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We report on a joint experimental and theoretical study of photoelectron circular dichroism (PECD) in methyloxirane. By detecting O 1s-photoelectrons in coincidence with fragment ions, we deduce the molecule's orientation and photoelectron emission direction in the laboratory frame. Thereby, we retrieve a fourfold differential PECD clearly beyond 50%. This strong chiral asymmetry is reproduced by ab initio electronic structure calculations. Providing such a pronounced contrast makes PECD of fixed-in-space chiral molecules an even more sensitive tool for chiral recognition in the gas phase., Comment: 5 figures

The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of trifluoromethyloxirane(TFMOx) is studied experimentally and theoretically for different photoelectron kinetic energies. The experiments were performed employing circularly polarized synchrotron radiation and coincidentelectron and fragment ion detection using Cold Target Recoil Ion Momentum Spectroscopy. The corresponding calculations were performed by means of the Single Center method within the relaxed-core Hartree-Fock approximation. We concentrate on the energy dependence of the differential PECD of uniaxially oriented TFMOx molecules, which is accessible through the employed coincident detection. We also compare results for differential PECD of TFMOx to those obtained for the equivalent fragmentation channel and similar photoelectron kinetic energy of methyloxirane (MOx), studied in our previous work. Thereby, we investigate the influence of the substitution of the methyl-group by the trifluoromethyl-group at the chiral center on the molecular chiral response. Finally, the presently obtained angular distribution parameters are compared to those available in literature., 6 figs

X-ray as well as electron diffraction are powerful tools for structure determination of molecules. Studies on randomly oriented molecules in the gas-phase address cases in which molecular crystals cannot be generated or the interaction-free molecular structure is to be addressed. Such studies usually yield partial geometrical information, such as interatomic distances. Here, we present a complementary approach, which allows obtaining insight to the structure, handedness and even detailed geometrical features of molecules in the gas phase. Our approach combines Coulomb explosion imaging, the information that is encoded in the molecular frame diffraction pattern of core-shell photoelectrons and ab initio computations. Using a loop-like analysis scheme we are able to deduce specific molecular coordinates with sensitivity even to the handedness of chiral molecules and the positions of individual atoms, as, e.g., protons., Comment: 13 pages, 3 figures

We report on a multiparticle coincidence experiment performed at the European X-ray Free-Electron Laser at the Small Quantum Systems instrument using a COLTRIMS reaction microscope. By measuring two ions and two electrons in coincidence, we investigate double core-hole generation in O_{2} molecules in the gas phase. Single-site and two-site double core holes have been identified and their molecular-frame electron angular distributions have been obtained for a breakup of the oxygen molecule into two doubly charged ions. The measured distributions are compared to results of calculations performed within the frozen- and relaxed-core Hartree-Fock approximations.

Author(s): Larsen, KA; Rescigno, TN; Severt, T; Streeter, ZL; Iskandar, W; Heck, S; Gatton, A; Champenois, EG; Strom, R; Jochim, B; Reedy, D; Call, D; Moshammer, R; Dorner, R; Landers, AL; Williams, JB; McCurdy, CW; Lucchese, RR; Ben-Itzhak, I; Slaughter, DS; Weber, T | Abstract: We report measurements on the H++H+ fragmentation channel following direct single-photon double ionization of neutral NH3 at 61.5 eV, where the two photoelectrons and two protons are measured in coincidence using three-dimensional (3D) momentum imaging. We identify four dication electronic states that contribute to H++H+ dissociation, based on our multireference configuration-interaction calculations of the dication potential energy surfaces. The extracted branching ratios between these four dication electronic states are presented. Of the four dication electronic states, three dissociate in a concerted process, while the fourth undergoes a sequential fragmentation mechanism. We find evidence that the neutral NH fragment or intermediate NH+ ion is markedly rovibrationally excited. We also identify differences in the relative emission angle between the two photoelectrons as a function of their energy sharing for the four different dication states, which bare some similarities to previous observations made on atomic targets.

