Table of contents

The XXVth ICPEAC in Freiburg marked a notable anniversary in collision physics: half a century ago the first conference in the series of International Conferences on the Physics of Electronic and Atomic Collisions (ICPEAC) was held in New York (1958). Since then, the development of electronic and atomic collision physics has seen tremendous progress. Starting during a time, when this field was regarded as somehow out-of-date, certainly not being in the main stream compared to particle and high-energy physics, it has expanded in a rather exceptional and unforeseen way. Over the years the original scope on electronic, atomic and heavy-ion collision physics was extended substantially to include upcoming expanding fields like synchrotron-radiation and strong-field laser-based atomic and molecular physics giving rise to a change of name to 'Photonic', Electronic and Atomic Collisions (ICPEAC) being used for the first time for the ICPEAC in Santa Fee in 2001. Nowadays, the ICPEAC has opened its agenda even more widely to other fields of atomic and molecular physics, such as interactions with clusters, bio-molecules and surfaces, to cold collisions, coherent control, femto- and attosecond physics and, with the Freiburg conference, to the application of free-electron lasers in the vacuum ultraviolet and soft x-ray regime, a field of potentially huge future impact in essentially all areas of science. In this larger context the XXVth ICPEAC in Freiburg with more than 800 participants set new standards. Representatives from all fields of Atomic, Molecular and Photon-based science came together and had very fruitful, inter-disciplinary discussions. This new forum of collision-based AMP physics will serve as a showcase example of future conferences, bridging not only the gap between different fields of collision physics but also, equally important, between different continents and cultures. The next ICPEAC is going to take place in Kalamazoo in North America, the one after that in Belfast back in Europe, and the subsequent one, 2013 in Lanzhou, will be the first one ever held in China. A great perspective for this ever-growing field of science!

Uwe Becker (Fritz-Haber-Institut, Berlin) Robert Moshammer (Max-Planck-Institut für Kernphysik, Heidelberg) Paul Mokler (Gesellschaft für Schwerionenforschung, Darmstadt) Joachim Ullrich (Max-Planck-Institut für Kernphysik, Heidelberg) Editors

Relaxed atmosphere for discussions during coffee breaks at ICPEAC XXV in Freiburg.

The PDF file contains details of previous conferences, sponsors, exhibitors and committees.

An overview of recent theoretical progress to accurately describe non dissociative and dissociative ionization of molecules exposed to synchrotron radiation and ultrashort uv/xuv laser pulses is presented. The success of recent theoretical approaches rely on their ability to account for both the electronic and nuclear degrees of freedom. This is essential to describe the delicate interplay between the electronic motion and the molecule's vibration, especially in those cases where ionization occurs in a time scale comparable to that of the vibrational motion. Some of the most successful applications and the new physics that has emerged by comparing with recent kinematically complete experiments on H₂ and D₂ will be discussed.

The talk contained examples of recent atomic physics experiments with stored and cooled ion beams from different ion storage-ring facilities. Here, we first introduce the principles of storage rings and electron cooling. A whole class of experiments exploits the cold electron beams available in the electron coolers and electron targets of storage rings. The recombination experiments have applications in fusion and astrophysical plasmas. Dielectronic resonances at meV to eV energies are measured with a resolution and absolute accuracy to much below a meV. The measurements of these resonances provide a serious challenge to theories for describing correlation, relativistic, quantum electrodynamical effects, and isotope shifts in highly ionized ions. Experiments with internal targets in storage rings use the high luminosity of cooled MeV ions for collisions. First measurements demonstrate the resolution with a He RIMS apparatus (He gas-jet Target for Recoil Ion Momentum Spectroscopy) in Thomas-like electron-transfer processes by protons. An outlook into the future with the new Facility for Anti-proton and Ion Research (FAIR) and the Stored Particle Atomic Research Collaboration (SPARC) is given.

We have studied the valence and inner-shell photoionization of free rare-gas clusters by means of angle and spin resolved photoelectron spectroscopy and momentum resolving electron-multi-ion coincidence spectroscopy. The electron measurements probe the evolution of the photoelectron angular distribution and spin polarization parameters as a function of photon energy and cluster size, and reveal a strong cluster size dependence of the photoelectron angular distributions in certain photon energy regions. In contrast, the spin polarization parameter of the cluster photoelectrons is found to be very close to the atomic value for all covered photon energies and cluster sizes. The ion imaging measurements, which probe the fragmentation dynamics of multiply charged van der Waals clusters, also exhibit a pronounced cluster size dependence.

The Layer-by-Layer (LbL) technique has recently been developed as a promising method for production of thin films functional molecular heterostructures since the interactions occurring, essentially ionic and hydrogen bonding patterns, are found to be identical to those observed in biological systems. Such films have been shown to be also potentially good mimics of biological membranes. Also, it is possible that a study of biological relevant molecules assembled in LbL films will provide a closer analogue to their role in cellular systems. Thin films of adenine (A) and the polyelectrolyte poly(vinylsulfonic acid sodium salt) (PVS), were prepared by cast and Layer-by-layer (LbL) techniques. In this article, the experimental results on the UV irradiation of adenine cast films are described and the effect of 140 nm irradiation, with an estimated dose of about 8.5 × 10⁻⁴ W/m², is evaluated at the molecular level.

An experimental study of the electronic excitation and ionic dissociation of two important classes of biomolecules–natural products (biogenic volatile organic compounds, VOCs, and volatile components of essential oils) and DNA and RNA constituents (aminoacids and bases) is here exemplified with recent results on the fragmentation of thymine and isoprene as induced by synchrotron radiation and fast electrons. Fragmentation of the thymine molecule was seen to dramatically increase as the photon energy increased from 21 to 300 eV and 450 eV. At the highest photon energy, simply and doubly charged N and O atoms were observed. The parent ion (m/z = 126) could be observed at all photon energies. The fragmentation pattern observed in the 1.0 keV electron impact mass spectrum of thymine resembled more closely the fragmentation observed with 21 eV photons. In isoprene, the dominant fragments observed at 21 eV and 310 eV photon energy as well as in the 1.0 keV electron impact mass spectrum were C₅H₇⁺(m/z = 67), C₄H₅⁺(m/z = 53), C₃H₃⁺(m/z = 39) and C₂H₃⁺(m/z = 27). Previously unreported fragments, namely H⁺, C⁺, CH⁺, CH₂⁺, and CH₃⁺ were observed at the high photon energies and at the electron impact mass spectrum.

