(1.54 eV) = 2.3 × 10^-20 m², respectively. Based on (i) a comparison of the resonance positions for the different bases and on (ii) high level ab initio calculations we can assign certain resonances to the site specific loss of hydrogen during the EA reaction. At higher electron energies, in the range between about 3 and 12 eV we observe further product anions (e.g., for U the ions CN^-, OCN^- and C₃H₂NO^-), however, with significantly lower cross section values.

We describe a new electron-scattering technique involving incident beam diffraction in electron-stimulated desorption (ESD) to probe ground-state surface bonding configurations of adsorbates and excited state configurations which lead to cation ESD. We present a qualitative theoretical description of diffraction in electron-stimulated desorption (DESD) and then demonstrate this process by studying the ESD of Cl⁺ from Cl terminated Si(111) (1 × 1) surfaces. The fine structure in the Cl⁺ ESD yields as a function of azimuth is due to interference of the incident electron-beam with the elastically scattered fraction. This interference forms a surface standing wave in the initial state of the desorption process. Comparison of the experimental data to the qualitative theory indicates that desorption is primarily initiated by an excitation localized on the Si atom bonded to Cl. The ~17 eV threshold energy is consistent with an ESD mechanism involving ionization of the Si 3s level followed by Auger cascading from the σ-bonding surface state and shake-up of an electron in the non-bonding surface π-level. The 2-hole, 1-electron final state produced can lead to the ESD of Cl⁺ due to the hole-hole Coulomb repulsion. Direct ionization of the Si–Cl σ-bonding state can also contribute to Cl⁺ ESD. In general, DESD provides a useful probe of the geometry and dynamics governing low-energy electron interactions with ordered complex targets.

Electronic states in designed geometries are attracting considerable attention. In this work, it is shown that it may be possible to induce ultrafast mechanical rotation in nanotubes using circularly polarized light. We predict a novel set of Rydberg-like electronic states around nanotubes having long lifetimes and exotic properties. Preliminary work on the interaction of slow electrons with an array of parallel nanotubes is discussed.

Electron–biomolecular ion collisions were studied using an electrostatic storage ring with a merging beam technique for singly protonated peptides and multiply deprotonated DNA ions. The neutral-particle production rates according to collisions were measured. For peptides, resonant neutral-particle emissions were observed at a relative energy of 6.5 eV with a shoulder at around 9 eV. These resonances are deduced to come from electron capture and a subsequent cleavage of peptide bonds. For DNAs, neutral-particle emissions start from definite threshold energies, which depend on the ion charges. The neutrals come from a break of DNAs, rather than electron detachment.

A rigorous full quantum mechanical wavepacket calculation is carried out for the protonium formation + H → p + e. The cross sections for the protonium formation are obtained at center-of-mass collision energies up to 10 eV. Also carried out is a semiclassical (quantum-classical hybrid) calculation to make a physical interpretation of the quantum mechanical result. An empirical fitting law is proposed for the relation between the formation probability and the turning point of the relative motion, and is found to be very useful in estimating the formation cross section at low energies.

In this work we present theoretical integral (ICS) and differential (DCS) cross section calculations for low-energy positron scattering by SF₆ and CO₂ molecules. Our scattering wave functions were obtained with the Schwinger Multichannel Method (SMC) at the static and static-plus-polarization levels of approximation. The elastic DCS were calculated over an energy interval ranging from 4 eV up to 50 eV for SF₆ and from 1 eV up to 20 eV for CO₂. The results show, in general, good agreement with recent experimental data. We found a Ramsauer–Townsend minimum in e⁺ - CO₂ calculations, even though no signature of virtual state formation was observed, contrasting with the well-known case of electron scattering.

An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.

