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Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method that is sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH3) following its strong field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH+3) undergoes an ultrafast geometrical transformation from a pyramidal (FHNH=107°) to planar (FHNH=120°) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar (FHNH=117±5°) field-dressed NH+3 molecular structure 7.8-9.8 femtoseconds after ionization. Our measured field-dressed NH+3 structure is in excellent agreement with our calculated equilibrium field dressed structure using quantum chemical ab initio calculations. J.B. and group acknowledge financial support from the European Research Council for ERC Advanced Grant “TRANSFORMER” (788218), ERC Proof of Concept Grant “miniX” (840010), FET-OPEN “PETACom” (829153), FET-OPEN “OPTOlogic” (899794), Laserlab- Europe (EU-H2020 654148), MINECO for Plan Nacional FIS2017-89536-P; AGAUR for 2017 SGR 1639, MINECO for “Severo Ochoa” (SEV- 2015-0522), Fundació Cellex Barcelona, CERCA Programme / Generalitat de Catalunya, and the Alexander von Humboldt Foundation for the Friedrich Wilhelm Bessel Prize. J.B., K.A. and R.Moszynski. acknowledge the Polish National Science Center within the project Symfonia, 2016/20/W/ST4/00314. J.B and B.B. acknowledge Severo Ochoa” (SEV- 2015-0522). J.B. and A.S. acknowledge funding from the Marie Sklodowska-Curie grant agreement No. 641272. C.D.L is supported in part by Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy under Grant No. DE-FG02-86ER13491. J.S. and S.G. highly acknowledges support from the European Research Council (ERC) for the ERC Consolidator Grant QUEM-CHEM (772676). The authors thank Alejandro Saenz for helpful discussions.

Charge transfer (CT) at avoided crossings of excited ionized states of argon dimers is observed using a two-color pump-probe experiment at the free-electron laser in Hamburg (FLASH). The process is initiated by the absorption of three 27-eV-photons from the pump pulse, which leads to the population of Ar2+*–Ar states. Due to nonadiabatic coupling between these one-site doubly ionized states and two-site doubly ionized states of the type Ar+*–Ar+, CT can take place leading to the population of the latter states. The onset of this process is probed by a delayed infrared (800 nm) laser pulse. The latter ionizes the dimers populating repulsive Ar2+ –Ar+ states, which then undergo a Coulomb explosion. From the delay-dependent yields of the obtained Ar2+ and Ar+ ions, the lifetime of the charge-transfer process is extracted. The obtained experimental value of (531 ± 136) fs agrees well with the theoretical value computed from Landau-Zener probabilities.

Physical review letters 122(7), 073001 (2019). doi:10.1103/PhysRevLett.122.073001, Time delays for atomic photoemission obtained in streaking or reconstruction of attosecond bursts by interference of two-photon transitions experiments originate from a combination of the quantum mechanical Wigner time and the Coulomb-laser coupling. While the former was investigated intensively theoretically as well as experimentally, the latter attracted less interest in experiments and has mostly been subject to calculations. Here, we present a measurement of the Coulomb-laser coupling-induced time shifts in photoionization of neon at 59.4 eV using a terahertz (THz) streaking field (λ=152 μm). Employing a reaction microscope at the THz beamline of the free-electron laser in Hamburg (FLASH), we have measured relative time shifts of up to 70 fs between the emission of 2p photoelectrons (∼38 eV) and low-energetic (, Published by APS, College Park, Md.