The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections (FDCS) are compared with Time Dependent Close Coupling (TDCC) calculations and the first-order helium-like model in the coplanar geometry for equal electron energy sharing. While the helium-like model gives a consistent agreement in shape and magnitude with experimental data, the comparison with the TDCC calculations highlights the limit of this model when the molecular axis' orientation is along the polarization direction. New evidence of changes in the FDCS with internuclear separation is reported for the coplanar geometry.

Angle resolved two-dimensional photoelectron spectroscopy has been used to study resonances corresponding to inner-shell excitation in hydrogen halide molecules. In HBr 3d → σ* excitations have been studied, whilst in HCl and DCl 2p → σ* resonances have been explored. In HBr the photon energy positions of the two 3d_5/2,3/2 → σ* resonances have been measured directly. Atomic Br is produced following dissociation of the σ* state and the asymmetric photon energy profile of Br atomic Auger lines is discussed. Angular distribution parameters of Br Auger lines have also been measured. In HCl and DCl the comprehensive nature of the analogous 2DPES reveal unexpected spectral features, the origin of which is considered.

When molecules are irradiated by x-rays complicated breakup reactions are initiated: inner shell excitation or ionization leads to a cascade of Auger decays resulting in multiple ionization and multiple bond breaks. During the last years high resolution electron - ion coincidence spectroscopy has developed into a standard tool to analyse molecular break-up reaction pathways. If ion pairs or the momenta of the fragment ions are also recorded, a very detailed analysis of the break-up reaction becomes possible. A manifold of interesting processes can be studied. Here we will discuss site specific fragmentation of CH₃F, F₃Si-CH₂-Si(CH₃)₃, F₃Si-CH₂-CH₂-Si(CH₃)₃ and Cl₃Si-Si(CH₃)₃ and electron transfer during the break up of CH₃F.

This work reports new theoretical and experimental results for the circular dichroism in electron angular distribution (CDAD) in the molecular frame of D₂ ionized by circularly polarized light in the region involving the resonant excitation of Q₁ and Q₂ doubly excited states. At the 32.5 eV photon energy, dissociative ionization of D₂ into the ground state limit D⁺ + D(1s) leads to an ion kinetic energy distribution extending over 7 eV, with a continuous energy sharing between the electron and the fragments. On the experimental side we used the vector correlation method to characterize the evolution of the molecular frame photoelectron angular distributions (MFPADs) as a function of the E_D+ kinetic energy. The CDAD is quantified by the θ_e polar angle dependence of the left right emission asymmetry maximum in the plane perpendicular to the light propagation axis k, for a space fixed molecule orthogonal to k. A remarkable variation of the CDAD is observed along the E_D+ energy distribution. The results are compared with the prediction of full four body ab initio calculations.

Experimental results obtained with our multi parameter multi-coincidence spectrometer are presented for the (e,3e) double ionisation of Ar and (e,2e) single ionisation of small molecules. The (e,3e) measurements are discussed in terms of competition between the two double ionisation processes present under the chosen kinematics, and qualitative conclusions are given. The results for the ionisation of H₂ and the outer orbital of N₂ are compared with the predictions of the most elaborate available theoretical models for description of the molecular ionisation process. Overall reasonable agreement is observed and tentative interpretations for the discrepancies are discussed.

Calculations are reported for four-body electron-helium collisions and positron-hydrogen collisions, in the S-wave model, using the time-independent propagating exterior complex scaling (PECS) method. The PECS S-wave calculations for three-body processes in electron-helium collisions compare favourably with previous convergent close-coupling (CCC) and time-dependent exterior complex scaling (ECS) calculations, and exhibit smooth cross section profiles. The PECS four-body double-excitation cross sections are significantly different from CCC calculations and highlight the need for an accurate representation of the resonant helium final-state wave functions when undertaking these calculations. Results are also presented for positron-hydrogen collisions in an S-wave model using an electron-positron potential of V₁₂ = − (8 + (r₁ − r₂)²)^−1/2. This model is representative of the full problem, and the results demonstrate that ECS-based methods can accurately calculate scattering, ionization and positronium formation cross sections in this three-body rearrangement collision.

We report on our progress in understanding the dynamics of atomic and molecular collision processes using time-dependent close-coupling methods

For several molecules which are important for plasma processing and gaseous dielectrics (CF₃I, CF₃Br, CH₃Br, and SF₆), we have studied the dependence of dissociative electron attachment (DEA) on both the electron energy and on the initial vibrational energy. With reference to electron swarm data, we determine highly-resolved absolute DEA cross sections over a broad energy range, using the Laser Photoelectron Attachment (LPA) method (E = 0–0.2 eV, δE ≈ 1–3 meV) and the EXtended Laser Photoelectron Attachment (EXLPA) method (0–2 eV, δE ≈ 15–30 meV). The experimental data are compared with the results of R-matrix calculations, involving ab initioinformation on the potential energy curves and semiempirical autodetachment widths. For CF₃I and CF₃Br, previous DEA cross sections are found to be substantially too high. For CH₃Br, the measurements confirm a predicted vibrational Feshbach resonance, associated with the v₃ = 4 vibrational threshold, and the value of the activation energy (due to an intermediate barrier) for this exothermic DEA process. For SF₆, we report absolute cross sections for SF₆⁻ as well as SF₅⁻ formation for vibrational temperatures ranging from 200 to 500 K. Moreover, the first absoluteDEA cross sections (SF₅₋ formation) for CO₂-laser excited SF₆ molecules have been obtained at different initial vibrational temperatures. The results indicate that the effect of the mode-selective energy input into the v₃-mode (predominantly v₃ = 1) on the enhancement of SF₅⁻ formation is very similar to that of a corresponding rise of the average vibrational energy by thermal heating; at E = 2 meV electron energy, the results indicate an activation energy of about 0.38 eV.

Fully differential cross sections for helium ionization by fast ion collisions are analyzed. The emitted-electron angular distributions for fixed momentum transfer and electron energies are described theoretically and compared to the experiment. The effect of uncertainties in the determination of the momentum is considered within the theoretical model. The cross sections are found to be extremely sensitive to the inclusion of the uncertainties. Important quantitative and qualitative modifications of the calculated cross sections are obtained by including small uncertainties in the determination of the momentum transferred by the projectile to the target.