Physical review research 2(3), 1-7 (2020). doi:10.1103/PhysRevResearch.2.033080, We report on kinematically complete measurements and ab initio nonperturbative calculations of double ionization of He and H2 by a single 800 eV circularly polarized photon. We confirm the quasifree mechanism of photoionization for H2 and show how it originates from the two-electron cusp in the ground state of a two-electron target. Our approach establishes a method for mapping electrons relative to each other and provides valuable insight into photoionization beyond the electric-dipole approximation., Published by APS, College Park, MD

We present state-selective measurements on the NH$_2^{+}$ + H$^{+}$ and NH$^{+}$ + H$^{+}$ + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH$_{3}$, where the two photoelectrons and two cations are measured in coincidence using 3-D momentum imaging. Three dication electronic states are identified to contribute to the NH$_2^{+}$ + H$^{+}$ dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold NH$_2^+$ fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited NH$_2^+$ fragment with roughly 1 eV of internal energy. The NH$^{+}$ + H$^{+}$ + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the NH$_2^{+}$ + H$^{+}$ channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states., Comment: 18 pages, 21 figures, 3 tables

We report experimental results on three-dimensional momentum imaging measurements of anions generated via dissociative electron attachment to gaseous formamide. From the momentum images, we analyze the angular and kinetic energy distributions for NH$_2^{-}$, O$^{-}$, and H$^{-}$ fragments and discuss the possible electron attachment and dissociation mechanisms for multiple resonances for two ranges of incident electron energies, from 5.3~eV to 6.8~eV, and from 10.0~eV to 11.5~eV. {\it Ab initio} theoretical results for the angular distributions of the NH$_2^{-}$ anion for $\sim$6~eV incident electrons, when compared with the experimental results, strongly suggest that one of the two resonances producing this fragment is a $^2$A$''$ Feshbach resonance., Comment: 10 pages, 6 figures

Physical review research 2(3), 033209 (2020). doi:10.1103/PhysRevResearch.2.033209, We show that the combination of two achiral components—an atomic or molecular target plus a circularly polarized photon—can yield chirally structured photoelectron angular distributions. For photoionization of CO, the angular distribution of carbon $K$-shell photoelectrons is chiral when the molecular axis is neither perpendicular nor (anti)parallel to the light propagation axis. In photo-double-ionization of He, the distribution of one electron is chiral if the other electron is oriented like the molecular axis in the former case and if the electrons are distinguishable by their energy. In both scenarios, the circularly polarized photon defines a plane with a sense of rotation and an additional axis is defined by the CO molecule or one electron. This is sufficient to establish an unambiguous coordinate frame of well-defined handedness. To produce a chirally structured electron angular distribution, such a coordinate frame is necessary but not sufficient. We show that additional electron-electron interaction or scattering processes are needed to create the chiral angular distribution., Published by APS, College Park, MD

Physical review / X Expanding access 10(2), 021052 (2020). doi:10.1103/PhysRevX.10.021052, A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved single-molecule imaging with atomic resolution. Here, we show that x-ray photoelectron diffraction—where a photoelectron emitted after x-ray absorption illuminates the molecular structure from within—can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O$_2$ molecule. By measuring the molecular-frame photoelectron emission patterns for a two-photon sequential $K$-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward time-resolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers., Published by APS, College Park, Md.

We present a combined experimental and theoretical investigation of the electron dynamics and body-frame angular dependence of valence photo-single ionization of CF4 and subsequent dissociation into CF3+ and F. Ionization from a valence t2 orbital shows overlapping shape resonances close to threshold that couple to the same total symmetry, leading to striking changes in the photoelectron angular distributions when viewed in the body-frame.

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.

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.