Angewandte Chemie / International edition 128(36), 10899 - 10903(2016). doi:10.1002/ange.201605335, The fragmentation of free tenfold protonated ubiquitin in intense 70 femtosecond pulses of 90 eV photons from the FLASH facility was investigated. Mass spectrometric investigation of the fragment cations produced after removal of many electrons revealed fragmentation predominantly into immonium ions and related ions, with yields increasing linearly with intensity. Ionization clearly triggers a localized molecular response that occurs before the excitation energy equilibrates. Consistent with this interpretation, the effect is almost unaffected by the charge state, as fragmentation of sixfold deprotonated ubiquitin leads to a very similar fragmentation pattern. Ubiquitin responds to EUV multiphoton ionization as an ensemble of small peptides., Published by Wiley-VCH9102, Weinheim

The migration of hydrogen atoms resulting in the isomerization of hydrocarbons is an important process which can occur on ultrafast timescales. Here, we visualize the light-induced hydrogen migration of acetylene to vinylidene in an ionic state using two synchronized 4 fs intense laser pulses. The first pulse induces hydrogen migration, and the second is used for monitoring transient structural changes via Coulomb explosion imaging. Varying the time delay between the pulses reveals the migration dynamics with a time constant of 54 ± 4 fs as observed in the H+ + H+ + CC+ channel. Due to the high temporal resolution, vibrational wave-packet motions along the CC- and CH-bonds are observed. Even though a maximum in isomerization yield for kinetic energy releases above 16 eV is measured, we find no indication for a backwards isomerization — in contrast to previous measurements. Here, we propose an alternative explanation for the maximum in isomerization yield, namely the surpassing of the transition state to the vinylidene configuration within the excited dication state.

Intense, multi-color laser fields permit the control of the ionization of atoms and the steering of electron dynamics. Here, we present the efficient collinear creation of the second and third harmonic of a 790 nm femtosecond laser followed by a versatile field synthesizer for the three color fields' composition. Using the device, we investigate the strong-field ionization of neon by fields composed of the fundamental, and the second or third harmonic. The three-color device offers sufficient flexibility for the coherent control of strong-field processes and for time-resolved pump-probe studies.

The first hundred attoseconds of the electron dynamics during strong field tunneling ionization are investigated. We quantify theoretically how the electron's classical trajectories in the continuum emerge from the tunneling process and test the results with those achieved in parallel from attoclock measurements. An especially high sensitivity on the tunneling barrier is accomplished here by comparing the momentum distributions of two atomic species of slightly deviating atomic potentials (argon and krypton) being ionized under absolutely identical conditions with near-infrared laser pulses (1300 nm). The agreement between experiment and theory provides clear evidence for a nonzero tunneling time delay and a nonvanishing longitudinal momentum of the electron at the ``tunnel exit.''

Interatomic Coulombic decay (ICD) is induced in helium nanodroplets by photoexciting the $n=2$ excited state of ${\mathrm{He}}^{+}$ using XUV synchrotron radiation. By recording multiple-coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic ${\mathrm{He}}^{+}$ ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow ${\mathrm{He}}^{+}$ ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of ${\mathrm{He}}_{k}{}^{+}$ complexes.

The ultrafast transfer of excitation energy from one atom to its neighbor is observed in singly charged argon dimers in a time-resolved extreme ultraviolet (XUV)-pump IR-probe experiment. In the pump step, bound 3s-hole states in the dimer are populated by single XUV-photon ionization. The excitation-energy transfer at avoided crossings of the potential-energy curves leads to dissociation of the dimer, which is experimentally observed by further ionization with a time-delayed IR-probe pulse. From the measured pump-probe delay-dependent kinetic-energy release of coincident Ar+ + Ar+ ions, we conclude that the transfer of energy occurs on a time scale of about 800fs . This mechanism represents a fast relaxation process below the energy threshold for interatomic Coulombic decay.

