While, at the same time, the correlated momenta of the entangled electron pair continue to exhibit quantum interference.

H 2 , the smallest and most abundant molecule in the universe, has a perfectly symmetric ground state. What does it take to break this symmetry? Here we show that the inversion symmetry can be broken by absorption of a linearly polarized photon, which itself has inversion symmetry.In particular, the emission of a photoelectron with subsequent dissociation of the remaining H + 2 fragment shows no symmetry with respect to the ionic H + and neutral H atomic fragments. This result is the consequence of the entanglement between symmetric and antisymmetric H + 2 states resulting from autoionization. The mechanisms behind this symmetry breaking are general for all molecules.

We have measured absolute triple differential cross sections for photo-double ionization of helium at 20 eV excess. The measurement covers the full ranges of energy sharing and emission angles of the two photoelectrons. We compare our data for selected geometries with the convergent close-coupling (CCC) calculations as well as 2SC calculations by Pont and Shakeshaft and 3C calculations by Maulbetsch and Briggs. In terms of the absolute magnitude and the trend in the shapes of the triple differential cross section for different geometries we find good agreement of the CCC and published 2SC calculations with our measurement, though differences with respect to the observed shape of individual patterns still exist.

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.

However, despite 80 years of theoretical attention, near exact calculations for such systems are only available for bound states. On the experimental side, the tests of these calculations are largely based upon level energies or single particle momentum 3 distributions. Very promising and challenging new classes of experiments are those which achieve a complete description of the outcome following the excitation of the ground state to an unbound continuum. The momenta, i.e. the set of vectors, of all the fragments of an atom or molecule break-up can be measured in coincidence with high precision using state-of-the-art imaging and timing techniques [16]. These asymptotic many-particle momentum distributions are determined by the interaction inducing the fragmentation, the bound initial state from which it emerged, and the interactions between the outgoing particles. Thus it is useful to the experimentalist to keep the interaction process as simple as possible and to choose a geometry where final state interactions are negligible or under control. In the present study we used the absorption of a single photon to fragment the deuterium molecule: hν + D 2 → 2 e -+ 2 d + Due to their heavy masses, the initial motion of the nuclei in the continuum can be assumed the same as in the ground state at the instant of the electronic transition (Born Oppenheimer approximation). Once the electrons have left the system, the motion of the nuclei is solely determined by their Coulomb repulsion; they accelerate to a Kinetic Energy Release (KER) which corresponds to the Coulomb potential associated with their initial separation. Quantum mechanically one maps the nuclear vibrational wave-function onto the Coulomb potential to yield a KER spectrum. Inverting this process determines the squared nuclear vibrational wave-function from the measured KER spectrum [17]. Furthermore, by selecting events that occur within a fixed subregion in the KER spectrum, one samples molecules for which the corresponding internuclear distance is defined much more precisely than the full extent of the initial nuclear wave-function. This allows us to show how the electronic continuum momentum distribution depends on the inter-nuclear separation in the molecule and its orientation with respect to the photon polarization. [18,19,20,21]. In brief, inside our momentum spectrometer, a supersonic D 2 -gas jet was crossed with the linear polarized photon beam from the LBNL Advanced Light Source (D 2 provides a higher target density than a comparable H 2 gas jet and data less contaminated by random coincidences from background H 2 O). The electrons and ions created in the intersection of the photons with

We measure fully differential cross sections for photo-double-ionization of helium at energies 1, 6, and 20 eV above threshold. The data have been obtained by measuring in coincidence the momentum vector of the He 2ϩ ion and one of the electrons. Using time-of-flight and imaging techniques, we cover a solid angle of 25-100 % 4 of the final-state continuum of all particles. Therefore the experiment is not confined to any particular set of angles or energy sharing, and allows for a reliable absolute calibration. We present momentum distributions of the ions and a comprehensive set of differential cross sections for electron emission. The latter are on an absolute scale and cover both equal and unequal energy sharing-for both the fast and the slow electron fixed-and a wide range of polar angles. We also present the first data for noncoplanar geometry. For all energies the cross section is sharply peaked around the coplanar emission, i.e., both electrons are preferentially emitted in the plane of the recoiling ion and the photon polarization direction. For most of the geometries the shape of the cross sections is well described by fourth-order Wannier theory calculations.

We demonstrate the use of a multiparticle coincidence technique to image the diffraction of an electron wave whose source is placed at a specific site in a free molecule. Core-level photoelectrons are used to illuminate the molecule from within. By measuring the vector momenta of two molecular fragments and the photoelectron, a richly structured electron diffraction pattern is obtained in a body-fixed frame of the randomly oriented molecule in the gas phase. We illustrate this technique for CO, creating a photoelectron from the C(1s) shell and scanning its energy from zero to 30 eV.

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