In the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration, the Trap group has been working on an efficient accumulation of antiprotons and production of ultra slow mono-energetic antiproton beam, which is realized by a combination of an RFQD (Radio Frequency Quadrupole Decelerator) and a large multi-ring trap (MRT) installed in a super-conducting solenoid. We have succeeded to accumulate several million antiprotons. A mono-energetic antiproton beam of 10 eV has been extracted and transported through a specially designed beam line, which has a high transport efficiency and at the same time enabling differential pumping of more than six orders of magnitude between the MRT and a collision chamber. This configuration was adopted to make atomic collision experiments like ionization and antiprotonic atom formation processes and also to study spectroscopic nature of various meta-stable antiprotonic atoms under single collision conditions, which has never been possible. A new scheme of efficient positron accumulation has been invented employing high-density electron plasma and an ion cloud, which fits quite well with the UHV requirements of antihydrogen synthesis as well as other applications. A so-called cusp trap configuration has been proposed as a new synthesizer of antihydrogen, where antiprotons and positrons can co-exist in the same place even at low temperature. Because of the inhomogeneous magnetic field distribution of the cusp trap, (1) a low energy component of the antihydrogen atom so formed can be trapped for a macroscopic time, and (2) a high energy component emerges as an intensity-enhanced and energy-filtered spin polarized antihydrogen beam, which is suitable for measurements of the hyperfine splitting of antihydrogen, and could provide the magnetic moment of antiproton with some ppm accuracy.

At ultralow temperatures, and in the presence of an electric field, the long-range interaction potentials between dipolar molecules are highly subject to manipulation. Most strikingly, a novel set of quasi-bound states, dubbed "field-linked" states, appear. In these states the molecules are held at intermolecular distances of tens to hundreds of atomic units, and can strongly influence resonant scattering of molecules.

Properties of alkali atoms (mainly rubidium and cesium) in a cold helium gas have been extensively studied. Experiments are performed at temperatures down to 1.2 K using alkali-metal vapor cells containing a helium buffer gas, with alkali atoms driven from the cell walls by one of three loading methods: laser ablation, light-induced atom desorption, and spark discharge. For alkali atoms loaded into the cold helium gas, we have found that the relaxation time of the electronic spin of optically polarized rubidium atoms in the ground state is about a minute, much longer than that achieved in conventional vapor cells at room temperature and above. We have also spectroscopically investigated the interactions of excited alkali atoms with helium atoms, especially the formation of alkali–He exciplexes.

We have calculated cross sections and rate coefficients for elastic scattering, ionization, and double excitation transfer in collision of two hydrogen atoms in the 2s state. We find that for collision energies between 5 × 10^-10 a.u. and 10^-6 a.u. elastic scattering has the largest cross section. For temperatures below 0.02 K ionization is the dominant loss process, while for higher temperatures double excitation transfer dominates. Our results for the total loss rate are found to be within a factor 2 or 3 of the error bars of recent measurements.

The pursuit of fermion superfluidity in cold atom traps has been touted as a worthy successor to the celebrated quest for Bose–Einstein condensation. Presently, the field appears to be closing in on this new target. This manuscript reviews some of the exciting prospects, the obstacles on the way, and the progress in overcoming these hurdles.

We have studied theoretically charge transfer and fragmentation in collisions of positively charged metal clusters (Na₉⁺, Li₃₁²⁺) and C₆₀ with various atomic targets (Cs and He²⁺). We show that the relative abundance of different fragments depends critically on cluster temperature and the spectrometer time of flight window (TOF). We have found good agreement with recent experimental results. For C₆₀^q+ species (q = 0, 1, 2) produced in collisions with He²⁺ projectiles, dissociation energies for dimer evaporation have been calculated within DFT. A very good agreement between theoretical estimates and available measurements is found.

State-resolved differential cross sections (DCSs) of NO (j'' = 0.5, Ω'' = 1/2) + Ar → NO (j', Ω' = 1/2, 3/2) + Ar have been measured at a collision energy of 516 cm^-1 by means of the crossed molecular beam velocity-map ion-imaging method. DCSs were obtained for twenty final (j', Ω') states including spin-orbit conserving (ΔΩ = 0) and changing (ΔΩ = 1) transitions, and compared with the results of full close-coupling scattering calculations based on ab initio potential energy surfaces. Remarkable agreement between theory and experiment was seen in the DCSs for the ΔΩ = 0 transitions, while small discrepancies were found for the ΔΩ = 1 transitions.