We report on simultaneous measurements of the continuum momentum distribution for RECC cusp electrons and bremsstrahlung in 90AMeV U⁸⁸⁺+ N₂ → U⁸⁸⁺+ {N₂^+*} + e_cusp(0⁰) + hv (RECC) collisions. We show that x-ray photons which appear coincident with RECC cusp electrons stem from the short-wavelength limit of the electron-nucleus bremsstrahlung. The observed pronounced asymmetric cusp shape is in good agreement with theory within the relativistic impulse approximation. We expect that when exploiting the full imaging properties of our forward electron spectrometer, fully differential cross sections at the tip region of the bremsstrahlung spectrum with near 10⁻⁴ electron energy resolution are possible in the near future.

The 'reaction microscope', an electron-ion coincidence momentum imaging spectrometer, represents a unique technique to measure multiply differential ionization cross sections for ionic projectiles. Observed discrepancies with theory for singly ionizing collision of 3.6 MeV/u Au⁵³⁺ and 100 MeV/u C⁶⁺ ions severely challenge theoretical models. We present a method based on event-generators, which allows to consistently incorporate instrumental effects into theory. The result of the convolution procedure suggests, that the observed discrepancies are not a result of the experimental resolution, but are of true physical nature.

An adaptation of the mapped Fourier grid method for the Dirac equation is applied to the problem of positron production in quasi-molecular collisions between fully stripped heavy ions with super-critical Coulomb fields. The resonance parameters for the 1S_σ vacancy which dives into the filled negative-energy continuum are calculated on the basis of the two similar analytic continuation methods of smooth exterior scaling and complex absorbing potential with Padé extrapolation. Previous results in monopole approximation for the U-Cf system as a function of internuclear separation are extended to a coupled-channel approximation to investigate resonance broadening. Calculations for dynamical positron production on the basis of the time-dependent Dirac equation for U-U collisions with artificially delayed nuclear trajectories are also presented, and explanations for the observed positron spectrum are provided.

The recent theoretical progress in studying the x-ray emission from highly-charged, few-electron ions is reviewed. These case studies show that relativistic, high-Z ions provide a unique tool for better understanding the interplay between the electron–photon and electron–electron interactions in strong fields. Most naturally, this interplay is probed by the radiative capture of a (quasi-) free electron into the bound states of projectile ions, and by varying the charge state and the energy of the projectiles. For the capture into initially hydrogen-and lithium-like ions, here we summarize the recent results for the angular distribution and polarization of the recombination photons as well as the subsequent K_α emission, if the electron is captured into an excited state of the ion.

The progress of differential studies of ion-atom/molecule collisions using the reaction microscope was reported. The state-selective and angular differential cross sections were measured for slow ion-atom collisions. In this paper we focus on the two-electron processes, namely double electron capture and transfer ionization, occurring in the collisions of ion impact on two-electron target system of helium and molecular hydrogen, respectively. The molecule Coulomb explosions were investigated by employing the negative ion characteristic. The transfer momentum dependence of two-electron processes for He²⁺ and H⁺ impact on He and H₂ were compared, and the kinetic energy release in Coulomb explosions induced by He²⁺ and H⁺ was measured.

Interferences caused by a single electron impacting on an independent double-center scatterer, which plays the role of an atomic-size double-slit system, are experimentally evidenced for the first time. The electron originates from the autoionization of doubly excited 2lnl' (n ⩾ 2) configurations of He following a double charge exchange process by He²⁺ ions impinging on H₂ molecules. Well-defined oscillations are visible in the angular distribution of the electrons emitted towards the receding H⁺ protons. The presence of these oscillations demonstrates that a single electron interferes with itself. This is analogous to the famous "thought" experiment imagined and discussed by Feynman in 1963, in which the quantum nature of the electron was illustrated by making it traverse an atomic-size double-slit arrangement.

We report on measurements of transfer excitation in collisions of 0.3-1.3 MeV protons with spatially oriented H₂ molecules. Evidences of two center interference are found in the angular distribution of the molecule after a transfer excitation process and directly in the projectile angular scattering distributions. These features can be explained in a way which is analogous to that for the interferences in Young's classical double slit experiment: The fast projectiles preferentially capture electrons close to either of the molecular nuclei, and thereby they change their momenta and de Broglie wavelengths. The waves emerging from the two 'slits' of the molecule interfere yielding the observed interference structure.

We have measured proton distributions from the collision systems Ar⁺, Kr⁺ on CH₄ molecular targets, searching for atom capture into the projectile continuum. Within the studied energy range (100 to 300 eV/u) we have not distinctive evidence of capture. A small contamination of ion beams with molecular ions as ArH⁺ or KrH⁺, have shown to be enough to produce peak shaped structures at the projectile velocity. We, therefore, concentrate our study on proton loss from molecular ions in collision with several targets.

Kinematically complete experiments lead to very substantial progress in the understanding of decay dynamics of multiply charged molecular ions. This paper is devoted to the study of dissociative ionization of water molecules and clusters induced by highly charged ions. We will present first how experimental results determined in the gas phase can provide information about the physical stage of liquid water radiolysis. Second, we will discuss strong bond cleavage selectivity in ion-induced dissociation of deuterium tagged water molecule. Finally, we will present first promising results about water cluster dissociative ionization. Stability, energetics and charge mobility in the charged cluster will be briefly discussed.

During the last years we have measured total detachment cross sections of atomic and cluster anions colliding with gases in the velocity range of 0.2 to 1.8 a.u. In particular, we measured negative atomic halogens incident on argon and molecular nitrogen. These last data are for the first time analyzed using the simple semi-classical model that we have developed. For that purpose, the values of elastic plus inelastic cross sections for impact of free electrons on Ar and N₂, the latter showing a shape resonance, convoluted with the anion's outermost electron momentum distribution yielded the overall shape of the anion cross sections. Inclusion of a velocity independent additive term, interpreted as an effective area of the collision region, led to accurate absolute cross section values. The high affinity of the halogens and the existence of a not well described resonance in the e-N₂ collision, are characteristics that may be used to delimit the scope and validity of the model.

An experiment for crossed beam imaging has been developed for kinematically complete studies of the reaction dynamics of ion-molecule collisions. Here we report on this technique and on recent results regarding the charge transfer reaction of Ar⁺ + N₂. We speculate that the experimental observation of N₂⁺ product ions with unexpectedly high vibrational excitation may be caused by a Feshbach scattering resonance and propose further theoretical work on this reaction. Furthermore we discuss how future collision experiments of molecular ions and clusters will utilize a radiofrequency 22pole trap for internal cooling of the ions prior to the interaction.