Author(s): Heck, S; Gatton, A; Larsen, KA; Iskandar, W; Champenois, EG; Strom, R; Landers, A; Reedy, D; Dailey, C; Williams, JB; Severt, T; Jochim, B; Ben-Itzhak, I; Moshammer, R; Dorner, R; Slaughter, DS; Weber, T | Abstract: We present an experimental investigation of symmetry breaking of H2 and D2 molecules after single photoionization due to the Coulomb field of the emitted slow electron interacting with the parent cation during dissociation. The experiments were carried out by measuring the three-dimensional momentum vectors of the photoelectron and recoiling ion in coincidence using a reaction microscope. For photon energies close to threshold, the low-energy photoelectron influences the dissociation process, which results in an asymmetric molecular frame photoelectron angular distribution. This can be explained by the retroaction of the Coulomb field of the photoelectron on its parent ion and has been recently experimentally demonstrated by M. Waitz et al. [Phys. Rev. Lett. 116, 043001 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.043001], confirming theoretical predictions by V. V. Serov and A. S. Kheifets [Phys. Rev. A 89, 031402(R) (2014)PLRAAN1050-294710.1103/PhysRevA.89.031402]. High-momentum resolution and a new series of photon energies just above the dissociation threshold enable the observation of a strong influence of the electron energy and nuclear kinetic energy on the electron localization process for energies below ∼100 meV, which so far has neither been observed nor discussed by theory. Exploring the limitations of the retroaction mechanism at our lowest photon energy, we are able to single out a sensitive testbed and present data of non-Born-Oppenheimer dynamics of the simplest molecular system for future benchmark computational treatments.

Author(s): Iskandar, W; Gatton, AS; Gaire, B; Sturm, FP; Larsen, KA; Champenois, EG; Shivaram, N; Moradmand, A; Williams, JB; Berry, B; Severt, T; Ben-Itzhak, I; Metz, D; Sann, H; Weller, M; Schoeffler, M; Jahnke, T; Dorner, R; Slaughter, D; Weber, T | Abstract: 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.

Proceedings of the National Academy of Sciences of the United States of America 113(51), 14651 – 14655 (2016). doi:10.1073/pnas.1610688113, Quantum tunneling is a ubiquitous phenomenon in nature and crucial for many technological applications. It allows quantum particles to reach regions in space which are energetically not accessible according to classical mechanics. In this “tunneling region,” the particle density is known to decay exponentially. This behavior is universal across all energy scales from nuclear physics to chemistry and solid state systems. Although typically only a small fraction of a particle wavefunction extends into the tunneling region, we present here an extreme quantum system: a gigantic molecule consisting of two helium atoms, with an 80% probability that its two nuclei will be found in this classical forbidden region. This circumstance allows us to directly image the exponentially decaying density of a tunneling particle, which we achieved for over two orders of magnitude. Imaging a tunneling particle shows one of the few features of our world that is truly universal: the probability to find one of the constituents of bound matter far away is never zero but decreases exponentially. The results were obtained by Coulomb explosion imaging using a free electron laser and furthermore yielded He2’s binding energy of 151.9 +-13.3 neV, which is in agreement with most recent calculations., Published by National Acad. of Sciences, Washington, DC

We report recent results of mass-resolved anion fragment momentum imaging experiments to investigate dissociative electron attachment to formic acid, for incident energies between 5 eV and 9 eV. A remarkable site-selectivity is found for a resonance at 8.5 eV by comparing anion fragment yields for two deuterated isotopologues of formic acid. This results in an H− fragment from the O-H bond of the transient anion, with negligible contribution from C-H break. In contrast, a lower-energy resonance at 7.1 eV dissociates by C–H or O–H break to produce H− and the neutral radicals HOCO or HCOO.

