Generation of entanglement using a short-wavelength seeded free-electron laser.

Quantum entanglement between the degrees of freedom encountered in the classical world is challenging to observe due to the surrounding environment. To elucidate this issue, we investigate the entanglement generated over ultrafast timescales in a bipartite quantum system comprising two massive particles: a free-moving photoelectron, which expands to a mesoscopic length scale, and a light-dressed atomic ion, which represents a hybrid state of light and matter. Although the photoelectron spectra are measured classically, the entanglement allows us to reveal information about the dressed-state dynamics of the ion and the femtosecond extreme ultraviolet pulses delivered by a seeded free-electron laser. The observed generation of entanglement is interpreted using the time-dependent von Neumann entropy. Our results unveil the potential for using short-wavelength coherent light pulses from free-electron lasers to generate entangled photoelectron and ion systems for studying spooky action at a distance.

Resonant Inner-Shell Photofragmentation of Adamantane (C10H16).

Adamantane, the smallest diamondoid molecule with a symmetrical cage, contains two distinct carbon sites, CH and CH2. The ionization/excitation of the molecule leads to the cage opening and strong structural reorganization. While theoretical predictions suggest that the carbon site CH primarily causes the cage opening, the role of the other CH2 site remains unclear. In this study, we used advanced experimental Auger electron-ion coincidence techniques and theoretical calculations to investigate the fragmentation dynamics of adamantane after resonant inner-shell photoexcitation. Our results demonstrate that some fragmentation channels exhibit site-sensitivity of the initial core-hole location, indicating that different carbon site excitations could lead to unique cage opening mechanisms.

Site-dependent nuclear dynamics in core-excited butadiene.

Symmetry breaking and competition between electronic decay and nuclear dynamics are major factors determining whether the memory of the initial core-hole localisation in a molecule is retained long enough to affect fragmentation. We investigate the fate of core holes localised at different sites in the free 1,3 trans butadiene molecule by using synchrotron radiation to selectively excite core electrons from different C 1s sites to π* orbitals. Fragmentation involving bonds localised at the site of the core hole provides clear evidence for preferential bond breaking for a core hole located at the terminal carbon site, while the signature of localisation is weak for a vacancy on the central carbon site. The origin of this difference is attributed to out-of-plane vibrations, and statistical evaporation of protons for vacancies located at the central carbon sites.

Coincidence study of core-ionized adamantane: site-sensitivity within a carbon cage?

We investigate the fragmentation dynamics of adamantane dications produced after core-ionization at the carbon edge followed by Auger decay. The combination of high-resolution electron spectroscopy, energy-resolved electron-ion multi-coincidence spectroscopy and different theoretical models allows us to give a complete characterization of the processes involved after ionization. We show that energy- and site-sensitivity is observed even for a highly-symmetric molecule that lacks any unique atomic site.

Attosecond dynamics of multi-channel single photon ionization.

Photoionization of atoms and molecules is one of the fastest processes in nature. The understanding of the ultrafast temporal dynamics of this process often requires the characterization of the different angular momentum channels over a broad energy range. Using a two-photon interferometry technique based on extreme ultraviolet and infrared ultrashort pulses, we measure the phase and amplitude of the individual angular momentum channels as a function of kinetic energy in the outer-shell photoionization of neon. This allows us to unravel the influence of channel interference as well as the effect of the short-range, Coulomb and centrifugal potentials, on the dynamics of the photoionization process.

Observation of Rabi dynamics with a short-wavelength free-electron laser.

Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field1. They are ubiquitous in physics with applications in different areas such as photonics2, nano-electronics3, electron microscopy4 and quantum information5. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light-matter interactions within the rotating-wave approximation6. Although intense nanometre-wavelength light sources have been available for more than a decade7-9, Rabi dynamics at such short wavelengths has not been directly observed. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser10 can drive Rabi dynamics between the ground state and an excited state in helium atoms. The measured photoelectron signal reveals an Autler-Townes doublet and an avoided crossing, phenomena that are both fundamental to coherent atom-field interactions11. Using an analytical model derived from perturbation theory on top of the Rabi model, we find that the ultrafast build-up of the doublet structure carries the signature of a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond12 and few-femtosecond13 extreme-ultraviolet pulses, our results unfold opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.

The origin of enhanced [Formula: see text] production from photoionized CO2 clusters.

CO2-rich planetary atmospheres are continuously exposed to ionising radiation driving major photochemical processes. In the Martian atmosphere, CO2 clusters are predicted to exist at high altitudes motivating a deeper understanding of their photochemistry. In this joint experimental-theoretical study, we investigate the photoreactions of CO2 clusters (≤2 nm) induced by soft X-ray ionisation. We observe dramatically enhanced production of [Formula: see text] from photoionized CO2 clusters compared to the case of the isolated molecule and identify two relevant reactions. Using quantum chemistry calculations and multi-coincidence mass spectrometry, we pinpoint the origin of this enhancement: A size-dependent structural transition of the clusters from a covalently bonded arrangement to a weakly bonded polyhedral geometry that activates an exothermic reaction producing [Formula: see text]. Our results unambiguously demonstrate that the photochemistry of small clusters/particles will likely have a strong influence on the ion balance in atmospheres.