By collision-energy/electron-energy-resolved 2D Penning ionization electron spectroscopy as well as by theoretical calculations of the collisional ionization dynamics, the outer shape of molecular orbitals for ionization to X(²Σ⁺) and A(²Π) states of CO⁺ was sensitively probed, and the anisotropy of the interaction between He^*(2³S) and CO was determined. Ionization to the B(²Σ⁺) state of CO⁺ was assigned to autoionization from superexcited states by angle-resolved electron spectroscopy and theoretical simulations.

The excitation and dissociation of polyatomic molecules by low-energy electron impact can be dominated by resonant collision processes. The formal resonance theory that has formed the basis for much of our understanding of these processes has, for the most part, treated the nuclear dynamics in one dimension. This talk will focus on dramatic effects in low energy electron scattering by small molecules that are purely polyatomic in origin and that can only be studied with a multi-dimensional treatment of the dissociation dynamics. Resonant vibrational excitation of CO₂ and dissociative electron attachment to formic acid are briefly described to illustrate the discussion. The talk will then concentrate on the recent progress that has been made in studying dissociative electron attachment to water, including a completely ab initio evaluation of the three complex-valued resonance potential surfaces involved as well as the dynamical studies, carried out in full dimensionality, that give the state-specific cross sections and branching ratios into various two- and three-body channels.

To illustrate our recent efforts to obtain electronic excitation cross sections of molecules by electron impact, we present in this paper results for the X ¹Σ a ³Π and A ¹Π transitions of CO obtained with the Schwinger multichannel method. Our results are in good agreement with other theoretical calculations, although not so good when compared with experiments. We also discuss the importance of inclusion of polarization effects to obtain electronic excitation cross sections of some molecules through an example using the C₂H₄ molecule, which has a triplet state with a low-energy threshold. Finally, we present a very simple rule to estimate integral electronic excitation cross sections using the differential cross section (DCS) at 90⁰, which can be useful to experimentalists using apparatus with difficulties to measure the DCS's at angles around 0 and 180 degrees. We show its efficiency for the present electronic excitation of the C₂H₄ molecule by electron impact.

Recent experimental investigations of dissociative electron attachment to unstable molecules and free radicals are reviewed along with observations of metastable negatively charged ions. Measurements have been made with a time-of-flight mass spectrometer in Belfast and also, for metastable ions, with a double focussing twin field mass spectrometer in Innsbruck. Electron attachment to unstable CS, for example, was found to be similar to electron attachment to the valence isoelectronic CO molecule with observation of S^- and C^- ions just above the thermodynamic threshold for S^- + C (³P) at 5.43 eV, C^-+ S at 6.40 eV and S^-+ C(¹D) at 6.70 eV with peak cross sections of ~ 0.025 Ų, 0.002 Ų and 0.003 Ų respectively. Slow fragmentation of metastable SF^-*₆ formed in low energy electron attachment to SF₆ has been observed on microsecond timescales in competition with autodetachment; processes SF^-*₆ → SF^-₅ + F and SF^-*₆ → SF₆ + e^- respectively. Fragmentation of metastable anions of benzene derivatives, such as 2,4-dinitro-toluene [CH₃.C₆H₃(NO₂)₂], has also been observed on microsecond timescales.

A theoretical framework is briefly reviewed that treats the multiple collisions that a quantum system undergoes with particles in its environment. The dynamics is governed by the quantum Lindblad master equation for the density matrix, which can be solved using a Monte Carlo technique. Its classical limit is identified and provides an opportunity to study the role of decoherence in the quantum to classical transition. Applications are discussed related to the dynamics of ions and atoms interacting with solids and gases, and Rydberg wavepackets subject to coloured noise.

We have developed methods to focus slow highly charged ions, MeV ions, and muon beams with various glass-made beam optics. (1) A focusing effect for an Ar⁸ + beam of 8 keV through a cm-long tapered capillary was obtained with a density enhancement of the transmitted beam compared with that of the input beam, which increases from 1 to 6 as the input current decreases from 30 pA to 0.8 pA. To study the stability of the transmitted beams through a glass capillary, we have measured the transmission of an 104 keV Ar⁸⁺ beam through a gap between a pair of parallel glass plates, and observed a precisely vibrational output current. (2) For 4 MeV He²⁺ beam, a 100 times density enhanced beam by a cm-long tapered capillary with a closed outlet was utilized to irradiate a cell in liquid. The range of the beam was controlled by the closed outlet with accuracy of ~1 μm. (3) Using 40 cm-long tapered glass tubes, a density enhancement of a factor of ≃2 was observed for both positive and negative muon beams with an energy of 13 MeV.

We study spin-dependent scattering and transport of low energy electrons (≤ 500 eV) through metals employing a classical transport theory within which electron trajectories are simulated as a sequence of stochastic scattering events. Elastic as well as spin-dependent inelastic processes are included in our model simulating the complete secondary electron cascade. We apply our model to spin-polarization measurements of electrons emitted from magnetized Fe after impact of unpolarized primary electrons. We find good agreement with experimental data.

Absolute cross sections for double electron transfer in H^- + H⁺ collisions have been measured for center-of-mass energies from 0.5 keV to 12 keV. Clear oscillations in the cross section are observed which are in excellent agreement with earlier measurements at lower energies by Brouillard et al (1979) as well as Peart and Dolder (1979). After an oscillation maximum at 3 keV center-of-mass energy the cross section decreases for increasing energy with no indication of further oscillations.

Photoelectron spectroscopy on free cold size-selected sodium clusters has led to a detailed knowledge of their electronic and geometric structures in a very broad size range. Even more information about the electronic structure of the clusters has been obtained now by angle resolved photoelectron spectroscopy, which in principle allows one to gain information about the character of the electronic wavefunctions. The results demonstrate that sodium clusters have an electronic structure close to that of an finite size free electron gas, which makes them perfect model systems for the study of many particle dynamics. One such measurement is the time-resolved study of the cooling of the hot electron gas in the cluster, which allowed to determine the size dependence of the electron phonon coupling strength.

Absolute keV x-ray measurements have been performed with nanometer size rare gas clusters submitted to femtosecond IR laser pulses at intensities I < 10¹⁷W cm^-2. Special care has been taken in the measurements, by controlling the different parameters, which play a role in laser-cluster dynamics, i.e., beam waist size, effective pulse length and energy profile of the laser, and the cluster size. In particular, the data obtained show evidence of low laser intensity threshold values for keV x-ray production, an optimum heating time when mapping the cluster dynamics by varying the pulse duration at constant laser energy and a saturation in the x-ray emission probability above a given cluster size.