Author(s): Reedy, D; Williams, JB; Gaire, B; Gatton, A; Weller, M; Menssen, A; Bauer, T; Henrichs, K; Burzynski, P; Berry, B; Streeter, ZL; Sartor, J; Ben-Itzhak, I; Jahnke, T; Dorner, R; Weber, T; Landers, AL | Abstract: We demonstrate the use of cold target recoil ion momentum spectroscopy to perform state-selective measurements of the dissociative channels following single-photon double-ionization of H2O. The two dominant dissociation channels observed lead to two-body (OH++H++2e-) and three-body (2H++O+2e-) ionic fragmentation channels. In the two-body case we observe the presence of an autoionization process with a double-differential cross section that is similar to the single-photon double-ionization of helium well above threshold. In the three-body case, momentum and energy correlation maps in conjunction with new classical trajectory calculations in the companion theory paper by Z. L. Streeter et al. [Phys. Rev. A 98, 053429 (2018)10.1103/PhysRevA.98.053429] lead to the determination of the eight populated dication states and their associated fragmentation geometry. For the latter case, state-specific relative cross sections, median kinetic energy releases, and median angles between asymptotic proton momenta are presented. This benchmark-level experiment demonstrates that, in principle, state-selective fixed-frame triple-differential cross sections can be measured for some dication states of the water molecule.

International audience; Almost ten years ago, energetic neutral hydrogen atoms were detected after a strong-field double ionization of H2. This process, called 'frustrated tunneling ionization', occurs when an ionized electron is recaptured after being driven back to its parent ion by the electric field of a femtosecond laser. In the present study we demonstrate that a related process naturally occurs in clusters without the need of an external field: we observe a charge hopping that occurs during a Coulomb explosion of a small helium cluster, which leads to an energetic neutral helium atom. This claim is supported by theoretical evidence. As an analog to 'frustrated tunneling ionization', we term this process 'frustrated Coulomb explosion'.

Journal of electron spectroscopy and related phenomena 204, 237 - 244 (2015). doi:10.1016/j.elspec.2015.06.012, Interatomic Coulombic decay (ICD) is a decay mechanism occurring in loosely bound matter, e.g. in systems bound by van der Waals-forces or hydrogen bonds. In many such cases the decay time is similar to the time scale of nuclear motion during the decay. As the efficiency of ICD strongly depends on the internuclear distance of the atoms or molecules involved in the decay, an overall non-trivial temporal decay behavior arises. The progress of examining the time-domain aspects of interatomic Coulombic decay is summarized in this short topical review with a special emphasis on experiments that are now feasible due to the developments of free-electron lasers., Published by Elsevier, New York, NY [u.a.]

Citation: McCurdy, C. W., Rescigno, T. N., Trevisan, C. S., Lucchese, R. R., Gaire, B., Menssen, A., . . . Weber, T. (2017). Unambiguous observation of F-atom core-hole localization in CF4 through body-frame photoelectron angular distributions. Physical Review A, 95(1). doi:10.1103/PhysRevA.95.011401

We have investigated interatomic Coulombic decay (ICD) after resonant Auger decay in Ar2, ArKr, and ArXe following 2p3/2 → 4s and 2p3/2 → 3d excitations in Ar, using momentum-resolved electron-ion-ion coincidence. The results illustrate that ICD induced by the resonant Auger decay is a well-controlled way of producing energy-selected slow electrons at a specific site.

Even though the study of ion-atom collisions is a mature field of atomic physics, large discrepancies between experiment and theoretical calculations are still common. Here we present experimental results with high momentum resolution on the single ionization of helium induced by 1-MeV protons, and we compare these to theoretical calculations. The overall agreement is strikingly good, and even the first Born approximation yields good agreement between theory and experiment. This has been expected for several decades, but so far has not been accomplished. The influence of projectile coherence effects on the measured data is briefly discussed in terms of an ongoing dispute on the existence of nodal structures in the electron angular emission distributions.

We investigate the dissociation of H_{2}^{+} into a proton and a H^{0} after single ionization with photons of an energy close to the threshold. We find that the p^{+} and the H^{0} do not emerge symmetrically in the case of the H_{2}^{+} dissociating along the 1sσ_{g} ground state. Instead, a preference for the ejection of the p^{+} in the direction of the escaping photoelectron can be observed. This symmetry breaking is strongest for very small electron energies. Our experiment is consistent with a recent prediction by Serov and Kheifets [Phys. Rev. A 89, 031402 (2014)]. In their model, which treats the photoelectron classically, the symmetry breaking is induced by the retroaction of the long-range Coulomb potential onto the dissociating H_{2}^{+}.