Results of molecular dynamics simulations of fission reactions Na₁₀²⁺ → Na₇⁺ + Na₃⁺ and Na₁₈²⁺ → 2Na₉⁺ are presented. The dependence of the fission barriers on the isomer structure of the parent cluster is analyzed. It is demonstrated that the energy necessary for removing homothetic groups of atoms from the parent cluster is largely independent of the isomer form of the parent cluster. The importance of rearrangement of the cluster structure during the fission process is elucidated. This rearrangement may include transition to another isomer state of the parent cluster before actual separation of the daughter fragments begins and/or forming a "neck" between the separating fragments. A novel algorithm for modeling the cluster growth process is described. This approach is based on dynamic search for the most stable cluster isomers and allows one to find the optimized cluster geometries, as well as their essential formation mechanisms. Cluster growth paths for Lennard–Jones clusters have been investigated for cluster sizes of up to 150 atoms. All known global minima structures of the Lennard–Jones clusters are found.

New absolute cross sections for ionization, capture and fragmentation of the C₆₀ molecule in collisions with 2 to 130 keV H_n⁺ (n = 1-3) hydrogen ions are presented together with the first electron spectroscopy of the C₆₀ molecule colliding with protons. A comparison with available C₆₀^r+ production cross sections indicate a good agreement for charges r higher than unity; a discrepancy of a factor of two between C₆₀⁺ cross sections is discussed. A CTMC calculation reproduces our measurements for r = 1 and 2 at 100 keV. An unusual electron energy distribution is measured which peaks generally above 10 eV, with a negligible contribution of very low energy electrons; these results are understood as a trapping of low energy electrons behind the centrifugal barrier. From these electron spectra, the averaged kinetic energy of the emitted electron is determined in the 20 to 120 keV energy range and, using the calculated mean energy deposit, the mean internal energy of the molecule is deduced comparing well with other experimental data.

The use of an electrostatic storage ring for the study of chromophore ions in the gas phase is demonstrated. In this way the intrinsic photophysics and photochemistry of "naked" chromophore ions without the "disturbance" of a solvation shell is elucidated. The ring is combined with an electrospray ion source which enables the production of large molecular ions. Experiments described here involve absorption spectroscopy of protein chromophores and measurements of lifetimes for dissociation processes and excited states of nucleotide and protoporphyrin ions. Comparisons with solution phase experiments allow us to address the perturbation of a solvent or a protein environment on the electronic structure of chromophores.

We studied multiply charged ion (MCI) induced ionization, excitation and fragmentation of the nucleobases uracil and thymine. Ions of different charge state at velocities between 0.2 and 0.4 atomic units were used as projectiles. By means of time-of-flight spectrometry of the collision products in a statistically representative mode as well as in triple coincidence mode, three different interaction types could be identified. Large impact parameters lead to "gentle" ionization of the molecules, whereas in close collisions mainly multifragmentation occurs. This gives rise to a bimodal mass-spectrum as found in fullerene fragmentation studies. An additional two-body breakup channel leading to intermediate fragment sizes was found for both nucleobases. From the coincidence data, the kinetic energy release of this channel could be obtained.

A review of recent progress in the theoretical description of electron capture for low-energy collisions of ions with molecules is given with an emphasis on molecular-orbital coupled-channel approaches. Results for non-dissociative single electron capture are presented for the O³⁺ + H₂ and H⁺ + CO collision systems.

Interference patterns have been identified and studied in the angle- and energy-dependent cross sections for electron emission measured for 60–70 MeV/u Kr^q+ ions impacting on H₂ molecules. The results show that interference effects are produced by the coherent emission of electrons from the two H atoms, in analogy with Young's two-slit experiment. Recent theoretical models provide good agreement with the observed variations of the oscillatory pattern with the electron observation angle. The analysis of the collected data indicates the presence of higher order interference effects in the electron emission from H₂ by fast ion impact.

The dynamics of CO interactions with water ice surfaces are studied using molecular beam techniques. The experimental method allows for detailed investigations of interaction between molecules and ice surfaces under single collision conditions, and collision dynamics and energy transfer are characterized. An ice surface is prepared by deposition of water vapor on a cold substrate and the surface is maintained in a vacuum chamber with a partial water vapor pressure up to 10^-4 mbar, which allows for experiments with ice surfaces in the temperature range 100–190 K. A molecular beam is directed towards the ice surface, and the molecular flux from the surface is detected by mass spectrometry. Angular-resolved intensity and time-of-flight distributions are measured, and the effects of surface temperature, incident translational energy and incident angle are investigated. The surface collisions are highly inelastic with large energy loss observed for the directly scattered flux, similar to the results for the previously studied Ar-ice and HCl-ice systems. The data for the energy loss as a function of scattering angle show that energy transfer is substantial both parallel and perpendicular to the surface plane. The trapping of CO on the surface is found to be very effective under typical thermal conditions. The molecules accommodate to the temperature of the ice, but rapidly leave the surface by desorption because of the low binding energy to the surface.