We study the interaction of an intense (I ≳ 10¹⁵ Wcm^-2) femtosecond laser pulse with a large (N > 10000 atoms) rare-gas cluster. The simulations are based on a mean-field classical transport approach. The electronic dynamics during the interaction with the laser are discussed in more detail. In particular we point out the difference in behavior between the fast electrons and the collectively moving slow electrons and try to shed light on the acceleration mechanisms behind the high energy tail of the electron energy distribution. The benchmark for our simulations is experimental X-ray spectroscopy data. We show a comparison with the experimentally found total X-ray yields and charge-state distributions.

We report the first direct observation of ultrafast dynamics in molecules induced by ionizing radiation. We use high harmonic upconversion process to generate soft-x-ray pulses of few femtoseconds duration, which photoionize N₂ molecule. This leads to the formation of highly excited N₂⁺ ions via inner-shell ionization and electron shakeup processes. We time resolve the unexplored fragmentation dynamics of the electron shakeup states with femtosecond resolution using a strong-field IR probe. The IR pulse promotes the dissociating N₂⁺ wavepacket to repulsive N₂²⁺ potential. We obtain kinetic energy release of N⁺/N⁺ channel as function of time delay and observe a rapid transition from spherically-symmetric molecular potential to a two-center potential within ~150 fs.

The first experimental results on photoionization of mass-selected endohedral Sc₃N@C₈₀⁺ and Ce@C₈₂⁺ fullerene ions are reported. The merged-beams technique was employed to measure photo-ion yield spectra as well as absolute cross sections. Comparing the results of endohedral fullerenes with those obtained using "empty" C₈₀⁺ and C₈₂⁺ ions provides insight into the mutual influence of the fullerene cage and the encapsulated atom or molecule on one another in photoabsorption processes.

We have studied the stabilities of multiply charged van der Waals dimers in slow Xe³⁰⁺ + [C₆₀]₂([C₆₀C₇₀], [C₇₀]₂) → Xe^(30-s)+ + [C₆₀]₂^r+([C₆₀C₇₀]^r+, [C₇₀]₂^r+) + (r-s)e^-electron-transfer collisions (v = 0.4 a.u. and r ≤ 7). The relative ionization cross sections display even-odd variations as functions of r for [C₆₀]₂^r+, [C₆₀C₇₀]^r+, and [C₇₀]₂^r+. This is in clear contrast to the typical smooth decreasing behavior of the cross sections for multiple ionization of fullerene monomers, which can be explained within the framework of the classical over-the-barrier model. In addition, we report the branching ratios and the kinetic energy releases for the dominant fragmentation processes that yield intact fullerenes. The experimental results are discussed in view of a simple electrostatic model for dimer ionization and recent results from high level density functional theory calculations.

We discuss some of our recent work on the near-threshold behavior of ultracold three-body collisions and their relation to a recent experiment [T. Kraemer, et al., Nature 440, 315 (2006)]. In particular, we discuss the role of Efimov physics in this experiment and other ultracold collisions and how this role can be understood within the adiabatic hyperspherical representation.

We study scattering of ultracold atoms by absorbing surfaces. Loss of flux through inelastic reactions and adsorption is described in an unambiguous and model-independent way by incoming boundary conditions in the semiclassical region near the surface. The near-threshold behaviour of the scattering amplitude is determined by a few parameters of the potential tail beyond the semiclassical region. Investigation of quantum reflection and scattering by flat and spherical surfaces shows that the curvature of the surface strongly influences the range to which the scattering amplitudes are sensitive in the atom-surface interaction.

A theoretical approach was developed that allows for a full numerical description of a pair of ultracold atoms trapped in a three-dimensional optical lattice. This approach includes the possible coupling between centre-of-mass and relative motion coordinates in a configuration-interaction type formulation. The atoms are allowed to interact by their full interaction potential that is, presently, only limited to be central. With the aid of the newly developed method deviations from the harmonic approximation are discussed for the heteronuclear ⁸⁷Rb-⁴⁰K pair.

We discuss the axial dynamics of laser-cooled relativistic C³⁺ ion beams at moderate bunching voltages. Schottky noise spectra measured at a beam energy of 122 MeV/u are compared to simulations of the axial beam dynamics. Ions confined in the bucket are addressed by the narrow-band force of a laser beam counter-propagating to the ion beam, while the laser frequency is detuned relatively to the cooling transition frequency in the rest frame of the bucket.

At large detuning comparable to the momentum acceptance of the bucket, the axial dynamics can be well explained by the secular motion of individual non-interacting ions. At small detuning, corresponding to a small axial momentum spread Δp_axial/p_axial < 10⁻⁶ of the ions, the measured Schottky noise spectra can no longer be explained using an approach which neglects the ion-ion interaction. Instead, the model fails when the ion bunch enters the space-charge dominatedregime, at which the mutual Coulomb-energy of the ions becomes comparable to the kinetic energy of the ions.

Negative carbon atomic ions and cluster ions were stored in an electrostatic ion storage ring. The lifetime of the metastable atomic ion is heavily affected by the blackbody radiation from the surrounding environment. By introducing rare gas target into the ring, collision-induced detachment cross sections of the atomic ion C^- were obtained. For cluster ions C_n^- (n = 2 ∼ 8), decay of metastable ions with sub-millisecond to millisecond lifetime was commonly observed.

We present a model for ab-initio calculations of the interaction of two-electron atoms and molecules with few-cycle pulses of intense linearly polarised Ti:sapphire laser radiation. In the model the center-of-mass motion of the two electrons is restricted along the polarisation direction axis, while its relative coordinate and, hence, the electron correlation is retained in its full dimensionality. Results of numerical simulations exhibit the two pathways to nonsequential double ionization, namely the emission of a highly correlated electron pair upon rescattering and a delayed electron emission from a previously excited ion. Comparisons with the results of the usual one-dimensional approximation, in which the direction of each electron is restricted to the polarisation axis, are given. Distributions of the center-of-mass momentum and the correlated electron momenta along the polarisation direction are in qualitative agreement with the experimental data.