Attosecond electron-spin dynamics in Xe 4d photoionization.

The photoionization of xenon atoms in the 70-100 eV range reveals several fascinating physical phenomena such as a giant resonance induced by the dynamic rearrangement of the electron cloud after photon absorption, an anomalous branching ratio between intermediate Xe+ states separated by the spin-orbit interaction and multiple Auger decay processes. These phenomena have been studied in the past, using in particular synchrotron radiation, but without access to real-time dynamics. Here, we study the dynamics of Xe 4d photoionization on its natural time scale combining attosecond interferometry and coincidence spectroscopy. A time-frequency analysis of the involved transitions allows us to identify two interfering ionization mechanisms: the broad giant dipole resonance with a fast decay time less than 50 as, and a narrow resonance at threshold induced by spin-flip transitions, with much longer decay times of several hundred as. Our results provide insight into the complex electron-spin dynamics of photo-induced phenomena.

Controlling photoionization using attosecond time-slit interferences.

When small quantum systems, atoms or molecules, absorb a high-energy photon, electrons are emitted with a well-defined energy and a highly symmetric angular distribution, ruled by energy quantization and parity conservation. These rules are based on approximations and symmetries which may break down when atoms are exposed to ultrashort and intense optical pulses. This raises the question of their universality for the simplest case of the photoelectric effect. Here we investigate photoionization of helium by a sequence of attosecond pulses in the presence of a weak infrared laser field. We continuously control the energy of the photoelectrons and introduce an asymmetry in their emission direction, at variance with the idealized rules mentioned above. This control, made possible by the extreme temporal confinement of the light-matter interaction, opens a road in attosecond science, namely, the manipulation of ultrafast processes with a tailored sequence of attosecond pulses.

Fission of charged nano-hydrated ammonia clusters - microscopic insights into the nucleation processes.

While largely studied on the macroscopic scale, the dynamics leading to nucleation and fission processes in atmospheric aerosols are still poorly understood at the molecular level. Here, we present a joint experimental-theoretical study of a model system consisting of hydrogen-bonded ammonia and water molecules. Experimentally, the clusters were produced via adiabatic co-expansion. Double ionization ionic products were prepared using synchrotron radiation and analyzed with coincidence mass- and 3D momentum spectroscopy. Calculations were carried out using ab initio molecular dynamics to understand the fragmentation within the first ∼500 fs. Further exploration of the potential energy surfaces was performed at a DFT level of theory to gain information on the energetics of the processes. Water was identified as an efficient nano-droplet stabilizer, and is found to have a significant effect even at low water content. On the molecular level, the stabilizing role of water can be related to an increase in the dissociation energy between ammonia molecules and the water enriched environment at the cluster surface. Furthermore, our results support the role of ammonium as a charge carrier in the solution, preferentially bound to surrounding ammonia molecules, which can influence the atmospheric nucleation process.

Fano's Propensity Rule in Angle-Resolved Attosecond Pump-Probe Photoionization.

In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. We generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular dependence of the photoionization time delays and the delay dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40 eV range.

Tracing charge transfer in argon dimers by XUV-pump IR-probe experiments at FLASH.

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.

Anisotropic photoemission time delays close to a Fano resonance.

Electron correlation and multielectron effects are fundamental interactions that govern many physical and chemical processes in atomic, molecular and solid state systems. The process of autoionization, induced by resonant excitation of electrons into discrete states present in the spectral continuum of atomic and molecular targets, is mediated by electron correlation. Here we investigate the attosecond photoemission dynamics in argon in the 20-40 eV spectral range, in the vicinity of the 3s-1np autoionizing resonances. We present measurements of the differential photoionization cross section and extract energy and angle-dependent atomic time delays with an attosecond interferometric method. With the support of a theoretical model, we are able to attribute a large part of the measured time delay anisotropy to the presence of autoionizing resonances, which not only distort the phase of the emitted photoelectron wave packet but also introduce an angular dependence.

The role of charge and proton transfer in fragmentation of hydrogen-bonded nanosystems: the breakup of ammonia clusters upon single photon multi-ionization.

The charge and proton dynamics in hydrogen-bonded networks are investigated using ammonia as a model system. The fragmentation dynamics of medium-sized clusters (1-2 nm) upon single photon multi-ionization is studied, by analyzing the momenta of small ionic fragments. The observed fragmentation pattern of the doubly- and triply-charged clusters reveals a spatial anisotropy of emission between fragments (back-to-back). Protonated fragments exhibit a distinct kinematic correlation, indicating a delay between ionization and fragmentation (fission). The different kinematics observed for channels containing protonated and unprotonated species provides possible insights into the prime mechanisms of charge and proton transfer, as well as proton hopping, in such a nanoscale system.

Dissociation of cyclopropane in double ionization continuum.