We have combined a tandem mass spectrometer with a 780 nm fs-laser system to study photoionization and photofragmentation of trapped protonated leucine enkephalin cations for laser intensities between 2 × 1013 W/cm2 and 1 × 1014 W/cm2 and pulse durations of 15 fs. In this intensity range, the transition from multiphoton ionization and excitation to tunneling ionization is expected to occur. The observed partial ion yield curves as a function of laser intensity exhibit a power-law dependence, indicating multiphoton absorption to be the dominating mechanism. Pump-probe studies were performed to investigate the time-evolution of the multiphoton ionization process. The partial ion yields of almost all fragmentation channels show a broad but distinct maximum at a delay-time of approximately 750 fs. The particularly flat appearance of the pump-probe curves suggests that not a single resonance, but a broad distribution of resonances is involved.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Different ionization processes were observed for acetylene. These time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

Visualizing chemical reactions as they occur requires atomic spatial and femtosecond temporal resolution. Here, we report imaging of the molecular structure of acetylene (C 2 H 2 ) 9 femtoseconds after ionization. Using mid-infrared laser–induced electron diffraction (LIED), we obtained snapshots as a proton departs the [C 2 H 2 ] 2+ ion. By introducing an additional laser field, we also demonstrate control over the ultrafast dissociation process and resolve different bond dynamics for molecules oriented parallel versus perpendicular to the LIED field. These measurements are in excellent agreement with a quantum chemical description of field-dressed molecular dynamics.

Temporal evolution of electronic and nuclear wave packets created in strong-field excitation of the carbon dioxide molecule is studied employing momentum-resolved ion spectroscopy and channel-selective Fourier analysis. Combining the data obtained with two different pump-probe set-ups, we observed signatures of vibrational dynamics in both, ionic and neutral states of the molecule. We consider far-off-resonance two-photon Raman scattering to be the most likely mechanism of vibrational excitation in the electronic ground state of the neutral CO2. Using the measured phase relation between the time-dependent yields of different fragmentation channels, which is consistent with the proposed mechanism, we suggest an intuitive picture of the underlying vibrational dynamics. For ionic states, we found signatures of both, electronic and vibrational excitations, which involve the ground and the first excited electronic states, depending on the particular final state of the fragmentation. While our results for ionic states are consistent with the recent observations by Erattupuzha et al. [J. Chem. Phys.144, 024306 (2016)], the neutral state contribution was not observed there, which we attribute to a larger bandwidth of the 8 fs pulses we used for this experiment. In a complementary measurement employing longer, 35 fs pulses in a 30 ps delay range, we study the influence of rotational excitation on our observables, and demonstrate how the coherent electronic wave packet created in the ground electronic state of the ion completely decays within 10 ps due to the coupling to rotational motion.

In order to probe molecular dynamics at few-femtosecond resolution we use laser-induced electron diffraction imaging driven by mid-IR electric waveforms. Combined with coincidence detection we can extract structural information of fragmentation pathways of acetylene.

X-ray free-electron lasers provide unique opportunities for exploring ultrafast dynamics and for imaging the structures of complex systems. Understanding the response of individual atoms to intense X-rays is essential for most free-electron laser applications. First experiments have shown that, for light atoms, the dominant interaction mechanism is ionization by sequential electron ejection, where the highest charge state produced is defined by the last ionic state that can be ionized with one photon. Here, we report an unprecedentedly high degree of ionization of xenon atoms by 1.5 keV free-electron laser pulses to charge states with ionization energies far exceeding the photon energy. Comparing ion charge-state distributions and fluorescence spectra with state-of-the-art calculations, we find that these surprisingly high charge states are created via excitation of transient resonances in highly charged ions, and predict resonance enhanced absorption to be a general phenomenon in the interaction of intense X-rays with systems containing high-Z constituents. Researchers create high ionization states, up to Xe36+, using 1.5 keV free-electron laser pulses. The higher than expected ionization may be due to transient resonance-enhanced absorption and the effect may play an important role in interactions of intense X-rays with high-Z elements and radiation damage.

