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In ultrafast spectroscopy, the temporal resolution of time-resolved experiments depends on the duration of the pump and probe pulses, and on the control and characterization of their relative synchronization. Free-electron lasers operating in the extreme ultraviolet and X-ray spectral regions deliver pulses with femtosecond and attosecond duration in a broad array of pump–probe configurations to study a wide range of physical processes. However, this flexibility, together with the large dimensions and high complexity of the experimental set-ups, limits control of the temporal delay to the femtosecond domain, thus precluding a time resolution below the optical cycle. Here we demonstrate a novel single-shot technique able to determine the relative synchronization between an attosecond pulse train—generated by a seeded free-electron laser—and the optical oscillations of a near-infrared field, with a resolution of one atomic unit (24 as). Using this attosecond timing tool, we report the first example of attosecond coherent control of photoionization in a two-colour field by manipulating the phase of high-order near-infrared transitions.Relative synchronization between free-electron laser pulses and a near-infrared field fields is achieved with 24 as resolution by using a correlation analysis of single-shot photoelectron spectra. It is applied to coherently control the photoionization process in neon atom on the attosecond timescale.

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 4 d 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. Here the authors report experiment and theory study of the photoionization of xenon inner shell 4d electron using attosecond pulses. They have identified two ionization paths - one corresponding to broad giant dipole resonance with short decay time and the other involving spin-flip transitions.

Photoinduced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic structure and molecular dynamics calculations of the excited- and ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state are shown to be driven by ballistic S–C bond extension and to be complete within 350 fs. Theory and experiment also enable visualization of the rich ground-state dynamics that involve the formation of, and interconversion between, ring-opened isomers and the cyclic structure, as well as fragmentation over much longer timescales.Photoinduced isomerization reactions, including ring-opening reactions, lie at the heart of many chemical processes in nature. The pathway and dynamics of the ring opening of a model heterocycle have now been investigated by femtosecond photoelectron spectroscopy combined with ab initio theory, enabling the visualization of rich dynamics in both the ground and excited electronic states.

The dissociative double ionization of sulphur hexafluoride, SF 6 , in the ionization energy range from threshold up to 48.4 eV has been examined in detail using a multiple coincidence electron-ion technique. The results are interpreted by comparison with molecular dynamics simulations, high level molecular structure calculations and with a statistical model of the ion breakdown. Comparison between the experimental breakdown pattern and the pattern derived on the basis of statistical theory indicates that the energy redistribution required for fully statistical behaviour is incomplete on the timescale of the dissociation reactions of , suggesting that the molecular size at which ergodic behaviour becomes dominant is larger for doubly and multiply charged ions than for neutral and singly ionized molecules.

Single photon double ionization of carbon diselenide ( ) has been investigated by means of multi-particle coincidence techniques. The interpretation of the experimental spectra is helped by post-Hartree-Fock computations at the Coupled Clusters and Multi-Reference Configuration-Interaction levels to determine the energetics and electronic state potentials of and its fragments. The lowest experimental double ionization energy of has been found to be 24.68 ± 0.20 eV, reflecting the ground state, and is in agreement with the theoretical vertical double ionization energy of 24.41 eV. Several fragmentation channels are reported including experimental appearance energies and kinetic energy releases in comparison to theoretical results on their characteristics. In particular, we identify several purely repulsive, Coulomb explosion fragmentation channels.

We have measured the core-valence double ionization spectra of carbon suboxide above both the O 1s and C 1s edges. Following several core-valence cases known in the literature, to begin with the spectra are compared with the well-known single ionization valence photoelectron spectrum of this system, from which they surprisingly differ quite strongly. This motivates a comparison to electronic structure calculations carried out within the sudden approximation where the overlap between the core and valence orbital is included, while still assuming decoupling of the core and valence ionization events. The substantially improved agreement indicates that this more complex description is needed to model the core-valence double ionization process adequately in the present case. Accordingly, assignments of spectral features are made by comparison of our experimental and numerical spectra. Auger decay following C 1s hole formation at the chemically distinct central and outer C atoms shows strong selectivity in the final multiply charged states produced, for initial core-valence ionization, being consistent with the Auger decay from single core ionization. Comparison to calculations allows for the identification of the initial core ionization site for the Auger decay following core-valence ionization.

When a molecule loses two electrons, Coulomb repulsion makes the resulting doubly charged system likely to fragment into two singly charged ions. These monocations can be detected in a correlated fashion using multiplex time-of-flight spectroscopy. The island shapes in the ion-ion coincidence maps derived from such two-body dissociations contain detailed information on the physical processes underlying the fragmentation. Here, a simple method is presented where a fit function is used to determine the anisotropy parameter β of the molecular distribution from the peak shape of the time-of-flight difference of the two ions. The validity of the method is demonstrated by performing fits to simulated peak shapes, recovering the β value of the input angular distribution, and by comparison of experimental peak shapes to β values known from the literature.

In this paper, we report X-ray absorption and core-level electron spectra of the nucleobase derivative 2-thiouracil at the sulfur L1- and L2,3-edges. We used soft X-rays from the free-electron laser FLASH2 for the excitation of isolated molecules and dispersed the outgoing electrons with a magnetic bottle spectrometer. We identified photoelectrons from the 2p core orbital, accompanied by an electron correlation satellite, as well as resonant and non-resonant Coster–Kronig and Auger–Meitner emission at the L1- and L2,3-edges, respectively. We used the electron yield to construct X-ray absorption spectra at the two edges. The experimental data obtained are put in the context of the literature currently available on sulfur core-level and 2-thiouracil spectroscopy.

Using time-of-flight multiple electron and ion coincidence techniques in combination with a helium gas discharge lamp and synchrotron radiation, the double ionisation spectrum of disulfur (S 2 ) and the subsequent fragmentation dynamics of its dication are investigated. The S 2 sample was produced by heating mercury sulfide (HgS), whose vapour at a suitably chosen temperature consists primarily of two constituents: S 2 and atomic Hg. A multi-particle-coincidence technique is thus particularly useful for retrieving spectra of S 2 from ionisation of the mixed vapour. The results obtained are compared with detailed calculations of the electronic structure and potential energy curves of S 2 2 + which are also presented. These computations are carried out using configuration interaction methodology. The experimental results are interpreted with and strongly supported by the computational results.

Attosecond photoelectron spectroscopy has opened up for studying light-matter interaction on ultrafast time scales. It is often performed with interferometric experimental setups that require outstanding stability. We demonstrate and characterize in detail an actively stabilized, versatile, high spectral resolution attosecond beamline based on a Mach-Zehnder interferometer. The active stabilization keeps the interferometer ultra-stable for several hours with an RMS stability of 13 as and a total pump-probe delay scanning range of  fs. A tunable femtosecond laser source to drive high-order harmonic generation allows for precisely addressing atomic and molecular resonances. Furthermore, the interferometer includes a spectral shaper in 4f-geometry in the probe arm as well as a tunable bandpass filter in the pump arm, which offer additional high flexibility in terms of tunability as well as narrowband or polychromatic probe pulses. We demonstrate the capabilities of the beamline via experiments using several variants of the RABBIT (reconstruction of attosecond beating by two photon transitions) technique. In this setup, the temporal-spectral resolution of photoelectron spectroscopy can reach a new level of accuracy and precision.