Dissociative double photoionization of cyclopropane is studied in the inner-valence region using tunable synchrotron radiation. With the aid of ab initio quantum chemical calculations the energies of dication states and their favoured fragmentation pathways are determined. These are compared to the experimental appearance energies of two-body fragmentation processes and to the kinetic energy released upon dissociation. Photon energy dependent state-selective dissociation in the 25-35 eV range is found. Calculations of dissociation pathways suggest that cyclopropane ring-deformation is selectively triggered at certain photon energies. The calculations suggest that initial ring deformation essentially determines the population of different dication states that function as gateways for particular dissociation channels. The measurements show that stepwise ionization processes populate dissociative 3e'-2 states via ring-opening and Jahn-Teller active states at photon energies below the double-ionization threshold. For energies above the double-ionization threshold the kinematics indicate that double ionization takes place predominantly within the Franck-Condon region populating 3e'-1 1e''-1 states.

Dissociative double-photoionization of butadiene in the 25-45 eV energy range using 3-D multi-coincidence ion momentum imaging spectrometry.

Dissociative double-photoionization of butadiene in the 25-45 eV energy range has been studied with tunable synchrotron radiation using full three-dimensional ion momentum imaging. Using ab initio calculations, the electronic states of the molecular dication below 33 eV are identified. The results of the measurement and calculation show that double ionization from π orbitals selectively triggers twisting about the terminal or central C-C bonds. We show that this conformational rearrangement depends upon the dication electronic state, which effectively acts as a gateway for the dissociation reaction pathway. For photon energies above 33 eV, three-body dissociation channels where neutral H-atom evaporation precedes C-C charge-separation in the dication species appear in the correlation map. The fragment angular distributions support a model where the dication species is initially aligned with the molecular backbone parallel to the polarization vector of the light, indicating a high probability for double-ionization to the "gateway states" for molecules with this orientation.

Molecular dynamics of NH3 induced by core-electron excitation.

Nuclear motion in the N1s(-1)4a core-excited state of ammonia is investigated by studying the angular anisotropy of fragments produced in the decay of the highly excited molecule and compared with predictions from ab initio calculations. Two different fragmentation channels (H(+)/NH2(+) and H(+)/NH(+)/H) reveal complex nuclear dynamics as the excitation photon energy is tuned through the 4a1 resonance. The well-defined angular anisotropy of the fragments produced in the dissociation of the molecular dication species suggests a very rapid nuclear motion and the time scale of the nuclear dynamics is limited to the low fs timescale.

Multi-purpose two- and three-dimensional momentum imaging of charged particles for attosecond experiments at 1 kHz repetition rate.

We report on the versatile design and operation of a two-sided spectrometer for the imaging of charged-particle momenta in two dimensions (2D) and three dimensions (3D). The benefits of 3D detection are to discern particles of different mass and to study correlations between fragments from multi-ionization processes, while 2D detectors are more efficient for single-ionization applications. Combining these detector types in one instrument allows us to detect positive and negative particles simultaneously and to reduce acquisition times by using the 2D detector at a higher ionization rate when the third dimension is not required. The combined access to electronic and nuclear dynamics available when both sides are used together is important for studying photoreactions in samples of increasing complexity. The possibilities and limitations of 3D momentum imaging of electrons or ions in the same spectrometer geometry are investigated analytically and three different modes of operation demonstrated experimentally, with infrared or extreme ultraviolet light and an atomic/molecular beam.

Rapid bond rearrangement in core-excited molecular water.

The angular anisotropy of fragments created in the dissociation of core-electron excited water molecules is studied to probe the correlation between fragmentation channels, kinematics and molecular geometry. We present fragment kinetic measurements for water molecules where the inner-shell oxygen electron is excited to the unoccupied 4a1 and 2b2 valence molecular orbitals. The kinematics of individual fragmentation channels are measured using fully three-dimensional momentum imaging of fragments. The results show that the geometry of the molecule and the kinetic energy of fragments are strongly coupled in the atomisation process. In addition we identify a fragmentation process arising from bond rearrangement evidenced by the H2(+)-O(+) ion pair which is accessible for resonant excitation of the 1s electron. In all of the two-body fragmentation processes the dissociation takes place along the potential-energy surface, while atomisation reveals both dissociation along the potential surface and Coulomb explosion. The angular distribution of fragments suggests that the bond rearrangement is very rapid; likely on a sub 10 fs time scale.

Core localization and sigma* delocalization in the O 1s core-excited sulfur dioxide molecule.

Electron-ion-ion coincidence measurements of sulfur dioxide at discrete resonances near the O 1s ionization edge are reported. The spectra are analyzed using a model based upon molecular symmetry and on the geometry of the molecule. We find clear evidence for molecular alignment that can be ascribed to symmetry properties of the ground and core-excited states. Configuration interaction (CI) calculations indicate geometry changes in accord with the measured spectra. For the SO(2) molecule, however, we find that the localized core hole does not produce measurable evidence for valence localization, since the transition dipole moment is not parallel to a breaking sigma* O-S bond, in contrast to the case of ozone. The dissociation behavior based upon the CI calculations using symmetry-broken orbitals while fixing a localized core-hole site is found to be nearly equivalent to that using symmetry-adapted orbitals. This implies that the core-localization effect is not strong enough to localize the sigma* valence orbital.