Studies of the simplest one-electron molecule, H₂⁺, are the first step towards understanding the interaction of ultrashort intense laser pulses with molecules. We conduct coincidence 3D imaging measurements of H₂⁺ beams following their exposure to intense ultrashort laser pulses. These measurements are compared with our time-dependent calculations as well as a simple model we recently proposed. Our findings include above threshold Coulomb explosion - a surprising structure in the energy spectrum near the ionization appearance intensity; above threshold dissociation (ATD) of the excited electronic states of H₂⁺; and enhanced high-order ATD - involving the net absorption of at least 3 photons - brought about by closing the 2-photon channel.

We discuss a computational method to study the dynamics in the laser-atom interactions. There are two key ingredients to the new method. Firstly, we transform the differential time-dependent Schrödinger equation into a time-integral equation, in which the dynamics related wavefunction is separated from the background wavefunction analytically to improve the numerical accuracy. Secondly, we divide the space into an inner region and an outer region, and propagate the inner-region wavefunction in the full Hamiltonian numerically and outer-region wavefunction in the momentum space analytically. In this way, we remove the physical boundary in space. To show the effectiveness of the method, we simulate the carrier-envelop phase dependent high energy above-threshold-ionization yields, which are in good agreement with the experimental observations. Furthermore, we investigate the rescattering electron momentum spectra and provide an intuitive rescattering picture from a full quantum non-perturbative calculation.

Strong optical fields induce multiple ionization in irradiated molecules. The ionization dynamics are governed by optical-field-induced distortions of molecular potential energy surfaces and molecular dissociation is the expected by-product. Recent experiments have even shown, quite counter-intuitively, that strong optical fields may even induce bond formation processes in molecules. All such processes are all manifestations of how intense light affects matter. In turn, matter also affects intense light. A visually dramatic manifestation of matter affecting light is obtained when ultrashort pulses of intense light propagate though condensed matter. The temporal and spatial properties of the incident light pulse are modified, and such modifications manifest themselves in an enlarged optical frequency sweep, resulting in the generation of broadband radiation (white light) known as supercontinuum production. Although the physics that governs supercontinuum generation is not properly understood, some recent progress is summarized. Novel applications of strong field phenomena are reported that are of relevance in the biomedical and life sciences.

The exact non-dipole minimal-coupling Hamiltonian for an atomic system interacting with an explicitly time- and space-dependent laser field is transformed into the rest frame of a classical free electron in the laser field, i.e., into the Kramers-Henneberger frame. The new form of the Hamiltonian has been used to study the non-dipole dynamics of atoms and molecules in intense XUV laser pulses. The time-dependent Schrödinger equation is solved without any simplifications.

We report on the high-resolution multidimensional real-time mapping of H₂⁺ and D₂⁺ nuclear wave packets performed employing time-resolved three-dimensional Coulomb explosion imaging with intense laser pulses. Exploiting a combination of a "reaction microscope" spectrometer and a pump-probe setup with two intense 6-7 fs laser pulses, we simultaneously visualize both vibrational and rotational motion of the molecule, and obtain a sequence of snapshots of the squared ro-vibrational wave function with time-step resolution of ~ 0.3 fs, allowing us to reconstruct a real-time movie of the ultrafast molecular motion. We observe fast dephasing, or 'collapse' of the vibrational wave packet and its subsequent revival, as well as signatures of rotational excitation. For D₂⁺ we resolve also the fractional revivals resulting from the interference between the counter-propagating parts of the wave packet.

We review recent applications of the convergent close-coupling (CCC) method to studies of two-photon double ionization (TPDI) of He. In a weak-field regime, the electron-photon interaction can be treated within the lowest order perturbation theory (LOPT) whereas the electron-electron interaction is included in full. The intermediate states of the target can either be represented by a discrete set of B-splines in a box or summed over with an average weight (closure approximation). In a non-perturbative regime, we solve the time-dependent Schrödinger equation on a square-integrable basis and project this solution on a set of CCC final states. In both regimes, we are able to calculate reliably the total intgrated and full differential TPDI cross-sections.

Besides purely numerical results, we introduce a convenient analytical parametrization of the TPDI amplitude in the manner similar to single photon double ionization. Aided with this parametrization, we observe two distinctly different modes of correlated motion of the photoelectron pair. We also derive the angular anisotropy parameters and the recoil ion momentum distribution for TPDI of He. The latter can be compared with recent experimental observations.

Strong optical laser fields modify the way x rays interact with matter. This allows us to use x rays to gain deeper insight into strong-field processes. Alternatively, optical lasers may be utilized to control the propagation of x rays through a medium. Gas-phase systems are particularly suitable for illustrating the basic principles underlying combined x-ray and laser interactions. Topics addressed include the impact of spin-orbit interaction on the alignment of atomic ions produced in a strong laser field, electromagnetically induced transparency in the x-ray regime, and laser-induced alignment of molecules.

We report on a physical mechanism of coherent control with intense shaped femtosecond laser pulses. We study photoelectron spectra from multi-photon ionization of potassium atoms and dimers using tailored femtosecond laser pulses. Our results are interpreted in terms of Selective Population of Dressed States (SPODS). Two realizations of SPODS by Photon Locking (PL) via pulse sequences and Rapid Adiabatic Passage (RAP) via chirped pulses are discussed. New physical mechanisms arise, when both PL and RAP are at play simultaneously. Control by the combined effect of PL and RAP is studied by mapping out a two-parameter Quantum Control Landscape (QCL) for selective population of dressed states.

We analyze the two-dimensional momentum distribution of electrons ionized by short laser pulses by solving the time-dependent Schrödinger equation. Lindner et al., [Phys. Rev. Lett. 95, 040401 (2005)] identified oscillations in the electron emission spectrum for ionization by a few-cycle pulse as a time-double slit interference. We extend this analysis to interference fringes in the momentum distributions. For longer pulses we find a complex two-dimensional interference pattern that resembles ATI rings at higher energies and displays Ramsauer - Townsend - type diffraction oscillations in the angular distribution near threshold.

The generation of echoes in the electric dipole moment of a Rydberg wavepacket precessing in an external electric field by reversal of the field is described. When the wavepacket experiences reversible dephasing, large echoes are observed pointing to strong refocusing of the wavepacket. The presence of irreversible dephasing leads to a reduction in the size of the echoes. The effect of irreversible dynamics on echoes is investigated using artificially synthesized noise. Methods to determine the decoherence rate are discussed.