Free-electron lasers are fourth-generation light sources that deliver extremely intense (>1012 photons per pulse), ultrashort (∼10−14 s = 10 fs) light pulses at up to kilohertz repetition rates with unprecedented coherence properties and span a broad wavelength regime from soft (∼10 eV) to hard X-ray energies (∼15 keV). They thus enable a whole suite of novel experiments in molecular physics and chemistry: Inspecting radiation-induced reactions in cold molecular ions provides unprecedented insight into the photochemistry of interstellar clouds and upper planetary atmospheres; double core-hole photoelectron spectroscopy offers enhanced sensitivity for chemical analysis; the dynamics of highly excited molecular states, pumped by vacuum ultraviolet pulses, can be inspected; and vacuum ultraviolet or X-ray probe pulses generally hold the promise to trace chemical reactions along an entire reaction coordinate with atomic spatial and temporal resolution. This review intends to provide a first overview on upcoming possibilities, emerging technologies, pioneering results, and future perspectives in this exciting field.

Single ionization of simple molecules, e.g. H2, CO2, by fast charged particle impact has been studied using a reaction microscope. By measuring the momenta of the emitted electron and the recoil ionic fragment in coincidence, channel-selective low-energy electron spectra have been recorded. The experimental cross sections will be presented, compared with the predictions of state-of-the-art CDW-EIS calculations and discussed in terms of molecular effects such as (i) autoionization and predissociation channels, (ii) interference patterns resulting from the two-center geometry of the diatomic molecule, in analogy to Young's double-slit experiment and (iii) dependence of the electron emission on the orientation of the molecular axis.

Very sharp, low-energy structures observed in photoelectron spectra reveal the population of autoionizing states in krypton and argon in strong laser fields over a large range of different wavelengths. The energies of the electrons, emitted by autoionization in a field-free environment, provide direct information about the spectrum of states involved. Despite their ability to resist ionization by the populating laser pulse, we demonstrate the possibility to promote the excited electrons into the continuum by subsequent absorption of a single photon. Thus, applying a classical pump-probe scheme, we are able to manipulate the autoionization contribution on a picosecond time scale. Different scenarios for the creation of autoionizing states in strong laser fields are discussed.

Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work, we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n = 2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants.

We present experimental and theoretical fully differential cross sections for the single ionization of He by heavy-ion impact for electrons emitted into the scattering plane. Data were obtained for 2 MeV amu−1 C6+ and 3.6 MeV amu−1 AuQ+ (Q = 24, 53) projectiles, corresponding to perturbations (projectile charge to velocity ratio) ranging from 0.7 to 4.4, a regime which is inaccessible for electron-impact ionization. We observe a decreasing recoil peak intensity (relative to the binary peak) and at the same time an increasing peak in the forward direction with increasing perturbations. Large discrepancies between the experimental data and theoretical predictions are found, which can at least be partly attributed to the use of hydrogenic wavefunctions.

The ionization and fragmentation dynamics of iodine molecules (I2) are traced using very intense (∼1014 W cm−2) ultra-short (∼60 fs) light pulses with 87 eV photons of the Free-electron LASer at Hamburg (FLASH) in combination with a synchronized femtosecond optical laser. Within a pump–probe scheme the IR pulse initiates a molecular fragmentation and then, after an adjustable time delay, the system is exposed to an intense FEL pulse. This way we follow the creation of highly-charged molecular fragments as a function of time, and probe the dynamics of multi-photon absorption during the transition from a molecule to individual atoms.

Laser-induced tunnel ionization from a coherent superposition of electronic states in Ar+ is studied in a kinematically complete experiment. Within a pump-probe scheme a spin-orbit wave packet is launched through the first ionization step from the neutral species. The multielectron coherent wave packet is probed as a function of time by the second pulse which ionizes the system to Ar++. By measuring delay-dependent electron momentum distributions we directly image the evolution of the nonstationary multielectron wave function. Comparing the results with simulations we test common assumptions about electron momentum distributions and the tunneling process itself.

Molecular two-center interferences in a collision induced excitation of ${\mathrm{H}}_{2}^{+}$ projectile ions, with simultaneous ionization of helium target atoms, are investigated in a kinematically complete experiment. In the process under investigation, the helium atom is singly ionized and simultaneously the molecular hydrogen ion is dissociated. Different collision mechanisms are identified and interference fringes emerging from a correlated first-order mechanism and from an independent second-order process are observed.

