We have conducted an experimental study on the photo double ionization (PDI) of carbon dioxide dimers and oxygen dimers, while focusing on the dissociation dynamics upon single photon absorption. The results in terms of the kinetic energy and angular distributions of the charged particles show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon dioxide dimers. In the oxygen dimer, the results show that ICD is accompanied by knock-off ionization mechanisms.

When a strong laser pulse induces the ionization of an atom, momentum conservation dictates that the absorbed photons transfer their momentum to the electron and its parent ion. The sharing of the photon momentum between the two particles and its underlying mechanism in strong-field ionization, occurring when the bound electron tunnels through the barrier created by the superposition of the atomic potential and the electric laser field, are still debated in theory1–4 after 30 years of research. Corresponding experiments are very challenging due to the extremely small photon momentum and their precision has been too limited, so far, to ultimately resolve this debate5–8. By utilizing an experimental approach relying on two counter-propagating laser pulses, we present a detailed study of the effects of the photon momentum in strong-field ionization. The high precision of the method and the intrinsically known zero momentum allow us to unambiguously demonstrate the action of the light's magnetic field on the electron while it is under the tunnel barrier, which has only been theoretically predicted so far1–3,9, thereby disproving opposing predictions5,10,11. Our results deepen the understanding of, for example, molecular imaging12,13 and time-resolved photoelectron holography14. Experiments with two counter-propagating laser beams report the observation that the photon momentum is shared between the electron and parent ion in strong-field ionization, which results from the photon's magnetic field acting on the electron.

We have conducted an experimental study on the photo double ionization (PDI) of carbon-dioxide dimers at photon energies of 37 and 55 eV and oxygen dimers at photon energies of 38, 41.5, and 46 eV, while focusing on the dissociation dynamics upon single-photon absorption. The investigation was performed by applying the cold-target recoil-ion momentum spectroscopy method in order to collect and record the three-dimensional momenta of the ionic fragments and emitted electrons from the dissociating dimer in coincidence. The kinetic-energy release upon fragmentation and the electron angular distributions in the laboratory and body-fixed frames, as well as the relative electron-electron emission angle, show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon-dioxide dimers upon photoionization below and above the double-ionization threshold of CO2 monomers. The PDI of oxygen dimers is less conclusive and shows contributions from ICD and knock-off ionization mechanisms. As for atomic dimers, the present results reveal that ICD in CO2 dimers after valence PDI can also serve as a source for low-energy electrons, known to be very relevant in biological systems, cells, and tissues.

We present a joint experimental and theoretical study of resonant interatomic Coulombic decay (RICD) in HeNe employing high resolution cold target recoil ion momentum spectroscopy and ab initio electronic structure and nuclear dynamics calculations. In particular, laboratory- and molecular-frame angular emission distributions of RICD electrons are examined in detail. The exciting-photon energy-dependent anisotropy parameter beta(omega), measured for decay events that populate bound HeNe+ ions, is in agreement with the calculations performed for the ground ionic state X-2 Sigma(+)(1/2). A contribution from the a(2)Pi(3/2) final ionic state is found to be negligible. For the He + Ne + fragmentation channel, the observed laboratory-frame angular distribution of RICD electrons is explained by a slow homogeneous dissociation of bound vibrational levels of the final ionic state Lambda(2)Pi(1/2) into vibrational continua of the lower lying states X-2 Sigma(+)(1/2) and a(2)Pi(3/2). Our calculations predict that the angular distributions of RICD electrons in the body-fixed dipole plane provide direct access to the electronic character (i.e., symmetry) of intermediate vibronic resonances. However, because of the very slow dissociation of the Lambda(2)Pi(1/2) state, the molecular-frame angular distributions of RICD electrons in the He + Ne+ fragmentation channel are inaccessible to our coincidence experiment.