Ultrafast hydrogen migration in deuterated acetylene dication (C₂D₂²⁺) is studied by the pump-probe Coulomb explosion imaging with few-cycle intense laser pulses (9 fs, 0.13 PW/cm², 800 nm). The temporal evolution of the momenta of the fragment ions produced by the three-body explosion, C₂D₂³⁺ → D⁺+ C⁺ + CD⁺, shows that the migration proceeds in a recurrent manner: The deuterium atom first shifts from one carbon site to the other in a short time scale (∼90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation.

The advent of mirror-less Free Electron Lasers emitting polarised and coherent 'laser-like' beams of high peak (> 1GW) and average (up to 100mW) powers in the extreme-UV (EUV) and X-ray bands of the electromagnetic spectrum heralds a new era in the study of the photoelectric effect. The unprecedented photon flux (∼10¹³ photons per pulse) opens up to scrutiny processes with cross sections considered hitherto unfeasibly small to probe with conventional EUV sources such as synchrotrons and laser plasmas. The peak intensity of the focussed pulse train (<10¹³ W/cm²), combined with the high photon energy, ports non-linear optics and spectroscopy into a regime where inner shell electrons can become the predominant mediator of the photon matter interaction. Few photon, few electron photoionization processes are made amenable to study for the first time and the wavelength tunability of the FEL permits resonances to come into play. In combination with ultrafast optical lasers, pump-probe experiments on atoms and molecules where both fields are of comparable high intensity but orders of magnitude different in photon energy become possible. In mid 2005 the 2nd phase of the Free Electron Laser project at DESY, Hamburg (FLASH) opened to users. In what follows I will attempt to illustrate at least some of the impressive progress that has been made by very brief descriptions of just a few of the pathfinder experiments that the growing Atomic and Molecular physics community at FLASH has undertaken in the intervening two years.

The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >10¹² photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100 m and 350 m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in the middle of 2010. The project design permits straightforward expansion of the LCLS to multiple undulators.

In vast regions of the universe highly charged ions (HCI [1, 2]) are the predominant form of visible matter. Their importance extends to high-temperature terrestrial plasmas, such as those used in fusion research. Yet, accurate prediction of their electronic structure remains a challenge for theory due to the strong electromagnetic field in which the remaining bound electrons dwell. Experimental accuracy has now reached the performance limits of conventional photon spectroscopy in the soft and hard x-ray regions. In this work [3], we report on the resonant laser excitation of the 2²S_1/2—2²P_1/2 transition of the Li-like Fe22P1/2²³⁺ ion at 48.6 eV, an energy range hitherto unattainable with powerful lasers. The HCI stored in an electron beam ion trap (EBIT [4]) were resonantly excited by ultra-brilliant radiation generated at the Free electron LASer in Hamburg (FLASH [5]). While yielding a relative statistical error of only 2.2·10⁻⁵, and extending laser spectroscopy on HCI from the near ultraviolet [6] to the soft X-ray region, this novel experiment demonstrates immediate potential to push the current limits of precision by orders of magnitude. Such experiments allow to verify predictions of quantum electrodynamics (QED) in a strong field environment where perturbation theory [7, 8] fails. Future EBIT experiments at upcoming x-ray free electron lasers (X-FEL) like the Stanford Linear Coherent Light Source (LCLS) or the European X-FEL will pave the way for laser spectroscopy into the hard x-ray region.

We consider two types of collisional processes with excited antiprotonic helium systems. (i) Transitions between hyperfine structure sublevels in (He⁺)_nLFJ are considered at T ≤ 25 K using the coupled-channels method and a model of scalar and tensor interactions between antiprotonic and ordinary He atoms. Inelastic cross sections for the single and double spin-flip transitions are less than elastic ones by four and seven orders of magnitude, respectively. The cross sections show a resonance behaviour at E ∼ 1–4 K, and therefore the relaxation rates, density shift and broadening of the M1 spectral lines decrease with increasing temperature from 3 to 25 K. (ii) Stark transitions and induced annihilation during collisions of the (He²⁺)_nl ion with He atoms at E ∼ 10 K are considered for all l-states at n ∼ 30 in the framework of the coupled-channels method. In order to take into account complex energy shifts of the ns and np-states, we introduce modified boundary conditions in these channels. The main contribution to elastic scattering, Stark transitions and induced annihilations during collisions comes from the long-range polarization interaction. Admixtures of ns and np-states to nl-states (l ≥ 2) during collisions induce the effective annihilation cross sections for the initial l up to 15, but don't affect the Stark cross sections for states with nearly-circular orbits. The calculated values for both types of processes are compatible with recent experimental data.

Until recently it has not been possible to determine differential cross sections for excitation of atoms by electron impact over the complete scattering geometry, due to the physical constraints of the apparatus. The invention in Manchester of the Magnetic Angle Changing (MAC) device which steers electrons to and from the interaction region has now changed this. By utilising super-elastic electron scattering from laser excited atoms within a MAC device, the differential cross sections for electron impact excitation of calcium atoms to the 4¹P₁ state have now been determined from near 0° to beyond 180°. The methods used in these experiments are discussed, and results are presented for the Natural frame parameter L_⊥ at energies of 45eV and 55eV. The results are compared to recent calculations using a distorted wave Born approximation.

The near-threshold behavior for positron-impact ionization (breakup) of hydrogen is considered. The two-centre convergent close-coupling method is used. We find that convergence in the cross section is only practically obtained if two near-complete expansions are used, one centred around H and the other around Ps. The contribution to the breakup cross section becomes the same for both centres as the threshold is approached. The calculations are found to be in good agreement with the threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)].

Coincidences between recoil ions-ejected electrons and recoil ions-scattered projectiles have been used to study the kinematics of electron and positron impact ionization. Triply Differential (TDCS) data for 500 eV positron and electron impact on Ar are presented here as function of scattering angle for a given range of energy losses. Binary and recoil interactions can be distinguished allowing us to determine the relative intensity between those interactions. Preliminary integration of the data indicate an enhancement of the binary region for positron interaction while for electron impact the intensity of the recoil and binary interactions is comparable.

Electron recombination of H₃⁺ has found a lot of attention due to its outstanding relevance for the chemistry of the interstellar medium (ISM) and its role as a benchmark for the treatment of dissociative recombination (DR) of polyatomic ions. We report DR measurements performed at the TSR storage ring utilizing a cryogenic ion trap injector. Furthermore, a chemical probing spectroscopy technique is described that allows for a very sensitive monitoring of the populated states inside the ion injector. Since H₃⁺ exists in two different nuclear spin modifications, a controlled manipulation of the ortho/para fraction is needed in order to perform state-selective measurements.