We present accurate extraction of multiple bond lengths for aligned and anti-aligned C2H2 using recollision electron diffraction. We measure bond lengths of1.20±0.06A (C≡C) and 1.06±0.08A (C-H).

We study electron-electron correlation effects in double ionization of He and Ne by 3.6 MeV/amu Au53+ impact by analysing the correlation function for back-to-back emission of electrons with equal energy. For this particular emission pattern we find the correlation function to be sensitive to the target while without selection of this pattern no significant dependence on the initial state is observed. Therefore, this method may open the possibility to sensitively map correlated many-electron bound states.

Single and multiple ionization of neon and argon atoms by 3.6 MeV/u Au{sup 53+} impact has been explored in kinematically complete experiments. Doubly differential cross sections for low-energy electron emission have been obtained for a defined charge state of the recoiling target ion and the receding projectile. Observed target specific structures in the electron continuum are attributable to the nodal structure of the initial bound state momentum distribution. The experimental data are in excellent accord with continuum-distorted-wave eikonal-initial-state single ionization calculations if multiple ionization is considered appropriately. (c) 1999 The American Physical Society.

We have measured and calculated fully differential cross sections for target ionization in 16-MeV O${}^{7+}$+He and 24-MeV O${}^{8+}$+Li collisions. As in previous studies, in the case of the He target we observe a pronounced forward shift in the angular distribution of the electrons relative to the direction of the momentum transfer q at small $q$ ($q$ 1 a.u.). An unexpected result is that we also find a strong forward shift at large $q$ ($q$ g 2 a.u.), while at intermediate $q$ this shift becomes very weak or even turns into a backward shift. For the Li target, in contrast, the forward shift monotonically increases with increasing $q$. These observations are qualitatively reproduced by our calculations. The comparison to theory suggests that at large $q$ the forward shift is due to the postcollision interaction between the outgoing projectile and the ejected electron, but at small $q$ it is mostly due to an interplay between the projectile-target core interaction and the electron-target core interaction.

We have measured the double differential cross sections (DDCS) ({ital d}{sup 2}{sigma}/{ital d}{var_epsilon}{sub {ital ed}}{Omega}{sub {ital e}}) of low-energy electron emission in the ionization of H{sub 2} bombarded by bare carbon ions of energy 30 MeV. The energy and angular distributions of the electron DDCS have been obtained for 12 different emission angles and for electron energies varying between 0.1 and 300 eV. We have also deduced the single differential and total ionization cross section from the measured DDCS. The data have been compared with the predictions of first Born approximations and the CDW-EIS (continuum distorted wave{endash}eikonal initial state) model. The CDW-EIS model provides an excellent agreement with the data. {copyright} {ital 1996 The American Physical Society.}

We demonstrate that the projectile coherence can have a major impact on atomic fragmentation processes. This has been overlooked for decades in formal scattering theory and may explain puzzling discrepancies between theoretical and experimental fully differential cross sections for single ionization.

We study ionization of molecules by an intense laser field over a broad wavelength regime, ranging from 0.8 to 1.5 μm experimentally and from 0.6 to 10 μm theoretically. A reaction microscope is combined with an optical parametric amplifier to achieve ionization yields in the near-infrared wavelength regime. Calculations are done using the strong-field S-matrix theory and agreement is found between experiment and theory, showing that ionization of many molecules is suppressed compared to the ionization of atoms with identical ionization potentials at near-infrared wavelengths at around 0.8 μm, but not at longest wavelengths (10 μm). This is due to interference effects in the electron emission that are effective at low photoelectron energies but tend to average out at higher energies. We observe the transition between suppression and nonsuppression of molecular ionization in the near-infrared wavelength regime (1–5 μm).

We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle (~10 fs) laser pulses. We find that less than 10 dopant atoms "ignite" the droplet to generate a non-spherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs.

