Abstract For photo-dissociation of a single hydrogen molecule ( $$H_2$$ H 2 ) with combined XUV and IR laser pulses, we demonstrate optical control of the emission direction of the photoelectron with respect to the outgoing neutral fragment (the H-atom). Depending on the relative delay between the two laser fields, adjustable with sub-femtosecond time resolution, the photoelectron is emitted into the same hemisphere as the H-atom or opposite. This emission asymmetry is a result of entanglement of the two-electron final-state involving the spatially separated bound and emitted electron.

Following up on a previous paper on two-color photoionization of Ar(3p) [Bharti et al., Phys. Rev. A 103 (2021) 022834], we present measurements and calculations for a modified three-sideband (3-SB) version of the "reconstruction of attosecond beating by interference of two-photon transitions" (RABBITT) configuration applied to He(1s). The 3-SB RABBITT approach allows us to explore interference effects between pathways involving different orders of transitions within the continuum. The relative differences in the retrieved oscillation phases of the three sidebands provide insights into the continuum-continuum transitions. The ground state of helium has zero orbital angular momentum, which simplifies the analysis of oscillation phases and their angle-dependence within the three sidebands. We find qualitative agreement between our experimental results and the theoretical predictions for many cases but also observe some significant quantitative discrepancies.

We report a joint experimental and theoretical study of a three-sideband (3-SB) modification of the reconstruc-tion of attosecond beating by interference of two-photon transitions (RABBIT) setup. The 3-SB RABBIT scheme makes it possible to investigate phases resulting from interference between transitions of different orders in the continuum. Furthermore, the strength of this method is its ability to focus on the atomic phases only, independent of a chirp in the harmonics, by comparing the RABBIT phases extracted from specific SB groups formed by two adjacent harmonics. We verify earlier predictions that the phases and the corresponding time delays in the three SBs extracted from angle-integrated measurements become similar with increasing photoelectron energy. A variation in the angle dependence of the RABBIT phases in the three SBs results from the distinct Wigner and continuum-continuum coupling phases associated with the individual angular-momentum channels. A qualitative explanation of this dependence is attempted by invoking a propensity rule. Comparison between the experimental data and predictions from an R matrix (close-coupling) with time dependence calculation shows qualitative agreement in most of the observed trends.

We report a joint experimental and theoretical study of a three-sideband (3-SB) modification of the “reconstruction of attosecond beating by interference of two-photon transitions” (RABBIT) setup. The 3-SB RABBIT scheme allows to investigate phases resulting from the interference between transitions of different orders in the ionization continuum. The strength of this method is its ability to focus on the angle-dependent atomic phases only, independent of a possible spectral chirp of the exiting EUV fields. Furthermore, with a traditional 1-SB RABBITT scheme, we observe an asymmetry of the electron emission direction with respect to the proton after few-photon dissociation of molecular hydrogen and present sub-femtosecond control of this emission direction.

In this paper, a 3-dimensional photoelectron/ion momentum spectrometer (reaction microscope) combined with a table-top attosecond beamline based on a high-repetition rate (49 kHz) laser source is presented. The beamline is designed to achieve a temporal stability below 50 attoseconds. Results from measurements on systems like molecular hydrogen and argon dimers demonstrate the capabilities of this setup in observing the attosecond dynamics in 3D while covering the full solid angle for ionization processes having low cross-sections.

We report a joint experimental and theoretical study of a three-sideband (3-SB) modification of the "reconstruction of attosecond beating by interference of two-photon transitions" (RABBIT) setup. The 3-SB RABBIT scheme makes it possible to investigate phases resulting from interference between transitions of different orders in the continuum. Furthermore, the strength of this method is its ability to focus on the atomic phases only, independent of a chirp in the harmonics, by comparing the RABBIT phases extracted from specific SB groups formed by two adjacent harmonics. We verify earlier predictions that the phases and the corresponding time delays in the three SBs extracted from angle-integrated measurements become similar with increasing photon electron energy. A variation in the angle dependence of the RABBIT phases in the three SBs results from the distinct Wigner and continuum-continuum coupling phases associated with the individual angular momentum channels. A qualitative explanation of this dependence is attempted by invoking a propensity rule. Comparison between the experimental data and predictions from an R-matrix (close-coupling) with time dependence calculation shows qualitative agreement in the observed trends.