Fullerenes are a direct link between atoms with discrete electronic energy levels and solids with a band structure and a well defined surface. In this paper, we report on our experiments on the resonant electron transfer in collisions between two fullerene ions as well as on the non-resonant transfer in collisions between fullerene ions and atomic ions. Absolute total cross sections have been measured for the reaction C₆₀⁺ + C₆₀²⁺ → C₆₀²⁺ + C₆₀⁺ at center-of-mass energies ranging from 27 keV to 69 keV. Surprisingly, within the error bars, these cross sections are identical to the respective cross sections for C₆₀⁺ + C₆₀ measured by Rohmund and Campbell (1997 J. Phys. B30 5293). For the non resonant system C₆₀⁺ + He²⁺ → C₆₀²⁺ + He⁺ absolute cross sections have been measured in the center-of-mass energy range from 38 keV to 196 keV.

Charge exchange (also referred to also charge transfer or electron capture) has been studied by the atomic collisions community for many years. Apart from the fundamental interest in the subject, much of the research has been driven by the needs of the fusion community where it plays an important role in radiative heat loss and diagnostics in tokamak plasmas. It is therefore surprising that it has generally not been applied as extensively in astrophysical plasmas. Atomic data for astrophysics includes comprehensive data for electron process but little for charge exchange, due to the fact that the data does not exist or is not considered important. However, it has been shown in the last few years that charge exchange is the dominant mechanism for x-ray production for a number of newly discovered x-ray sources in the solar system. These results have demonstrated the need to understand this mechanism for the particular species of interest. Recent experimental work in our laboratory, complimented by other groups, has shown that the relative intensities of these x-ray and extreme ultra-violet lines can be highly dependent on the nature of the neutral target and the collision velocity.

Using a single-particle picture that is based on time-dependent density functional theory we analyze the He⁺–Ne collision system, in which the projectile electron as well as the target electrons undergo inelastic transitions. It is shown that many experimental cross sections can be understood if the model is applied carefully and consistently. In particular, we consider (i) the problem of nonorthogonal propagated target and projectile orbitals, (ii) the antisymmetry of the many-electron wave function, and (iii) dynamical screening effects.

Experimental and theoretical study of energy spectra of electrons ejected in H^-–He, H₂ collisions has been performed. Comparison of the calculations using the Sturmian and advanced adiabatic theories with experimental data reveals existence of a new, correlated S-detachment mechanism responsible for production of high-energy part of ejected electron energy distribution. This mechanism is associated with dynamical energy transfer to the loosely bound 1s-electron of H^- in the course of superpromotion of the inner 1s-electron.

We consider multiple ionization of Ne by fast H₂⁺, with and without projectile electron loss. Measurements of the total cross section for the direct ionization channel support the similarity of ionization of Ne by an H₂⁺ projectile that does not break and Ne ionization by an equi-velocity proton. The multiple target ionization with projectile electron loss is analyzed within a proposed model, adapted from the independent particle model, taking into account post-collisional decay channels following the ionization of Ne 2s electrons. We point out that, for the H₂⁺ + Ne collision system, the post-collisional decay channels have a prominent role in the multiple target ionization associated with the antiscreening mode (two-center electron-electron correlation).

Using cold target recoil ion momentum spectroscopy (COLTRIMS) we have investigated the production of one free electron in slow He²⁺ + He(1s²) collisions. Kinematically complete data have been measured for all possible final states of the second electron, which is either still bound at the target or transferred to the projectile. The result will be discussed within the molecular orbital approach. Strong correlation between the bound electron final state and the electron emission pattern has been observed. This shows that the description of the electron emission from multi electron systems by a single active electron approach is not sufficient.