In a recent article, McGovern et al. [Phys. Rev. A 81, 042704 (2010)] suggested that the normalization of our measured fully differential cross section for ionization of helium by Au{sup 53+} needs to be checked. In this comment we confirm that the normalization of the published data is correct. Furthermore, we point out that, for a conclusive comparison between experiment and theory, an accurate inclusion of the experimental resolution using correct experimental parameters in the calculation is important.

We report on first proof-of-principles results on non-sequential double ionization of argon and neon achieved by using a newly developed long-cavity Ti:sapphire femtosecond oscillator with a pulse duration of 45 fs and a repetition of 6.2 MHz combined with a dedicated reaction microscope. Under optimized experimental conditions, peak intensities larger than 2.310(14) W/cm(2) have been achieved. Ion momentum distributions were recorded for both rare gases and show significantly different features for single as well as for double ionization. For single ionization of neon a spike of zero-momentum electrons is found when decreasing the laser intensity towards the lowest ionization rate we can measure which is attributed to a non-resonant ionization channel. As to double ionization, the longitudinal momentum distribution for Ne(2+) displays a clear double-hump structure whereas this feature is found to be smoothened out with a maximum at zero momentum for Ar(2+).

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of ${\mathrm{Ne}}^{2+}$ production reveals the dominance of nonsequential two-photon double ionization at intensities of $Il6\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ and significant contributions of three-photon ionization as $I$ increases. ${\mathrm{Ne}}^{2+}$ recoil-ion-momentum distributions suggest that two electrons absorbing ``instantaneously'' two photons are ejected most likely into opposite hemispheres with similar energies.

Citation: Corlin, P., Fischer, A., Schonwald, M., Sperl, A., Mizuno, T., Thumm, U., . . . Moshammer, R. (2015). Probing O+ 2 potential curves with an XUV-IR pump-probe experiment. 635(11). doi:10.1088/1742-6596/635/11/112060

We experimentally investigated the asymmetry of photo-ion and photoelectron momentum spectra of argon and nitrogen by ionizing an argon-nitrogen gas mixture in a strong two-color (400 nm + 800 nm) laser field. By changing the time delay between the 400 nm laser pulse and the 800 nm pulse, the fan-like stripes in low-energy electron momentum shift from negative momentum to positive for both argon and nitrogen. A clear difference of asymmetric electron emission between Ar and N2 is observed.

We present three-dimensional fully differential cross sections for single ionization of helium by ion impact at very small and very large perturbations. Clear signatures of the projectile–target nucleus interaction are identified; it manifests itself, however, through qualitatively different features in the cross sections depending on the perturbation.

We study helium single ionization with emission of low-energy $({\ensuremath{\varepsilon}}_{k}l10\mathrm{eV})$ electrons in collisions with $2\ensuremath{-}\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}$ ${\mathrm{C}}^{6+}$ ions. We explore, both experimentally and theoretically, longitudinal and transverse momentum distributions of the final reaction products. We present in-depth discussion of mechanisms resulting in the forward-backward asymmetry in the longitudinal spectra of emitted electrons and recoil ions. By comparing our experimental data and calculations we display clear signatures of the interaction between the projectile and the target core (the $n\ensuremath{-}n$ interaction) for the transverse distributions of the recoil ion and projectile. For the collision system in question the $n\ensuremath{-}n$ interaction is shown to represent an important mechanism of the momentum exchange.

To understand the physical processes that occur in nature we need to obtain a solid concept about the ‘fundamental’ forces acting between pairs of elementary particles. It is also necessary to describe the temporal and spatial evolution of many mutually interacting particles under the influence of these forces. This latter step, known as the few-body problem, remains an important unsolved problem in physics. Experiments involving atomic collisions represent a useful testing ground for studying the few-body problem. For the single ionization of a helium atom by charged particle impact, kinematically complete experiments have been performed1,2,3,4,5,6 since 1969 (ref. 7). The theoretical analysis of such experiments was thought to yield a complete picture of the basic features of the collision process, at least for large collision energies8,9,10,11,12,13,14. These conclusions are, however, almost exclusively based on studies of restricted electron-emission geometries1,2,3. Here, we report three-dimensional images of the complete electron emission pattern for the single ionization of helium by the impact of C6+ ions of energy 100 MeV per a.m.u. (a four-body system) and observe features that have not been predicted by any published theoretical model. We propose a higher-order ionization mechanism, involving the interaction between the projectile and the target nucleus, to explain these features.