We report on a kinematically complete experiment on strong-field double ionization of helium using laser pulses with a wavelength of 394 nm and intensities of 3.5-5.7 x 10(14) W/cm(2). Our experiment reaches the most complete level of detail, which previously has been reached only for single-photon double ionization. We give an overview of the observables on many levels of integration, from the ratio of double to single ionization, the individual electron, and ion momentum distributions over the joint momentum and energy distributions to fully differential cross sections showing the correlated angular momentum distributions. Within the studied intensity range the ratio of double to single ionization changes from 2 x 10(-4) to 1.5 x 10(-3). We find the momentum distributions of the He2+ ions and the correlated two electron momentum distributions to vary substantially. Only at the highest intensity are both electrons emitted in the same direction, while at the lowest intensity back-to-back emission dominates. The joint energy distribution of the electrons shows discrete structures from the energy quantization of the photon field, which allows us to count the number of absorbed photons and thus access the parity of the final state. We find the energy of the individual electron to show a peak structure indicating a quantized sharing of the overall energy absorbed from the field. The joint angular momentum distributions of the two electrons show the highly directed emission of both electrons along the polarization axis as well as clear imprints of electron repulsion. They strongly change with the energy sharing between electrons. The aspect of selection rules in double ionization which are also visible in the presented data set is the subject of a preceding publication [Henrichs et al., Phys. Rev. A 97, 031405(R) (2018)].

We report on the observation of phase space modulations in the correlated electron emission after strong field double ionization of helium using laser pulses with a wavelength of 394~nm and an intensity of $3\cdot10^{14}$W/cm$^2$. Those modulations are identified as direct results of quantum mechanical selection rules predicted by many theoretical calculations. They only occur for an odd number of absorbed photons. By that we attribute this effect to the parity of the continuum wave function.

We study the decay of a helium/neon dimer after ionization and simultaneous excitation of either the neon or the helium atom using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS). We find that, depending on the decaying state, either direct Interatomic Coulombic Decay (ICD) (i.e. mediated by a virtual photon exchange), exchange ICD (mediated by electron exchange) or radiative charge transfer occurs. The corresponding channels are identified.

In this work we present a comprehensive and detailed study of Interatomic Coulombic Decay (ICD) occurring after irradiating argon dimers with XUV-synchrotron radiation. A manifold of different decay channels is observed and the corresponding initial and final states are assigned. Additionally, the effect of nuclear dynamics on the ICD electron spectrum is examined for one specific decay channel. The internuclear distance-dependent width Γ(R) of the decay is obtained from the measured kinetic energy release distribution of the ions employing a classical nuclear dynamics model.

clusters M. Kircher∗1, F. Trinter∗, M. Weller∗, J. B. Williams†, N. Sisourat‡, S. Kazandjian‡, M. Waitz∗, C. Goihl∗, A. Hartung∗, H. Sann∗, A. Schottelius∗, G. Kastirke∗, M. Pitzer§, D. You¶, T. Deselaers∗, Y. Herrmann∗, M. Tia∗, M. Schöffler∗, R. Dörner∗, T. Jahnke∗2 ∗ Institut für Kernphysik, Goethe-Universität, D-60438 Frankfurt am Main, Germany † Department of Physics, University of Nevada, Reno, NV 89557, USA ‡ Sorbonne Université. UPMC University Paris 06, CNRS, Laboratoire de Chimie Physique Matière et Rayonnement, F-75005 Paris, France § Experimental Physics IV, University of Kassel, D-3413 Kassel, Germany ¶ Institute of Multidisciplinary Research for Advanced Materials, Tohoku Univ. (IMRAM), Sendai 980-8577, Japan

We experimentally study 2p photoionization of neon dimers (Ne_{2}) at a photon energy of hν=36.56  eV. By postselection of ionization events which lead to a dissociation into Ne^{+}+Ne we obtain the photoelectron angular emission distribution in the molecular frame. This distribution is symmetric with respect to the direction of the charged vs neutral fragment. It shows an inverted Cohen-Fano double slit interference pattern of two spherical waves emitted coherently but with opposite phases from the two atoms of the dimer.