We investigate the photodouble ionization of H2 molecules with 400 eV photons. We find that the emitted electrons do not show any sign of two-center interference fringes in their angular emission distributions if considered separately. In contrast, the quasiparticle consisting of both electrons (i.e., the "dielectron") does. The work highlights the fact that nonlocal effects are embedded everywhere in nature where many-particle processes are involved, This work was funded by the Deutsche Forschungsgemeinschaft (DFG), the BMBF, the European COST Action Grant No. XLIC CM1204, the European Research Council Advanced XCHEM Grant No. 290853, and the MINECO Project Grant No. FIS2013-42002-R. J. L. thanks the DFG for support. We are grateful to the staff of PETRA III for excellent support during the beam time. Raw data are archived at the Goethe-University Frankfurt am Main and are available on request

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction—an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Nature Communications 9(1), 2259 (2018). doi:10.1038/s41467-018-04740-5, The toolbox for imaging molecules is well-equipped today. Some techniques visualize the geometrical structure, others the electron density or electron orbitals. Molecules are many-body systems for which the correlation between the constituents is decisive and the spatial and the momentum distribution of one electron depends on those of the other electrons and the nuclei. Such correlations have escaped direct observation by imaging techniques so far. Here, we implement an imaging scheme which visualizes correlations between electrons by coincident detection of the reaction fragments after high energy photofragmentation. With this technique, we examine the H2 two-electron wave function in which electron–electron correlation beyond the mean-field level is prominent. We visualize the dependence of the wave function on the internuclear distance. High energy photoelectrons are shown to be a powerful tool for molecular imaging. Our study paves the way for future time resolved correlation imaging at FELs and laser based X-ray sources., Published by Nature Publishing Group UK, [London]

Although valence electrons are clearly delocalized in molecular bonding frameworks, chemists and physicists have long debated the question of whether the core vacancy created in a homonuclear diatomic molecule by absorption of a single x-ray photon is localized on one atom or delocalized over both. We have been able to clarify this question with an experiment that uses Auger electron angular emission patterns from molecular nitrogen after inner-shell ionization as an ultrafast probe of hole localization. The experiment, along with the accompanying theory, shows that observation of symmetry breaking (localization) or preservation (delocalization) depends on how the quantum entangled Bell state created by Auger decay is detected by the measurement.

The fragmentation pathways and dynamics of ethylene molecules after core ionization are explored using coincident measurements of the Auger electron and fragment ions by employing the cold target recoil-ion momentum spectroscopy method. The influence of several factors on the dynamics and kinematics of the dissociation is studied. These include propensity rules, ionization mechanisms, symmetry of the orbitals from which the Auger electrons originate, multiple scattering, conical intersections, interference, and possible core-hole localization for the double ionization of this polyatomic molecule. Energy correlation maps allow probing the multidimensional potential energy surfaces and, in combination with our multiconfiguration self-consistent field calculations, identifying the populated electronic states of the dissociating dication. The measured angular distributions of the Auger electrons in the molecular frame further support and augment these assignments. The deprotonation and molecular hydrogen ion elimination channels show a nearly isotropic Auger electron angular distribution with a small elongation along the direction perpendicular to the molecular axis. For the symmetric breakup the angular distributions show a clear influence of multiple scattering on the outgoing electrons. The lowest kinetic energy release feature of the symmetric breakup channel displays a fingerprint of entangled Auger and photoelectron motion in the angular emission pattern identifying this transition as an excellent candidate to probe core-hole localization at a conical intersection of a polyatomic molecule.