By studying ion-pair formation in electron recombination with molecular ions, fundamental knowledge on the molecular dynamics can be obtained. In order to study these types of reactions, both the electron recombination as well as the dynamics all the way to the asymptotic limits must be well described. We have used the wave packet technique to study ion-pair formation in electron recombination with HeH⁺, HD⁺, H₃⁺ and HF⁺. We here discuss what will determine the general shape of the ion-pair cross section, the threshold effects, possible interference effects as well as the ratio of the cross sections of ion-pair formation to dissociative recombination.

We report measurements of resonant processes in electron collisions with very highly charged heavy ions made using an electron beam ion trap. By measuring the ion abundance ratio in the trap at the equilibrium condition as a function of electron energy, we have observed resonant processes such as dielectronic recombination and resonant excitation double autoionization very clearly. Remarkable relativistic effects due to the generalized Breit interaction have been clearly shown in dielectronic recombination for highly charged heavy ions.

Negative ion resonances overlapping the ionisation continuum were studied by spin-polarised electron impact on zinc atoms. The measured integrated Stokes parameters of the 636.2 nm and 468.1 nm photons from the respective 3d¹⁰4s4d¹D₂ and 3d¹⁰4s5s³S₁ states show Fano-equivalent profiles reflecting the effects of spin-orbit interaction and electron exchange in the resonant states which have an open 3d core shell.

Alcohols and nitriles not only play an important role as templates for synthesis of larger molecules in the interstellar medium and planetary atmospheres, but they can also be regarded as precursors for biomolecules. Alcohols can form carbohydrates through reaction with HCO and nitriles can be hydrolysed to amino acids in aqueous solutions, which is the final step of the well-known Strecker synthesis. Therefore the question of the pathways of formation of alcohols and nitriles and the efficiency and the product distribution of their subsequent degradation reactions in the above-mentioned astrophysical environments is of great interest. In both processes dissociative recombination reactions of protonated nitriles and alcohols may play a major role and are included in models of interstellar clouds and planetary atmospheres. However, the reaction rate coefficients and product branching ratios for the majority of these processes are so far still unknown, which adversely affects the quality of predictions of model calculations. In this Contribution, we therefore present branching ratios and rate constants of the dissociative recombination of protonated methanol (CH₃OH₂), as well as protonated acetonitrile (CH₃CNH⁺), acrylonitrile (C₂H₃CNH⁺) and cyanoacetylene (HC³NH⁺). The impact of the obtained new data on model calculations of abundances of important interstellar molecules in dark clouds is discussed.

Collisions between electrons and neutral species play an important role in the energy balance of upper atmospheres throughout the Solar System. These processes, and the subsequent photon emissions produced, provide one of the primary means for probing, diagnosing and understanding the dynamics of these environments. Modelling of these plasmas and interpretation of optical observations requires accurate knowledge of atomic/molecular parameters such as oscillator strengths and predissociation yields along with cross sections for electron impact excitation (and emission) with the constituent species of these atmospheres. In this paper, our recent work involving electron collisions with two of the most important atmospheric species in our Solar System, namely N₂ and H₂, will be reviewed. Preliminary differential cross sections for excitation of the B ¹Σ⁺_u and C ¹Π_u states out of the X ¹Σ_g⁺(v^'' = 1) ground state level of H₂ are presented.

We discuss and illustrate our recent work on dielectronic recombination, photoionization & opacities, and electron-impact excitation, with a particular view to its application for plasma modelling.

Interpreting cosmic spectra from ionized atomic gas hinges on our understanding the underlying physical processes which produce the observed spectra. Of particular importance are electron-ion recombination and electron impact ionization. These processes control the charge state distribution (CSD) of the gas. The CSD is intimately tied in to the observed spectral features and can also affect the thermal structure of the plasma. Heavy ion storage rings play a crucial role in improving our knowledge of electron-ion recombination and electron impact ionization, thereby deepening our understanding of the cosmos. Here we will review some of the astrophysical motivation behind laboratory astrophysics research with atomic ions at heavy ion storage rings. We also present some recent results, discuss some of the astrophysical implications, and present future astrophysical needs.

We report on results of computational studies of the interaction of slow electrons with the purine and pyrimidine bases of DNA, as well as with their associated nucleosides and nucleotides. The calculations focus on characterisation of the π* resonances associated with the bases and also provide general information on the scattering of slow electrons by these targets. High-level studies of the π* resonances in pyrazine, a close analogue of the pyrimidine bases, indicate that the higher-energy π* resonances in these bases may in fact contain large admixtures of core-excited character built on low-lying triplet states. Decay into such triplet states may provide a mechanism for damage to DNA.

We show that functional group dependence exists in dissociative attachment of electrons to molecules and this leads to site and bond selectivity in fragmenting organic molecules at the N-H, O-H and C-H sites using electron energy as a control parameter. This phenomenon is investigated further by measuring the momentum distribution of hydride ions using the newly developed ion momentum imaging technique. We find that while the electron attachment at the O-site leads to a two-body fragmentation, attachment at the C-site leads to few-body fragmentation. Several new phenomena like 'bond orientation dependent electron attachment' and direct screening of one part of the molecule by another part to the incoming electron are unravelled in the very rich momentum distribution data of the hydride ions that we have obtained at various resonances.

Extensive results from measurements on short DNA strands impinged on by 1-30 eV electrons indicate that the damage they induce is due to the chemical nature of the nucleic bases and/or their sequence. The strong variation of effective cross sections for plasmid DNA single-strand breaks with incident electron energy and the resonant enhancement at 1 eV suggest that considerable damage is inflicted by very low-energy electrons to DNA, and it indicates the important role of π* shape resonances formed on different constituents of DNA. Recent results of vibrational and electronic excitation of thin condensed films of adenine and thymidine by electrons of energy from 1 to 12 eV are presented.

We present detailed free electron attachment measurements on nitroaromatic compounds in the gas phase. It turns out that dissociative electron attachment can act as a selective and sensitive probe for the identification of isomeric forms of nitrotoluene and dinitrobenzene. Rich fragmentation patterns have been observed for both nitroaromates and all fragments above the detection limit of our instrument have been investigated with a high energy resolution electron monochromator in the energy range of about 0 to 15 eV. It is shown that relative attachment cross section curves can act as fingerprints for the particular molecule and its isomers. Additionally it has been observed that numerous fragments arise from surprisingly complex structural and electronic rearrangements.