We investigate the temporal evolution of molecular frame angular distributions of Auger electrons emitted during ultrafast dissociation of HCl following a resonant single-photon excitation. The electron emission pattern changes its shape from that of a molecular σ orbital to that of an atomic p state as the system evolves from a molecule into two separated atoms.

APS/123-QED Hydrogen and fluorine migration in photo-double-ionization of 1,1-difluoroethylene (1,1-C 2 H 2 F 2 ) near and above threshold B. Gaire, 1 I. Bocharova, 1 F. P. Sturm, 1, 2 N. Gehrken, 1, 2 J. Rist, 1, 2 H. Sann, 2 M. Kunitski, 2 J. Williams, 2 M. S. Sch¨ offler, 2 T. Jahnke, 2 B. Berry, 3 M. Zohrabi, 3 M. Keiling, 2, 4 A. Moradmand, A. L. Landers, 4 A. Belkacem, 1 R. D¨orner, 2 I. Ben-Itzhak, 3 and Th. Weber 1 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Institut f¨ ur Kernphysik, Goethe-Universit¨ at, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA Department of Physics, Auburn University, AL 36849, USA (Dated: February 26, 2014) We have studied the nondissociative and dissociative photo-double-ionization of 1,1- difluoroethylene using single photons of energies ranging from 40 to 70 eV. Applying a coincident electron-ion three-dimensional-momentum imaging technique, kinematically complete measurements have been achieved. We present the branching ratios of the six reaction channels identified in the experiment. Electron-ion energy maps and relative electron emission angles are used to distinguish between direct and indirect photo-double-ionization mechanisms at a few different photon energies. The influence of selection and propensity rules is discussed. Threshold energies of double-ionization are extracted from the sum of the kinetic energies of the electrons, which hint to the involvement of different manifolds of states. The dissociative ionization channels with two ionic fragments are explored in detail by measuring the kinetic energy release of the fragment ions, sum of the kinetic energies as well as the energy sharing of the two emitted electrons. We investigate the migration of hydrogen and fluorine atoms and compare the experimental results to the photo-double-ionization of centro-symmetric linear and planar hydrocarbons (C 2 H 2 and C 2 H 4 ) whenever possible. PACS numbers: 33.80.Eh, 33.90.+h I. INTRODUCTION Photo-double-ionization (PDI) is a process in which two electrons are removed from an atom or a molecular target with a single photon. Studies of PDI lead to a better understanding of the correlation between the elec- trons, the ionization mechanisms leading to the ejection of the two electrons, selection and propensity rules pro- hibiting transitions, and the molecular dynamics during the transition from the neutral ground state to the re- spective dication or the ionic fragments. The ionization to the dication states can occur either through a direct or an indirect process. In the direct process (sometimes also referred to as two-step-one, TS1) the two electrons are ejected simultaneously. In the indirect process (some- times also referred to as a sequential process) photoe- jection of one electron leads to an intermediate cation state, which later decays by autoionization or other pro- cesses (e.g. Auger decay, fluorescence etc.). After a cou- ple of decades of studying the PDI of simple atoms and diatomic molecules (e.g. He, H 2 , N 2 , CO) in great de- tail [1–9], the investigation has been extended to poly- atomic molecules in order to gain a general understand- ing of the double ionization process in more complex sys- tems. Simple hydrocarbon molecules are an ideal test bed for a series of studies with increasing complexity. Here we choose the 1,1-difluoroethylene (1,1-C 2 H 2 F 2 , H H >C= C C= H F C C= C< 2 2 2 F F H We expect differences in the PDI of the valance elec- trons of 1,1-C 2 H 2 F 2 as compared to C 2 H 4 . For exam- ple, the propensity rule proposed for the PDI of centro- symmetric molecules [9, 11, 12], which states that the triplet gerade and singlet ungerade electronic states of the dications are likely to be populated, is no longer valid for 1,1-C 2 H 2 F 2 . Hence, the nondissociative ionization (NDI) of these two species can be very different. In ad- dition, a variety of reaction channels can be expected in the dissociative ionization (DI) of 1,1-C 2 H 2 F 2 . For in- stance, in C 2 H 4 the migration of H atoms from one side of the C=C double bond to the other simply cannot be distinguished (at least in our experiments). In contrast, a migration of atoms from the opposite sides of the double bond in 1,1-C 2 H 2 F 2 leads to distinguishable conforma- tions of the molecule. The symmetric breakup channel of 1,1-C 2 H 2 F 2 leading to two CHF + fragment ions, on the other hand, is very complex since it would require multiple bond breaking and a subsequent rearrangement of the constituent atoms, which is obviously not the case for C 2 H 4 and C 2 H 2 due to their mirror symmetry. In the past Ibuki and coworkers explored the PDI of 1,1-C 2 H 2 F 2 and 1,1-C 2 H 2 D 2 in the photon energy range of 37-85 eV using a photoion-photoion coincidence (PIPICO) technique [13]. They measured the branching