Intermolecular Coulombic decay transfers excess energy to neighbouring molecules, which then lose a low-energy (and, hence, genotoxic) electron; here the process is experimentally confirmed to be site-selective and highly efficient, possibly enabling more targeted radiation therapy. The irradiation of matter with light tends to electronically excite atoms and molecules. What happens to the resulting excitation energy depends on the nature of the relaxation pathway and the energy of the electrons and ions produced. In one such pathway, known as intermolecular Coulombic decay (ICD), excess energy is transferred to neighbouring atoms or molecules that then lose an electron and become ionized. ICD electrons have relatively low energy, prompting suggestions that they might be harnessed as a form of Auger therapy — cancer treatment that uses large numbers of genotoxic low-energy electrons to damage cancer cells. In a pair of papers [in this issue of Nature] published online this week, Gokhberg et al. propose that ICD can be triggered upon relaxation of an initial resonant core excitation, and Trinter et al. confirm the existence of the proposed excitation experimentally. The efficiency of this relaxation cascade and the fact that it can be tuned to directly control the generation site and the energy of the electrons raise the prospect of the development of more targeted cancer radiotherapy, and possibly new spectroscopic techniques. In 1997, it was predicted1 that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen-bonded or van-der-Waals-bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low-energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon2,3,4,5,6,7,8,9,10,11,12, raising questions about its role in DNA damage induced by ionizing radiation, in which low-energy electrons are known to play an important part13,14. It was recently suggested15 that ICD can be triggered efficiently and site-selectively by resonantly core-exciting a target atom, which then transforms through Auger decay into an ionic species with sufficiently high excitation energy to permit ICD to occur. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant-Auger-driven ICD cascade, we find that ICD occurs in less time than the 20 femtoseconds it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X-ray excitation schemes16,17 for more localized and targeted cancer radiation therapy.

We investigate the dynamics of ultra-low kinetic energy photoelectrons. Many experimental techniques employed for the detection of photoelectrons require the presence of (more or less) weak electric extraction fields in order to perform the measurement. Our studies show that ultra-low energy photoelectrons exhibit a characteristic shift in their apparent measured momentum when the target system is exposed to such static electric fields. Already fields as weak as 1 V cm(-1) have an observable influence on the detected electron momentum. This apparent shift is demonstrated by an experiment on zero energy photoelectrons emitted from He and explained through theoretical model calculations.

Helium shows fascinating quantum phenomena unseen in any other element. In its liquid phase, it is the only known superfluid. The smallest aggregates of helium, the dimer (He2) and the trimer (He3) are, in their predicted structure, unique natural quantum objects. While one might intuitively expect the structure of (4)He3 to be an equilateral triangle, a manifold of predictions on its shape have yielded an ongoing dispute for more than 20 years. These predictions range from (4)He3 being mainly linear to being mainly an equilateral triangle. Here we show experimental images of the wave functions of (4)He3 and (3)He(4)He2 obtained by Coulomb explosion imaging of mass-selected clusters. We propose that (4)He3 is a structureless random cloud and that (3)He(4)He2 exists as a quantum halo state.

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

The photon-ion merged-beams technique has been employed at the new Photon-Ion spectrometer at PETRA III (PIPE) for measuring multiple photoionization of Xe$^{q+}$ (q=1-5) ions. Total ionization cross sections have been obtained on an absolute scale for the dominant ionization reactions of the type h\nu + Xe$^{q+}$ $\to$ Xe$^{r+}$ + (q-r) e$^-$ with product charge states q+2 $\le$ r $\le$ q+5. Prominent ionization features are observed in the photon-energy range 650-750 eV, which are associated with excitation or ionization of an inner-shell 3d electron. Single-configuration Dirac-Fock calculations agree quantitatively with the experimental cross sections for non-resonant photoabsorption, but fail to reproduce all details of the measured ionization resonance structures., Comment: 20 pages, 12 figures, submitted for publication to J. Phys. B

We present kinematically complete measurements of the photo-double-ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double-ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photoelectrons and the nuclear kinetic-energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels. The reason for such a striking difference is explained in part on the basis of a propensity rule that influences the population of states in the photo-double-ionization of a centrosymmetric closed-shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential-energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

Journal of physics / Conference Series 488(14), 142006 (2014). doi:10.1088/1742-6596/488/14/142006, A photon-ion merged beam endstation has been set up at the variable polarization XUV-beamline P04 of PETRA III in Hamburg. In a commissioning experiment first results could be obtained for multiple photoionization of Xe$^{q+}$ ions (q = 1,2,..,5) at photon energies around the 3d ionization threshold., Published by IOP Publ., Bristol