Recent photon correlation studies for Radiative Electron Capture into high-Z projectiles are reviewed. Emphasis is given to the investigation of polarization phenomena which are now accessible due to recent developments in position sensitive solid-states detectors. It is shown, that REC may provide a tool for the diagnostics and detection of the spin–polarization of particles involved in atomic collisions. Also the impact of REC studies for atomic structure studies is outlined. Here the strong alignment of excited states induced by REC allowed us to observe an interference between competing decay branches for the case of the Lyman-α₁ transition in hydrogen-like ions.

First measurements of angular distributions of Uranium dioxide (UO₂) sputtering induced by slow highly charged Xenon ions (81 keV, v < 0.16 a.u.) have been performed using a catcher technique. The angular distributions are found to be non-isotropic. A clear projectile charge state effect on the sputtering process is observed. From the angular distributions, the total sputtering yield can be deduced. It increases with the projectile potential energy. Varying the projectile incidence angle from 0° with respect to the surface normal to 60°, the sputtering yield increases in the beam forward direction. The projectile charge effect on the angular distribution becomes less prominent at 60° than at 0°. In addition, angular sputtering distributions were studied with Xe²⁵⁺ at 8 keV (v < 0.02 a.u.), where the potential and kinetic energies of the projectile are equal.

We present classical calculations and experimental results for the scattering of H₂ (v_i = 0, J_i = 0/1) molecules from the Pd(111) surface, at incident energies between 100 and 150 meV. In spite of the low elastic reflectivity at a surface temperature of 430 K (~0.5%), in-plane and out-of-plane diffraction peaks could be resolved in the experiment. Surprisingly, the angular distributions show that out-of-plane diffraction is comparable to in-plane diffraction, in agreement with our theoretical calculations. This suggests that in this very reactive system, measurement of out-of-plane diffraction may provide a clue for understanding the H₂ dissociation dynamics.

Electron emission after grazing ion-surface collisions is studied for high impact velocities. We have focused on glancing angles of electron emission where the dominant mechanism is the ionization from atomic bound states. To describe this process, we introduce a quantum model called field distorted-wave (FDW) approximation, which takes into account the effect of the surface interaction on the electronic transition.

The FDW model is applied to analyze electron distributions produced by impact of protons on Al and LiF surfaces, which are metal and insulator materials respectively. In the case of metals, we also evaluate the contibution coming from the valence band by employing the binary collisional formalism. Calculated electron emission yields are in reasonable agreement with the available experimental data. We find that the maximum of the convoy electron distribution is accelerated for Al and decelerated for LiF, with respect to its position in ion-atom collisions, in quantitative accordance with experiments.

Recent measurements of projectile neutralization during 120° binary collision backscattering from Au(110) and RbI(100), performed at the ORNL Multicharged Ion Research Facility, are described. A common feature of the Au(110) and RbI(100) results is that the observed scattered charge state distributions show significant dependences on the target azimuthal orientation, even for normal or close to normal incidence conditions. The observed target azimuth dependences originate in part from the fact that for the 120° backscattering geometry, quasi-binary double collisions make significant contributions to the total binary scattering collision flux. For the Au(110) target, the types of possible quasi-binary collisions vary with target azimuth because of the corrugation of the reconstructed surface and the number of layers of atoms exposed in a (110) fcc lattice. For the RbI(100) surface, quasi-binary collisions vary with azimuth because of the ordered, two-component nature of the surface even when scattering from deeper layers is blocked. Sample data for few-keV incident F, Ne, and Ar multicharged incident projectiles are presented to illustrate the underlying concepts.

We present a progress report on the use of large-scale time-dependent simulations methods to address an extensive variety of problems in atomic, molecular, and optical physics. This includes identification of relevant problems, a short summary of methods, and a few representative examples.

Gigawatt level radiation pulses at wavelengths between 80 and 120 nm and a pulse duration below 100 fs have recently been produced by a single-pass free electron laser at DESY. The radiation has been characterized with respect to pulse energy, statistical properties, spectral distribution and coherence. It has also been used for first experiments on solids and gases demonstrating the potential of the new source. The facility is currently upgraded to higher energy and is planned to start user operation in 2004. Based on this work a large free electron laser laboratory with several experimental stations for the hard X-ray regime down to 1 Å will be built at DESY which could start user operation at the beginning of the next decade.