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range 19-40 eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the C2H2+ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the 4π-collection geometry of the velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

Entanglement is one of the most fascinating aspects distinguishing quantum from classical physics. It is the backbone of quantum information processing which relies on engineered quantum systems. It also exists in natural systems such as atoms and molecules, showcased in many experimental instances mostly in the form of entangled photon pairs and a few examples of entanglement between massive particles. Nevertheless, the control of entanglement in natural systems has never been demonstrated. In artificially prepared quantum systems, on the other hand, the creation and manipulation of entanglement lies at the heart of quantum computing currently implemented in a wide array of two-level systems (e.g. trapped ions, superconducting and semiconductor systems). These processes are, however, relatively slow: the time scale of the entanglement generation and control ranges from a couple of {\mu}s in case of trapped-ion quantum systems down to tens of ns in superconducting systems. In this letter, we show ultrafast optical control of entanglement between massive fundamental particles in a natural system on a time scale faster than that available to engineered systems. We demonstrate the sub-femtosecond control of electronic entangled states in a single hydrogen molecule by applying few-photon interactions with adjustable relative delays. This molecular entanglement is revealed in the asymmetric electron emission with respect to the ejected proton in the photodissociation of H2. We anticipate that these results open the way to entanglement-based operations at THz speed.

Complete measurements of photoionization include both time-resolved and angle-resolved mapping of the atomic and molecular dynamics. The setup described in this work is suited to be used for attosecond pump-probe experiments as well as providing insights into the angular dependencies of photoionization.

We present a novel high-repetition rate (143 kHz) attosecond beamline in combination with a reaction microscope in order to study time, 3D-momentum and channel resolved photoionization and dissociation processes. We present channel resolved RABBITT measurements.

Embedded atoms or molecules in a photoexcited He nanodroplet are well-known to be ionized through inter-atomic relaxation in a Penning process. In this work, we investigate the Penning ionization of acetylene oligomers occurring from the photoexcitation bands of He nanodroplets. In close analogy to conventional Penning electron spectroscopy by thermal atomic collisions, the n = 2 photoexcitation band plays the role of the metastable atomic 1s2s 3,1S He*. This facilitates electron spectroscopy of acetylene aggregates in the sub-Kelvin He environment, providing the following insight into their structure: the molecules in the dopant cluster are loosely bound van der Waals complexes rather than forming covalent compounds. In addition, this work reveals a Penning process stemming from the n = 4 band where charge-transfer from autoionized He in the droplets is known to be the dominant relaxation channel. This allows for excited states of the remnant dopant oligomer Penning-ions to be studied. Hence, we demonstrate Penning ionization electron spectroscopy of doped droplets as an effective technique for investigating dopant oligomers which are easily formed by attachment to the host cluster.

Helium nanodroplets are a unique system of quantum fluid clusters possessing several intriguing properties. From the perspective of atomic systems, they have been predominantly viewed as a spectroscopic matrix hosting other molecules and systems of interest. In this report, we draw particular attention to select electronic processes in He aggregates hosting other atoms, in particular, Rb. From this perspective, we present the details of single- and multi-electron processes occurring in this alkali-He system interacting with single extreme ultraviolet and soft X-ray photons. The features brought out in this study are generic and pertinent to systems with similar design on the atomic scale.

Photoionization process of acetylene doped in helium nanodroplets is studied with EUV synchrotron radiation with photon energies between 20 and 26 eV by Photoelectron-Photoion Coincidence (PEPICO) experiment by detecting photoelectrons in coincidence with the photoions using electron velocity map imaging (VMI) spectrometer and ion time of flight (TOF) spectrometer. Acetylene is ionized in the droplet via Penning ionization at 21.6 eV photon energy. For photon energy of 23.9 eV and above the photoionization threshold of He, charge transfer ionization occurs in acetylene following autoionization and direct ionization in the droplet respectively.