We have measured fully differential cross sections for ionization of He by 3 MeV proton impact. A projectile beam with a coherence length larger than typical atomic scales was prepared at the Test Storage Ring taking advantage of electron cooling. Pronounced effects due to the projectile coherence are observed which are absent for an incoherent 100 MeV u−1 C6 + projectile beam and the present data are in much better agreement with theory. Thus, these results represent a major advancement in resolving the so far unexplained discrepancies between experiment and theory for the 100 MeV u−1 C6 + case.

Sequential and direct two-photon double ionization (DI) of D2 molecule is studied experimentally and theoretically at a photon energy of 38.8 eV. Experimental and theoretical kinetic energy releases of D++D+ fragments, consisting of the contributions of sequential DI via the D2+(1ssg) state and direct DI via a virtual state, agree well with each other.

Multiple ionization (MI), induced by a few EUV photons at energies of 28.2 eV, 38 eV and 44 eV from FLASH (the free-electron laser at Hamburg), has been studied for atoms (He, Ne, Ar) and N2 molecules utilizing our multi-hit coincident technology?the reaction microscope. At comparably low intensities of I 1011?1013 W cm?2 we find the non-sequential (NS) MI mechanism dominating Ar3+ and Ar4+ production. Inspecting recoil ion and electron momentum distributions evidence is provided (i) for preferential back-to-back emission of electrons for NS double ionization of He and (ii) for angular entanglement between two outgoing electrons in sequential ionization (SI). In contrast to atoms, SI is observed to be most effective for MI of N2 molecules at an intensity of ~1013 W cm?2 leading, among others, to N2+2 ? N+ + N+, N3+2 ? N2+ + N+, N4+2 ? N2+ + N2+ Coulomb explosion channels. Fragment ion momentum distributions are investigated and are demonstrated to allow tracing SI pathways.

In strong laser fields, sub-femtosecond control of chemical reactions with the carrier-envelope phase (CEP) becomes feasible. We have studied the control of reaction dynamics of acetylene and allene in intense few-cycle laser pulses at 750 nm, where ionic fragments are recorded with a reaction microscope. We find that by varying the CEP and intensity of the laser pulses it is possible to steer the motion of protons in the molecular dications, enabling control over deprotonation and isomerization reactions. The experimental results are compared to predictions from a quantum dynamical model, where the control is based on the manipulation of the phases of a vibrational wave packet by the laser waveform. The measured intensity dependence in the CEP-controlled deprotonation of acetylene is well captured by the model. In the case of the isomerization of acetylene, however, we find differences in the intensity dependence between experiment and theory. For the isomerization of allene, an inversion of the CEP-dependent asymmetry is observed when the intensity is varied, which we discuss in light of the quantum dynamical model. The inversion of the asymmetry is found to be consistent with a transition from non-sequential to sequential double ionization., Comment: 15 pages, 8 figures

We study ionization of molecules and atoms with same IP by intense laser field from 0.6-10 μm. A trend from ionization-suppression to non-suppression is found for many molecules as a function of wavelength.

We apply laser-induced electron diffraction imaging driven by mid-IR electric waveforms towards probing molecular dynamics at few-femtosecond temporal resolutions. Combined with coincidence detection we can extract structural information of fragmentation pathways of acetylene.

We present the simultaneous measurement of the C-C and C-H bond lengths of aligned acetylene. Our approach combines an ultrafast 160 kHz mid-IR source with 3D electron-ion coincidence detection towards imaging of molecular dynamics.