Electron impact dissociation of CH₄ into non-emissive neutral fragments, i.e., CH₃ and CH₂ radicals, has been studied near threshold of the lowest excited states. The neutral fragments produced are discriminated by means of the threshold ionization technique, which is based on the difference in ionization threshold energy between the parent molecule (14.3 eV, CH₄ + e → CH₃⁺ + H + 2e) and the produced fragments (9.8 eV, CH₃ + e → CH₃⁺ + 2e). Electron impact ionization spectrum of the CH₃ radical, which agrees well with the previous measurements, were obtained. The excitation function of CH₃ and CH₂ radicals by electron impact dissociation of CH₄ showed that threshold energies for both CH₃ and CH₂ radical production were about 8.5 eV. Consideration from the comparison of the threshold energies for CH₃ and CH₂ radical production with the electron energy loss spectra of CH₄, the dissociation channel of the lowest excited states into CH₃ radicals play an important role, which is clearly different from the photodissociation.

Localization and manipulation of Rydberg wave packets using half cycle pulses are discussed. Rydberg wave packets can be localized by applying a train of half-cycle pulses equispaced in time. The localization in this "kicked" Rydberg atom can be due to trapping the wave packet inside the stable islands seen in the classical mixed phase space structure, or the scarred wave function representing quantum mechanical localization while the classical atoms show fast ionization. Chirping the frequency of a train of pulses and modulating the kick strength modify the phase space structure in time and the localized wave packet can be driven to the preferred location in phase space by properly adjusting the time evolution of the phase space structure.

List of members of the Executive Committee, General Committee and Local Organizing Committee.

The XXIII International Conference on Photonic, Electronic and Atomic Collisions (23rd ICPEAC) was held in Stockholm, Sweden, from July 23 to 29, 2003, following ICPEAC in Japan in 1999 and the United States in 2001. This was the second ICPEAC in Scandanavia, the first being the 18th ICPEAC in Aarhus, Denmark in 1993.

The XXIII ICPEAC was hosted by Stockholm University (SU) and the Royal Institute of Technology (KTH), Stockholm. It was attended by 708 scientific participants, including 149 students, and in addition 132 accompanying persons. Thus, XXIII ICPEAC was the second largest so far. The number of accepted abstracts was 1025. The local organizers encountered a few uncertainty factors in estimating the expected number of participants. Our new electronic abstract submission could change the established conversion factor from submitted to presented abstracts. Also, the conference was held in a time of large political and health issue disturbances. This could have led some participants to cancel their trip to the XXIII ICPEAC. Still, the high participant number could be partly due to the fact that the conference was placed in Europe. It could also be due to the photon community taking the change, since the previous ICPEAC, from Physics to Photonic in the ICPEAC acronym, seriously.

The XXIII was the first ICPEAC that used a completely web-based registration, abstract handling, and participant correspondence system. Invitation letters could be requested on the web with personal information as well as title of an abstract included. The abstract system allowed internet abstract submission, automatic receipt of abstracts acknowledgement, sorting abstracts into fields and sub fields, generation of an address data base, preparing lists and sorting for the selection of special reports. It also made the abstract assignments to poster sessions and created the book of abstracts, in electronic form (CD and online) and a printed version. This is the first ICPEAC where abstracts can be searched and downloaded online before and after the conference at http://www.physto.se/icpeac. This internet abstract search service has been used nearly 40000 times to date.

The XXIII ICPEAC program committee, consisting of the executive committee and general committee members, met two times before the conference to select the invited speakers. A year before, the 5 plenary speakers, 6 review talks, and 61 progress reports were selected. Around three months before XXIII ICPEAC, a meeting was held to scan the more than 1000 submitted abstracts and to choose from them 20 special reports. Almost all speakers accepted our invitation to give a talk. The program also included two evening lectures. One, the public lecture, was given by A Bárány entitled `The Nobel prize and Stockholm-A guided tour for pedestrians'. The other, a Sheldon Datz memorial symposium, contained a lecture by J P Toennies.

We thank the participants for the high scientific level and quality of the presentations, which is reflected in these publications of the majority of the invited lectures.