We present an experimental investigation into the angular and energy distributions of electrons set free by the interaction of hydrogen molecular ions with strong laser fields. The results extend on those presented previously [M. Odenweller et al., Phys. Rev. Lett. 107, 143004 (2011)] for circularly polarized light. Pulses of laser light (lambda = 780 nm, I approximate to 6 x 10(14) W cm(-2), tau approximate to 40 fs) of both linear and circular polarization are focused onto an H-2(+) molecular ion beam. The momenta of the proton and electron fragments are determined in a coincidence experiment, enabling electron momentum emission patterns in the molecular frame to be deduced for specific values of internuclear distances.

We have measured the ionization and fragmentation of Helium, Neon and Argon Dimers induced by ion impact and observed two different pathways, the sequential ionization on each atom and the interatomic Coulombic decay.

, 1096 (2013); 341 Science et al. Martin Pitzer Phase by Coulomb Explosion Imaging Direct Determination of Absolute Molecular Stereochemistry in Gas This copy is for your personal, non-commercial use only. clicking here. colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to others here. following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles ): September 2, 2014 www.sciencemag.org (this information is current as of The following resources related to this article are available online at http://www.sciencemag.org/content/341/6150/1096.full.html version of this article at: including high-resolution figures, can be found in the online Updated information and services, http://www.sciencemag.org/content/suppl/2013/09/04/341.6150.1096.DC1.html can be found at: Supporting Online Material http://www.sciencemag.org/content/341/6150/1096.full.html#ref-list-1 , 1 of which can be accessed free: cites 22 articles This article http://www.sciencemag.org/content/341/6150/1096.full.html#related-urls 1 articles hosted by HighWire Press; see: cited by This article has been http://www.sciencemag.org/cgi/collection/chemistry Chemistry subject collections: This article appears in the following

We report on the observation of discrete structures in the electron energy distribution for strong field double ionization of argon at 394 nm. The experimental conditions were chosen in order to ensure a nonsequential ejection of both electrons with an intermediate rescattering step. We have found discrete above-threshold ionization like peaks in the sum energy of both electrons, as predicted by all quantum mechanical calculations. More surprisingly, however, is the observation of two above-threshold ionization combs in the energy distribution of the individual electrons.

Interatomic Coulombic Decay in helium dimers has been measured time-resolved for the first time.

Electron motion in chemical bonds occurs on an attosecond timescale. This ultrafast motion can be driven by strong laser fields. Ultrashort asymmetric laser pulses are known to direct electrons to a certain direction. But do symmetric laser pulses destroy symmetry in breaking chemical bonds? Here we answer this question in the affirmative by employing a two-particle coincidence technique to investigate the ionization and fragmentation of H 2 by a long circularly polarized multicycle femtosecond laser pulse. Angular streaking and the coincidence detection of electrons and ions are employed to recover the phase of the electric field, at the instant of ionization and in the molecular frame, revealing a phase-dependent anisotropy in the angular distribution of H + fragments. Our results show that electron localization and asymmetrical breaking of molecular bonds are ubiquitous, even in symmetric laser pulses. The technique we describe is robust and provides a powerful tool for ultrafast science.