We investigate the onset of photoionization shakeup induced interatomic Coulombic decay (ICD) in He2 at the He+*(n = 2) threshold by detecting two He+ ions in coincidence. We find this threshold to be shifted towards higher energies compared to the same threshold in the monomer. The shifted onset of ion pairs created by ICD is attributed to a recapture of the threshold photoelectron after the emission of the faster ICD electron., Comment: 5 Pages, 2 Figures

During the past 15 years a novel decay mechanism of excited atoms has been discovered and investigated. This so-called interatomic Coulombic decay (ICD) involves the chemical environment of the electronically excited atom: the excitation energy is transferred (in many cases over long distances) to a neighbor of the initially excited particle usually ionizing that neighbor. It turned out that ICD is a very common decay route in nature as it occurs across van der Waals and hydrogen bonds. The time evolution of ICD is predicted to be highly complex, as its efficiency strongly depends on the distance of the atoms involved and this distance typically changes during the decay. Here we present the first direct measurement of the temporal evolution of ICD using a novel experimental approach.

An ion-momentum spectrometer is used to study the dissociative dynamics of electron attachment to molecules. A skimmed, supersonic gas jet is crossed with a pulsed beam of low-energy electrons, and the resulting negative ions are extracted toward a time- and position-sensitive detector. Calculations of the momentum in three dimensions may be used to determine the angular dependence of dissociative attachment as well as the energetics of the reaction.

Molecular frame photoelectron angular distributions (MFPADs) are measured in electron-ion momentum imaging experiments and compared with complex Kohn variational calculations for carbon K-shell ionization of carbon tetrafluoride (CF4) ,e thane(C2H6) and 1,1-difluoroethylene (C2H2F2). While in ethane the polarization averaged MFPADs show a tendency at low energies for the photoelectron to be emitted in the directions of the bonds, the opposite effect is seen in CF4 .A combination of these behaviors is seen in difluoroethylene where ionization from the two carbons can be distinguished experimentally because of their different K-shell ionization potentials. Excellent agreement is found between experiment and simple static-exchange or coupled two- channel theoretical calculations. However, simple electrostatics do not provide an adequate explanation of the suggestively simple angular distributions at low electron ejection energies.

We demonstrate a method for determining the full three-dimensional molecular-frame photoelectron angular distribution in polyatomic molecules using methane as a prototype. Simultaneous double Auger decay and subsequent dissociation allow measurement of the initial momentum vectors of the ionic fragments and the photoelectron in coincidence, allowing full orientation by observing a three-ion decay pathway, (H þ , H þ , CH þ ). We find the striking result that at low photoelectron energies the molecule is effectively imaged by the focusing of photoelectrons along bond directions.

Theoretical two-center interference patterns produced (i) by the $K$-shell photoionization process of the ${\mathrm{N}}_{2}$ molecule and (ii) by the Auger decay process of the $K$-shell hole state of the ${\mathrm{N}}_{2}$ molecule are compared for the case of equal photo- and Auger-electron energies of about 360 eV. The comparison shows that both the angular distribution of the photoelectrons and the angular distribution of the Auger electrons of equal energy in the molecular frame are primarily defined by the Young interference. The experimental data for the angular resolved $K$-shell Auger electrons as a function of the kinetic-energy release (KER) obtained earlier [Phys. Rev. A 81, 043426 (2010)] have been renormalized in order to visualize the angular variation in the regions of low Auger-electron intensities. That renormalized data are compared with the corresponding theoretical results. From the known behavior of the potential energy curves, the connection between the KER and the internuclear distance can be established. Since the Young interference pattern is sensitive to the internuclear distance in the molecule, from the measured KER dependence of the Young interference pattern one can trace the behavior of the Auger-electron angular distribution for different molecular terms as a function of internuclear distance. The results of that analysis are in a good agreement with the corresponding theoretical predictions.